ACTAUNIVERSITATIS

UPSALIENSISUPPSALA

2020

Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Pharmacy 283

Palladium(0)-Catalyzed Synthesisof Spirocycles and SupercriticalChemistry using a ResistivelyHeated Flow Reactor

AHMED ADEYEMI

ISSN 1651-6192ISBN 978-91-513-0855-5urn:nbn:se:uu:diva-402202

Dissertation presented at Uppsala University to be publicly examined in A1:107a, UppsalaBiomedical Centre (BMC), Husargatan 3, Uppsala, Friday, 6 March 2020 at 09:15 for thedegree of Doctor of Philosophy (Faculty of Pharmacy). The examination will be conductedin English. Faculty examiner: Professor Kenneth Wärnmark (Centre for Analysis andSynthesis, Lund University).

AbstractAdeyemi, A. 2020. Palladium(0)-Catalyzed Synthesis of Spirocycles and SupercriticalChemistry using a Resistively Heated Flow Reactor. Digital Comprehensive Summaries ofUppsala Dissertations from the Faculty of Pharmacy 283. 93 pp. Uppsala: Acta UniversitatisUpsaliensis. ISBN 978-91-513-0855-5.

This doctoral thesis focusses on an effective and selective approach to the synthesis ofspirocycles using palladium(0)-catalyzed Mizoroki-Heck reactions. In addition, selective andefficient chemistry was highlighted by the design and evaluation of a novel resistively heatedsystem for continuous flow (CF) synthesis for high-temperature and high-pressure applications.

Paper I described the design and evaluation of a novel resistively heated CF system. Thedesign of a low-cost, simple, robust, and effective CF system involving a resistively heatedsteel reactor capable of delivering 400 °C and 200 bar was reported. The reactor was evaluatedwith esterification, transesterification and direct carboxylic acid to nitrile conversions usingsupercritical ethanol, methanol and acetonitrile respectively. Diels-Alder reactions under neatconditions were also carried out at high temperature and pressure.

Paper II reported the synthesis of spirooxindoles by a selective application of thepalladium(0)-catalyzed Mizoroki-Heck spirocyclization. The precursors for the reaction weresynthesized by coupling 2-iodoanilines with esters derived from enantiomerically pure (+)-Vince lactam decorated with the bulky, directing 2,5-dimethylpyrrole protecting group. Tendifferent spirooxindoles were reported with good yields and high regio- and stereoselectivity.Functionalization of a synthesized spirooxindole was done by a palladium(0)-catalyzedalkoxycarbonylation, followed by selective deprotections.

In Paper III, ether precursors were synthesized from (+)-Vince lactam, via a Mitsunobureaction with the corresponding iodophenols. The precursors were later subjected to conditionsfor intramolecular Mizoroki-Heck reaction. Overall, 12 spiroethers were synthesized in useableyields, regioselectivity up to 98% and with excellent diastereoselectivity (d.e.>98%). Furtherfunctionalization to mono-protected rigidified amino acids was also demonstrated.

Keywords: Continuous Flow, Resistive heating, Supercritical fluids, high-temperaturesynthesis, Mizoroki-Heck reaction, Spirooxindoles, Spiroethers, Spirobenzofuranes,Unnatural Amino Acids

Ahmed Adeyemi, Department of Medicinal Chemistry, Organic Pharmaceutical Chemistry,Box 574, Uppsala University, SE-75123 Uppsala, Sweden.

© Ahmed Adeyemi 2020

ISSN 1651-6192ISBN 978-91-513-0855-5urn:nbn:se:uu:diva-402202 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-402202)

Si obi, ebi, iyawo ati awon omo mi.

List of Papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals.

I Adeyemi, A., Bergman, J., Brånalt, J., Sävmaker, J., Larhed, M. (2017) Continuous Flow Synthesis under High-Tempera-ture/High-Pressure Conditions Using a Resistively Heated Flow Reactor. Org. Process Res. Dev. 21(7): 947-955.

II Adeyemi, A., Wetzel, A., Bergman, J., Brånalt, J., Larhed, M. (2019) Regio- and Stereoselective Synthesis of Spirooxindoles via Mizoroki–Heck Coupling of Aryl Iodides. Synlett, 30(01): 82-88.

III Adeyemi, A., Odell, L. R., Larhed, M. (2020) Regio- and Stereo-Selective Synthesis of Allylic Spiroethers (Spirobenzofuranes) via Intramolecular Mizoroki-Heck Reaction. J. Org. Chem (Manuscript under revision)

Reprints were made with permission from the respective publishers.

Contents

1 Introduction ......................................................................................... 11

1.1 Catalysis in Drug Discovery ........................................................... 11

1.2 Spirocycles in Medicinal Chemistry ............................................... 22

1.3 Unnatural Amino Acids - Tools for Fine-tuning Peptideomimetics ..................................................................................... 25

2. CF Chemistry - Introduction ................................................................ 27

3. Aims of Present Study ......................................................................... 35

4. CF Synthesis under High-Temperature/High-Pressure Conditions Using a Resistively Heated Flow Reactor (Paper I)...................................... 36

Objectives of the Project .......................................................................... 36

Evaluation of the CF reactor .................................................................... 40

Conclusion and Projections ...................................................................... 54

4. Regio- and Stereoselective Synthesis of Spirooxindoles via Mizoroki–Heck Coupling of Aryl Iodides (Paper II) .................................... 56

a. Aims and Objectives ....................................................................... 56

b. Results and Discussions .................................................................. 56

c. Conclusion ...................................................................................... 64

5. Regio- and Stereo-Selective Synthesis of Allylic Spiroethers (Spirobenzofuranes) via Intramolecular Mizoroki-Heck Reaction (Paper III) ............................................................................................................. 65

a. Aims and Objectives ....................................................................... 65

b. Results and Discussions ............................................................. 65

c. Conclusion ...................................................................................... 75

6. Acknowledgements.............................................................................. 76

7. References ........................................................................................... 78

Abbreviations

BPR Backpressure regulator

CF

DC

d.e.

DIBAL-H

DIAD

Continuous flow

Direct current

Diastereomeric excess

Diisobutylaluminium hydride

Diisopropyl azodicarboxylate

DMF Dimethylformamide

DPPF 1,1’-Ferrocenediyl-bis(di-phe-nylphosphine),

GC Gas chromatography

GC-MS Gas chromatography-mass spectrom-etry

JohnPhos (2-Biphenyl)di-tert-butylphosphine

L Ligand

LC Liquid chromatography

MW Microwave

NMP N-methyl-2-pyrrolidione

ORTEP Oak ridge thermal ellipsoid plot

RCM Ring closing metathesis

ScEtOH Supercritical ethanol

ScMeOH Supercritical methanol

ScMeCN Supercritical acetonitrile

SCFs Supercritical fluids

TLC Thin layer chromatography

UAAs Unnatural amino acids

Xphos 2-Di-cyclohexylphosphino-2’,4’,6’-triisopropylbiphenyl

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1 Introduction

1.1 Catalysis in Drug Discovery The synthesis of new drug molecules is one key aspect of drug discovery and development. In order to keep up with rapidly demanding need for new com-pounds, constant improvements must be made to synthetic methods. Develop-ment of new methods can be advantageous in giving more selective transfor-mations, cleaner reactions, ‘greener’ synthetic options, possible cost reduction and easy access to hitherto difficult reactions.

One fundamental transformation in organic synthesis is the formation of a car-bon-carbon bond (C-C bond).1,2 Over the past 50 years, a lot of interests have been focused on the formation of new C-C bonds from methods involving transition metal catalysis.

In order to be synthetically efficient, a method has to have favorable atom economy, good selectivity, operate in reasonably mild conditions and provide synthetically useful compounds.1,3 In constructing new C-C bonds, the need for improved selectivity has perhaps favored the use of transition metal catal-ysis. Interestingly, catalysis was first highlighted in an annual report by J. J. Berzelius to the Swedish Academy of Science in 1835, in which he reported:

“It is given then, that many, both simple and complex compounds, both in sol-ubilized and solid form, have the ability to, without necessarily contributing with its own constituents, produce transformation of other compounds. This is a new power, able to produce chemical activity, belonging to both inorganic and organic nature, which is surely more extensive than we have hitherto be-lieved and the nature of which is hidden to us. When I call it a new power, I do not mean that it is a force independent of the electrochemical properties of matter. On the contrary, I am unable to suppose that this is anything other than a kind of special manifestation of these, but as long as we are unable to discover their relationship, it will simplify our research to regard it as a separate power. It will also make it easier for us to refer to it if it possesses a name of its own. I shall therefore, using a derivation well known in chemistry, call it the cata-lytic power of substances, and decomposition by means of this power catalysis, similar to how we use the word analysis to denote separation of the component parts of bodies by means of ordinary chemical forces. The catalytic power ap-pears to constitute the ability of substances that, just by their mere presence, and not by its reactivity, awaken reactions that otherwise would be slumbering at the observed conditions.” Jöns Jakob Berzelius.4 Årsberättelse om framste-gen i Fysik och Kemi; P. A. Nordstedt: Stockholm, 1835.4

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1.1.1.1 Transition Metal Catalysis - A Viable Option With over 90% of all chemicals made involving at least one form of catalysis or another, it is no surprise that major advances in methods involving catalysis have dominated the chemistry literature space over the past few decades.5 Transition metals have partially filled d-orbitals in at least one of the stable cations formed. Aside having partially filled d-orbitals, transition metals also have vacant s and p- orbitals and sometimes vacant anti-bonding π* orbitals. These vacant orbitals largely define the characteristic properties of transition metal elements. The ions from these transition metals can thus bind with lig-ands to make metal-ligand complexes.

It should be noted that although all transition metals reside in the ‘d-block’, not all elements in the ‘d-block’ exhibit transition metal-like properties. After all, the name ‘transition’ denotes that these elements oscillate between elec-tropositive ‘s-block’ properties and electronegative ‘p-block’ properties.

Transition metal catalysis have been useful in important synthetic chemistry reactions such as cycloisomerizations, allylic alkylations, radical polymeriza-tion and in nucleophilic fluorination amongst others.6,7 Transition metal catal-ysis is also being combined with organocatalysis to a greater extent.8,9

However, there seems to have been a somewhat paradigmatic shift since the discovery of palladium catalysis in the 1970s.10,11 This led to a large interest and resulted in the 2010 Nobel Prize in Chemistry.

1.1.1.2 Palladium Catalysis - State of the art Palladium catalyzed reactions have been useful in a lot of areas such as drug development12, agrochemical development12, fragrances and terpenoid devel-opment13, polymer synthesis14, and more recently application in nanomedi-cine.15 Palladium has all the unique properties of transition metals, which in-cludes variable oxidation state, a property that accounts for the important cat-alytic applications.

Moreover, it is privileged as it is in the 2nd row in the late transition metal series, and can form both tetrahedral d10 and square planar d8 complexes in low oxidation states Pd(0) and Pd(II) respectively. Palladium compounds of high valency (Pd(IV)) have also been well reported for their use in catalysis.16 These variable oxidation states impose the unique properties of electron do-nation and electron withdrawal on palladium, making it a valuable transition metal.

Palladium catalysis has expanded in large proportions since the late 1960s.10,17 The applications of palladium catalysis can be attributed to three major oxida-

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tion states of the metal: Pd(0), Pd(II) and Pd(IV).18 Of these states, Pd(0)-ca-talysis has overwhelmingly found the most applications.19 The reactions are advantageous in many ways including:19

- Easily accessible substrates

- Generally good yields and good selectivity can be achieved

- Low catalyst amounts, with some reaction carried out with or without ligands

- Broad functional group compatibility in most of the reactions

- Ease of scale up and industrial applications

1.1.1.3 Pd(0)-Catalyzed Reactions One major importance of Pd(0)-catalysis is in reactions that form new carbon-carbon bonds through a cross-coupling reaction. The new C-C bond is formed via a palladium-carbon σ-complex intermediate, which brings together the re-acting species, thus allowing for a smooth transmetallation and a subsequent reductive elimination to generate the product. The reacting species (one elec-trophilic and the other nucleophilic) then form a new C-C bond, and in the process regenerate the palladium catalyst. The reactions are usually classed/named based on the nucleophilic species, but generally proceed as shown in Scheme 1.1.

Scheme 1.1. General representation of palladium(0)-catalyzed cross-coupling reac-tions.

Although the electrophilic specie R-X is less explored, and limited to few or-ganic halides and a number of alkylsulfonates such as triflates (OTf), the nu-cleophilic R’-M scope is being widened as more investigations into the reac-tions are carried out. The class of nucleophile R’-M is usually used to group or name the reaction subclass. The Suzuki reaction uses alkyl or aryl nucleo-philes, where R’-M are boronic esters or acids. The Stille coupling makes use of organotin compounds as nucleophile, while Negishi cross-coupling uses organozinc compounds. Other nucleophiles used includes organomagnesium compounds in the Kumada coupling, organosilicates in the Hiyama coupling, and organolithium derivatives reported by Feringa and co-workers over 6 years ago.20–23 Broadly speaking, these reactions proceed according to the cat-alytic cycle shown in Scheme 1.2.

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Scheme 1.2. A general representation of the catalytic cycle for palladium(0)-cata-lyzed cross-coupling reactions.

Apart from using organometallic nucleophiles, palladium(0) catalysis have also been applied to classic nucleophiles in reactions such as carbonylation, cyanation and Buchwald-Hartwig amination. Carbonylation reactions typi-cally involve a carbon monoxide (CO) source coupling to an aryl or akyl hal-ide or pseudo halide to form a carbonyl derivative of the nucleophile.24,25 Cy-anation reactions can also be catalyzed by Pd(0), using zinc or silyl cyanates as the CN source.26 The Buchwald-Hartwig reaction utilizes the deprotonated metal salt of amines.27 Coupling reactions involving alkynes can also be cata-lyzed by Pd(0), the Sonogashira reaction.28 Direct palladium(0)-catalyzed vi-nylic substitution using alkenes and organic halides (or pseudohalides) are named Mizoroki-Heck reactions.

1.1.1.4 The Mizoroki-Heck reaction The Mizoroki-Heck reaction (represented in Scheme 1.3) has become the most important method for vinylic substitution reactions.29,30 The reaction was in-dependently discovered by Tsutomu Mizoroki and Richard Heck working in-dependently on olefenes in the late 1960s and early 1970s.10,11,31 Since then, the reaction has been further investigated and developed to meet the ever-de-manding needs for an assortment of C-C bond formations. The wide applica-bility of the Mizoroki-Heck reaction to meet the needs in targeted C-C bond formation earned it a ‘sleeping beauty’ description by Oestreich in his 2007 review, although since then, there have some ‘awakening’ with more reported applications and a Nobel Prize.32

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Scheme 1.3. Mizoroki-Heck reaction involves an alkyl/aryl halide or pseudohalide and an alkene.

The reaction typically involves a vinyl or aryl halide or pseudo halide, reacting with an alkene to form a new C-C bond to the aryl/vinyl substrate, while main-taining the double bond as shown in Scheme 1.3.

The mechanism of the Mizoroki-Heck reaction is summarized into 5 major steps which are expatiated below.

1. Formation of palladium(0) from precatalysts.

The Mizoroki-Heck reaction is quite advantageous in that it can be carried out using a wide variety of palladium sources if the conditions are well opti-mized.33 Precatalysts such as PdOAc2, Pd(dba)2, PdCl2, are usually combined with phosphine ligands (L) to generate the active catalytic system. Immobi-lized molecular Pd(II) complexes have also been reported as precatalysts.19,33 Precatalysts, which are salts, are first reduced from the higher oxidation state Pd(II) to generate the active Pd(0)L2 specie 1.1a (Scheme 1.4 shows an exam-ple with Pd(OAc)2).

Scheme 1.4. Reduction of a palladium precatalysts to Pd(0), with Pd(OAc)2 as exam-ple.

The reduction of these Pd(II) precatalysts to Pd(0) can be achieved by different reaction components such as phosphine ligands or bases. The structure and reactivity of the Pd(0) complex 1.1a is affected by the ligand and the solvent used in the reaction.29 This in turn affects the efficiency (catalyst turnover) and selectivity of the reaction. Alkenes, and tertiary amines are amongst spe-cies that can also reduce precatalysts in situ.29,34

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2. Oxidative addition

The catalytic steps will be illustrated with a reaction between a linear alkene and an aryl halide. The active Pd(0) complex 1.1a is added to the aryl halide to form an Ar-Pd-X -complex 1.1b in Scheme 1.5. The oxidative addition is more favored with electron poor aryl halides. Furthermore, aryl iodides are most reactive for this step, while aryl chlorides are typically sluggish at this step. Hence, aryl halides with electron withdrawing groups in para positions are more favored than electron donating groups in this positions.29

Scheme 1.5. Oxidative addition of aryl halide to Pd(0) complex 1.1a.

On the other hand, the oxidative addition step is enhanced with bulky electron donating phosphine ligands.

3. Syn insertion of vinyl compound

The Ar-Pd-X -complex 1.1b next undergoes a dissociation process before coordination to the olefinic bond. There are two possibilities for the dissocia-tion depending on the type of ligand and/or halide as shown in Scheme 1.6. In few instances, the halide can dissociate, leading to a cationic intermedi-ate.29,35,36 Cationic intermediates like 1.1d are more favored in reaction sys-tems that accommodate cations better, such as polar solvents. On the other hand, the ligand can dissociate, without disrupting the neutrality of the aryl palladium halide complex formed (1.1c), hence it is termed the neutral path-way.35

Scheme 1.6. Different possibilities for association/dissociation can lead to cationic or neutral pathways.

The intermediate complex formed by either of the paths then reacts with the alkene to form vinyl-palladium π-intermediate (Scheme 1.7).

17

Scheme 1.7. The addition of the alkene to the Pd-complex can lead to two possible outcomes- affecting the regioselectivity outcome of the Mizoroki-Heck reaction.29

The π-complex intermediate then undergoes a syn insertion of the alkene as in Scheme 1.8. This reaction is also termed migratory insertion or carbopallada-tion. The syn insertion of the alkene provides to a new Pd(II) σ-complex 1.1e or 1.1f.29,35,36 This reaction step creates a new Pd-C bond; hence, it is often referred to as the carbopalladation step. The possibility of carbopalladation to form different π-complexes is the reason for possibly different regioselective outcome for the Mizoroki-Heck reaction (generating terminal or internal prod-ucts). The intermediates then undergoes an internal C-C rotation to reposition the β-hydrogen into a syn position with the palladium in 1.1g and 1.1h (Scheme 1.8).

Scheme 1.8. Migratory insertion leads to a Pd(II) σ-complex, which then undergoes an internal C-C bond rotation.

4. β-hydride elimination.

The next step after the internal C-C bond rotation is usually a β-hydride elim-ination. This reaction step is reversible, especially in instances of phosphine-free Mizoroki-Heck reactions.29 The β-hydride elimination is also face selec-

18

tive as it proceeds as a syn elimination as shown in Scheme 1.9. The elimina-tion reaction leads to the formation of an alkene and a hydridopalladium(II) halide (HPdXL2) complexing to it.

Scheme 1.9. β-hydride elimination leads to the formation of new double bond.

The reversibility of this β-hydride elimination means there is a possibility of readdition of the hydridopalladium(II) halide to the newly formed olefinic bond. This might lead to isomerization of the alkene product, providing a mix-ture of (E)/(Z) isomers and double bond migration products.

5. Product dissociation, reductive elimination, catalyst regeneration

The hydridopalladium(II) halide dissociates from the arylated product (in Scheme 1.9) and undergoes reductive elimination (Scheme 1.10). The pres-ence of a base drives this reductive elimination reaction. The base next depro-tonates the hydridopalladium(II)-halide to regenerate the catalytically active Pd(0)-ligand complex.

HPdXL2Base

Pd(0)L 2 + BaseH+X- Scheme 1.10. Catalyst regeneration.

1.1.1.5 Inter and Intramolecular application of Mizoroki-Heck Reaction

In terms of the reacting aryl halide (or pseudohalide) and the alkene substrate, there are two major types of the Mizoroki-Heck reaction: the intermolecular and intramolecular Mizoroki-Heck reaction as shown in Scheme 1.11. The intermolecular version involves two separate. However, in the intramolecular version, the aryl halide and vinyl functionality is incorporated in one com-pound. Intramolecular Mizoroki-Heck reaction is an extensively used method for cyclization of double bond containing compounds.37

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Scheme 1.11. Inter38 and intramolecular37,39 Mizoroki-Heck reaction examples. PG= protecting group.

One major drawback of the intermolecular Mizoroki-Heck reaction is the dif-ficulty in controlling regio- and stereoselectivity in the reaction with neutral and electron rich olefins, especially when cyclic alkens are involved. This is due to a lack of control on the insertion site for the aryl halide (or pseudohal-ide) into the olefinic bond (Scheme 1.8) subsequent double bond migra-tions.40,41 Another limitation of the intermolecular Mizoroki-Heck reaction is the limitation on the vinyl coupling partner to mono and di substituted al-kenes.42 Over the years, efforts have been made to enhance the reactivity and selectivity of the intermolecular Mizoroki-Heck reaction by selectively choos-ing aryl halides (or pseudohalides), while developing new fine-tuned ligands, and by evaluating different additives and reaction conditions.17,43,44

On the other hand, the intramolecular Mizoroki-Heck offers greater control on ring size in annulation reactions.45 The defined proximity of the aryl halide to the double bond reduces the number of possibilities for the insertion of the aryl halide into the olefinic bond. This in turn creates an opportunity for better reaction control and higher selectivity.

Moreover, the intramolecular version of the Mizoroki-Heck reaction can be applied to mono, di, tri and tetra substituted species, thus enhancing the pos-sibility of constructing congested stereogenic centers after syn β-hydride elim-ination and exo-cyclization.32 The reaction has therefore emerged as a very important tool in the formation of enantioselective C-C bonds in cyclization reactions.36,46

1.1.1.6 Selectivity in Intramolecular Mizoroki-Heck Reaction Selectivity in the intramolecular Mizoroki-Heck reaction can be of different types. The products formed can differ in the ring sizes, position of the double bond, and/or different enantio-, stereo- or diastereoisomers. The general

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mechanism for the intramolecular Mizoroki-Heck reaction is presented in Scheme 1.12.

Scheme 1.12.Mechanism for intramolecular Mizoroki-Heck exo-cyclization reaction.

Regioselectivity: Regioselectivity in intramolecular Mizoroki-Heck cycliza-tion is largely governed by the type and size of the ring formed during inser-tion.47,48 Cyclized products with 5-, 6- and 7-membered rings have all been investigated.47 In instances where the 5-exo-trig and 6-endo-trig are possible, the 5-exo-trig is preferably formed (Scheme 1.13).47–49 The cyclization prod-ucts on a 6-endo-trig have been reported in compounds that had amide nitro-gen in benzofused rings, prompting the author to suggest that indoline-based substrates generally favor 6-endo-trig pathway.50 In general, for Mizoroki-Heck cyclizations, the exo-trig is more favourable than the endo-trig. Moreo-ver, 5- membered rings are more kinetically favorable than 6-membered rings, and 6-membered rings more favorable than 7-membered rings.48 An example of two cyclization products is shown in Scheme 1.13.

N

OBr

N

O

+Pd(OAc)2

85%

N

O

A B

A:B = 1:1.4 Scheme 1.13. Different regiospecific outcome of the intramolecular Mizoroki-Heck reaction showing 6-endo-trig and 5-exo-trig cyclization products.50

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The nature of the substrate and reaction conditions can also affect the position of the double bond in the product, as shown in Scheme 1.14. In other instances, the hydridopalladium(II)specie (HPdX) could reengage the newly formed al-kene. This will lead to poor control of the double bond isomerization. Addi-tives such as silver salts have been reported to minimize the double bond isom-erization.45,49

Scheme 1.14. Possibility of regioselectivity emergence by double bond migration. X = halide, L= ligand.

Regiocontrol in the insertion step can also be achieved by decorating the al-kene with an auxiliary coordinating group. Hallberg and coworkers pioneered the use of chelation control by using a heteroatom directing group to influence the regioselectivity of the Mizoroki-Heck reaction.51

Stereoselectivity: As with control of regioselectivity, the direction from which insertion of the aryl palladium complex to the double bond can determine the stereochemical outcome of the reaction.35 Overman and Poon had previously demonstrated the impact of silver salt (Ag3PO4) in imposing stereoselectivity on the Mizoroki-Heck 5-exo spirocyclization of aryl iodide (Scheme 1.15).36,52 The ligand (R)-BINAP has been used to direct the stereoselectivity in the re-action.53 Stereoselectivity has been reported to also be affected by the type of base used in the reaction.35 Steric effect also plays a major role in diastereose-lectivity of the Mizoroki-Heck reaction.44,54

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Scheme 1.15. Different stereochemical outcomes in intramolecular Mizoroki-Heck reaction.36

Coordination to heteroatoms in groups such as hydroxyl or amino groups has been reported to control the stereoselectivity in Mizoroki-heck reaction of cy-clic olefins.42 Other factors such as the geometry of the intermediate formed after insertion, steric impacts of ligands, solvents and preference for cationic or neutral pathways may influence the stereoselectivity of the reaction. Bulky groups present on the susbtrates for the Mizoroki-Heck reaction have also been demonstrated to influence the stereoselectivity of the Mizoroki-Heck re-action.37–39 In the intermolecular version of the reaction, Wetzel and cowork-ers have demonstrated stereocontrol with the use of different amino protecting groups.38

1.2 Spirocycles in Medicinal Chemistry Spirocycles are polycyclic compounds formed when two rings are connected by one atom, called the spiroatom.55–58 The spiroatom acts as a quartenary centre connecting at least two cyclic structures as can be seen in Figure 1.1. Spirocycles are priviledged scaffolds in synthetic chemistry due to their inher-ent 3-dimensionality and uniqueness.55 Unlike the traditional methods of con-ferring defined 3-dimensionality on target compounds which involves cou-pling of multiple planar molecules, spirocycles provide an inherent 3-dimen-sionality in a single compound.59

Figure 1.1. Spirocycles are made when two rings are connected with one atom, called the spiroatom.

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This 3-dimensionality imposes rigidity on the compound, and leads to spa-tially well-defined positions of substituents on the spirocyclic scaffold.59 This rigidity is a useful tool in modification and design in drug delivery. There is an abundance of spirocycles in natural products and medicinal products.58,60–

64 Research into spirocyclic compounds with medicinal chemistry value has been on the rise over the years. This is in part due to the rigidity offered by spirocycles, as well as the novelty of spirocycles; a rich ground for patency applications.

Spirocycles come in various forms and compositions as shown in Figure 1.2. Different ring sizes and heteroatoms are well accommodated. Tricyclic spiro-cycles (AM-5262 in Figure 1.2) combining a 5 membered and 3- membered ring has been reported in the development of a potent GPR40 full agonist.65 Spirocyclic structures can be seen more frequently in yearly list of approved drugs. Ubrogepant, which features a spirocyclic core, was among small mol-ecules approved in 2019. Eplerenone and spironolactone are aldosterone blockers used in management of chronic heart failure, hypertension and high blood pressure.66,67 Buspirone (for treatment of anxiety disorders) and Irbesar-tan are drug molecules which also feature spirocyclic core.

Figure 1.2. Examples of drug molecules which have spirocyclic core.

Spirocycles are usually named according to the ring type and sizes. Some common subclass of spirocycles include spiroxindoles, spiroketals, triazaspi-rocycles, and spiroethers. The construction of spirocycles is synthetically challenging as it involves the creation of a new quaternary center, and in most cases a new carbon-carbon bond.68 Common synthetic methods used in con-structing spirocycles includes: ring closing metathesis (RCM), cycloadditions, rearrangements and metal catalyzed annulations.68–70 As this thesis is focused on spirooxindoles and spiroethers, only the synthesis of these sub-classes of spirocycles is discussed.

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Spirooxindoles have been used in drug discovery for novel anti-cancer agents by taking advantage of the privileged spirooxindole substructure.71,72 They have also been promising antituberculosis, antiviral, and antimalarial com-pounds (see Figure 1.3).39,73,74

Figure 1.3. Examples of spirooxindoles used as antimalarial, antiviral, antitubercu-losis and in migraine treatment.

Synthetic methods that deliver spirooxindoles are of importance to the medic-inal chemistry community. The construction of spirooxindoles with high ste-reoselectivity remains a challenge in synthetic chemistry.75–79 Until recently, majority of the methods reported for the synthesis of spirooxindoles were cen-tered around modification of isatin dericatives.60,72,77,78,80 This includes spiro-cyclization at different position of the indole skeleton. Other spirocyclization methods have also used palladium-catalyzed reactions. A palladium catalyzed Heck tandem reaction involving a cycloamidation and nucleophilic allylic substitution was employed by Kamisaki and co-workers in the synthesis of elacomine derivatives and other spirooxindoles (Scheme 1.16).81,82

Scheme 1.16. Kamisaki demonstrated the use of Heck tandem reaction in the synthe-sis of elacomine derivatives.

25

Other groups have also effectively applied palladium catalyzed intramolecular reactions to synthesize spirooxindoles.83–86 This is unsurprising as intramolec-ular Mizoroki-Heck reactions are well suited for the formation of stereogenic centres, which is always the challenge in spirooxindole synthesis.

Spiroethers are another important subclass of spirocycles. Unlike spirooxin-doles, preparation of spiroethers are much less investigated, despite the uniqueness of the presence of oxygen containing heterocycle in the spiroether core. Spiroethers however are very promising in medicinal chemistry, with examples of applications in antibacterial and anti-tumor investigations. Gris-eofulvin (shown in Figure 1.4), a commonly used antifungal drug, bears a spi-roether scaffold. Theaspirane, found in some flavorings, as well as grindelic acid, a diterpene carboxylic acid both bear the spiroether structure. Spiroethers are also included in the design of compounds that mimic natural aminoglyco-sides, like the third structure in Figure 1.4. This compound has been investi-gated for its antibacterial site affinity after binding to decoding center of bac-terial ribosome.87,88 Spiroethers can also be suitable as bioisosteric replace-ment for other spirocycles, such as spiroindolines.

Figure 1.4. Examples of naturally occurring and synthesized spiroethers for medici-nal chemistry.

There have been many interesting approaches to the synthesis of spiroethers.89 Hunter and co-workers devised a method for stereoselective trapping of rho-dium carbenoids with gold activated alkynols to synthesize spiroethers.75 Lankri and coworkers have also used palladium catalyzed cascade of diene-alcohols in the synthesis of tricyclic spiroethers.90

1.3 Unnatural Amino Acids - Tools for Fine-tuning Peptideomimetics

Unnatural amino acids (also called unusual amino acids), UAAs, broadly re-fers to non-proteogenic amino acids, that is, outside of the naturally occurring

26

20 amino acids.91 They can be naturally occurring or synthesized and are usu-ally incorporated into peptides to enhance or induce certain characteristics in the peptides and proteins.92–96 Some naturally occurring UAAs are hydroxy-proline (Hyp), Citrulline (cit), 3-nitrotyrosine, and pyroglutamic acid (Pyr). Many UAAs are also synthesized for medicinal chemistry research functions. Spirocyclic LY2934747 in Scheme 1.17 was investigated for its activity as a selective agonist for metabolic glutamate receptors.91 The 3-hdroxy-4-methyl-L-Glu (in Scheme 1.17) is naturally occurring, while L-DOPA is a natural precursor of dopamine. Modification have been made to L-DOPA to make it into an imaging probe (6'-[18F]-L-DOPA) in Scheme 1.17.

Improved knowledge of structure-activity relationship of peptides have in-creased the focus on unnatural amino acids, as it allows chemists to synthesize peptides that deliver specific properties.97 These UAAs can also offer in-creased activity and stability in peptides while allowing a greater control over the chemical structure expressed by proteins.96 UAAs have also been used as probes of protein structures and functions, including as imaging probes (6'-[18F]-L-DOPA as an example).91,98,99

Scheme 1.17. Some natural and synthesized unnatural amino acids (UAAs).

One important aspect which synthetic chemists make use of UAAs is to use UAAs as a means of imposing enhanced metabolic stability and rigidity in peptides. Structural rigidity is one of the core determinants of peptide and pro-tein functions.100 It may affect the potency, specificity, and even half-life of the peptide in the body.101 It is therefore of importance for chemists who de-sign modified peptides or peptidomimetics to be able to impose rigidity one way or another.102 One of such ways is to incorporate rigidified amino acids, in the form of UAAs.

Rigidified amino acids have received many attentions over the past 25 years.64,103,104 Having a defined rigid conformer, the interactions of a particular amino acid is controlled, thus avoiding unwanted protein-protein interac-tions.105 One method of imposing this rigidity is by introducing a form of ring constrain on cyclic UAAs, or having a rigidified scaffold on which the amino acid is mounted.106

27

2. CF Chemistry - Introduction

The unabating demands for new and improved chemical compounds is a major incentive for advancing process chemistry. These advances in chemical trans-formation have however, not always been met with advances in reaction meth-odology. Most chemical reactions continue to be carried out in the ‘conven-tional’ way: a reaction flask stirred and/or heated on a heating block. Though popular, this conventional method has certain setbacks, mainly prolonged re-action times due to ineffective heating, mixing of reactants or gradual decrease in concentration of the yield determining reactant. With conventional heating methods, fine control of these parameters is troublesome.

Figure 2.1. General representation of a CF system

Inspired by analytical chemistry applications, the field of organic synthesis have evolved to carry out reactions in a continuous stream as opposed to the conventional method.107 Reagents are in continuous movement in a stream, and the product collected at the end of the stream. This method is now referred to as continuous flow (CF) chemistry. CF chemistry as a process involves re-agents passing through a CF reactor and then collected for analysis and further purification if needed (represented in Figure 2.1). The reaction proceeds inside the reactor with subsequent analysis of the product. Over the past few decades, greater focus has been on the development of CF for improved process chem-istry.108–112

Earlier reported applications of CF was in solid-phase peptide synthesis.113 Since then, different systems have been developed to meet synthetic chemistry

28

demands.110,111,114–116 Importantly, CF synthesis is being used frequently in the industrial development of active pharmaceutical ingredients.117–121 Moreover, CF synthesis aligns well with the principles of green chemistry by providing efficient reactions, minimizing waste, and relatively cheaper energy require-ment.122,123

Advantages of CF systems

- Faster reaction: Reaction times greatly improved in CF synthesis112

- Controlled and quicker optimization: Reaction parameters are con-trolled, so, reaction turnover can be improved.

- Better and efficient control of heating time and mixing time

- Cleaner products, as unwanted side products formation is minimized

- In-line monitoring

- Ease of step-up and scale up

- Access new chemical transformations typically hindered in batch re-actions.

- Allows for safer reaction of hitherto unsafe compounds or routes with unstable intermediates

- Faster library collation from improved reaction conditions

Limitations of CF systems

- Clogging

- Need for homogeneity

- Need for expertise in operation

- Does not solve purification issues

- Costs of set-up, parts and replacements

In the design and application of CF systems, the key differentiating factors that allow for fine control of the various chemical reaction demands are: the type and size of reactor, temperature, pressure, heating/energy sources for the reactor and the post transformation accessories.

29

1. CF reactors system A CF system is typically an assemblage of a pump, reactor and a collection point, as seen in Figure 2.1. Reactants are pumped into the reactor, usually using traditional syringe pumps or LC pumps. In some instances, a premixing chamber is placed just before the reactor. Pre-mixing can lead to better yield and higher efficiency.124 However, in most cases, the reactants are pumped directly into the CF reactor at a set flow rate.

With the exception of heat transfer between the reactor and the processed re-agents within, reaction temperature can be controlled by heat input to the re-actor. The products are collected at the other end of the reactor, often after chemical or thermal quenching. Advancements in CF systems have also seen the use of in-line analysis of the products from a CF reactor by a tandem con-nection to analytical instrument.125–128

2. CF reactors As with the emerging technology of CF, the reactor types have also continued to evolve. The reactors must be inert to the reacting species, while being able to withstand the reaction conditions. The reactor type ranges from miniature reactors to large, industrial sized reactors. Reactors with internal diameter less than 1 millimeter (1 mm) are called microreactors and remain very popular in preparative chemistry. Although reactors of smaller sizes have also been re-ported, microreactors are still popular. These microreactors are advantageous in ensuring good heat transfer and efficient mixing, though a major drawback is the increased likelihood of clogging as the internal diameter narrows.129

The diameter and length of the reactor, together with the flow rate, are factors that typically define the residence time in the reactor. The balance between the reactor sizes and expected residence time are important factors to consider when building new CF reactors.

CF reactors are usually tube-like, and mostly coiled. The CF reactor can be housed in a temperature controlled ‘chamber’ or directly heated by heating sources explained in the next section. Tubular reactors made of perfluoroal-koyl materials, Teflon tubes, stainless steel, borosilicate glass and microwave friendly silicon carbide reactors are common in reactor designs.130–134

The type of reactor material chosen depends on the type of reaction the reactor is intended for. Metal catalyzed reactions, for example, can create metal pre-cipitations, hotspots and some reaction failures that can affect the reactor. For example, Konda and co-workers found that a borosilicate reactor can be dam-aged when used for a Mizoroki-Heck reaction due to palladium precipita-tions.133 In choosing reactor material, it is thus important to consider replace-ment parts and spares.

30

3. Heating Sources for CF reactors. Merits and limitations Some reactions need to be carried out at low temperatures, thus will need a reactor that can be used at low temperature.109,135 However, in most of syn-thetic chemistry, the reactions are driven by heat energy. Thus, most CF reac-tors are designed to operate at temperatures above room temperature.

The choice and suitability of the heating source typically defines the merits and limits of the application of such CF reactor. CF reactors have been heated in water baths, oil baths, heating mantles, GC ovens, and even with MW heating.134 These heating sources however have their limitations. For example, water baths cannot be suitable for heating reactors that provide temperatures above 100 °C. Oil baths are able to go over 100 °C, but this raises major safety issues regarding handling of oil baths at such high temperatures. Carrying out synthesis at tem-peratures above 300 °C demands a high degree of safety awareness, and high-energy input. The direct conversion of carboxylic acid to nitriles was demon-strated by Kappe and coworker using a CF reactor at 350 °C that was heated in a GC-oven.136

CF chemistry at high temperature thus presents the challenge of suitability and sustainability of the energy source. The number of CF systems that handle temperatures above 300 °C is limited, despite the advantages of accessing more chemical transformations.137 The few systems available are expensive and require a degree of expertise to operate.

Heat transfer is important at these elevated temperatures, to translate the mon-itored temperatures to the reacting substances. In the case of MW heated CF reactors, MW transparent reactors are needed.138,139 Resistive heating is one option that provides a greener source of energy, while being capable of heating CF reactors to temperatures above 300 °C effectively. The evolution of resis-tive heating is shown in Figure 2.2.

Figure 2.2. Evolution of resistive heating from hot plates in the conventional heat-ing, to steel coils in CF synthesis.

31

Resistive heating involves the use of electrical resistance to heat up a reaction. It has been used for many years in chemistry labs in indirect heating of reac-tion flasks on hot plates. Kunz and co-workers reported direct heating of a resistively heated CF reactor, albeit operational at lower temperature than cur-rently commercially available reactors.140 Apart from the ability to provide high temperature through direct heating, resistively heated reactors also show remarkably higher heating rates when compared to other heating sources at high temperature. Singh and co-workers devised a method of wrapping a re-actor coil around a heated mantle that was resistively heated (middle image in Figure 2.2).141 Resistive heating offers a promising alternative for develop-ment of CF reactor systems that are capable of reaching higher temperatures.

4. Pressure control in CF The flow of reactants within the reactor is often driven by the regular syringe pump or LC pump. This allows for easy monitoring and adjustment of the flow rate. More importantly, the fact that the flow can be controlled enables the pressure within the CF reactor to be increased by restricting the outflow of the reactants from the CF reactor. A backpressure regulator (BPR) is usually used to increase and stabilize the pressure within the CF reactor.130,131,133 This offers a simple two-way system of delivering and controlling the pressure (us-ing the LC pump or BPR), without elaborate pressure kits. Pressure within the CF reactor will also be affected by other factors, such as temperature, solvent expansion and formation of precipitates.

5. High-Temperature and High-Pressure reactions in CF Synthesis Chemical reactions that require high temperature and pressure are numerous and valuable. Perhaps, many transformations can be accessed if the right re-action temperature and pressure can be reached. The uniqueness and im-portance of these transformations make CF synthesis an important tool. How-ever, the relatively low number of systems for high-temperature and pressure CF could be due to the need for specially designed CF systems and materials.

Reactions such as catalytic Fischer indole synthesis, Claisen rearrangements and Diels-Alder reaction are synthetic transformations that can be improved if carried out above 200 °C. Other high-temperature and high-pressure trans-formations like the example shown in Scheme 2.1 have also been carried out using CF reactors (see works of Oliver Kappe and co-workers).136–138,142,143 The significance of high temperature and pressure can be seen in the lowering of reaction times from 9 weeks to 1 second in the condensation reaction in Scheme 2.1. Recently, the synthesis of heterocycles using high-temperature and high-pressure CF systems have been reported.144

32

Temperature (°C) Pressure (bar) Time

25 - 9 weeks

60 - 3 days

100 - 5 h

130 2 1 h

160 4 10 min

200 9 3 min

270 29 1 s

Scheme 2.1. High temperature and increased pressure significantly improve reac-tion time for the condensation of o-phenylenediamine with acetic acid.145

Another class of reactions that can be reached by high-temperature and high-pressure CF synthesis are reactions at supercritical conditions.

6. Organic Synthesis in Supercritical Fluid Supercritical state of a fluid is reached when it is subjected to a defined state of the fluid above its critical point (critical temperature and pressure). At this supercritical state, several properties of the solvents appear like an intermedi-ate between the gaseous and liquid states.146 These supercritical conditions are usually achieved at high temperature and high pressure. This, in turn, creates many safety concerns.

The benefits of using supercritical fluids (SCFs) are however numerous; rang-ing from solvent extraction, to the many applications of supercritical carbon dioxide in hydrogenation, oxidation, and even palladium catalyzed coupling reactions.147 It therefore remains an exciting challenge to develop safer means of carrying out synthetic chemistry transformations at supercritical conditions. Solvent expansion, diffusion, and possibility of auto-ignition of SCFs are im-portant factors to keep in mind when working at supercritical temperatures.

SCFs exhibit properties such as high diffusivity, low viscosity, and interme-diate density, all of which can remedy major issues of mass transfer faced in heterogeneous reactions.146,148–151 Thus, the use of SCFs as against ‘normal’ solvents can lead to remarkable increase in reaction rates, especially in reac-tions where the rate limiting step is hindered by ineffective mass transfer, low

33

miscibility or phase differences.148 The use of SCFs in heterogeneous catalysis can also improve selectivity by increasing the reaction rate and turnover of the selectivity determining intermediate steps.146,148 Kamat and coworkers demon-strated this selectivity in the biocatalytic synthesis of acrylates in SCFs.152 Im-proved selectivity was also observed during the amination of propanediols in supercritical ammonia.153

Photoisomerization of trans-stilbene in supercritical carbon dioxide makes use of the difference in viscosity requirements between the cis-stilbene and trans-stilbene photoisomerization reactions.151 Friedel-Crafts alkylation of mesity-lene and anisole has been reported using supercritical propene or carbon diox-ide, in a reaction that could give full selectivity for a mono-alkylated prod-uct.154,155 Other variants of alkylation reactions have also been reported using SCFs.156–159 Moreover, SCFs can themselves effect chemical transformations. Esterification, transesterification, and direct nitrile formation are some exam-ples of transformations achievable with methanol, ethanol and acetonitrile re-spectively at supercritical conditions.134

Other advantages of SCFs include:

• SCFs have been proven to be environmentally friendly,149 with su-percritical carbon dioxide offering a non-toxic option without harming the ozone layer.160

• The extraction and separation properties of SCFs are superior to common organic solvents.146

• Higher yield in reactions,150 although recent investigations with SN1 reactions dispute this.161

• Ease of solvent removal.150

• SCFs can be used in reaction control (e.g in nanoparticle synthe-sis).162

34

Table 2.1. Common organic solvents and their supercritical conditions163 Solvents Boiling Point

(°C) Critical Tem-perature (°C)

Critical Pres-sure (bar)

Autoignition Temperature (°C)

Methanol 64 240 79.6 470

Ethanol 78 243 63.9 419

isopropanol 82 235 47.6 425

Toluene 110.6 319 42.2 480

Acetone 56 235 48 465

Common organic solvents such as methanol, ethanol and acetone reach super-critical states between 250 °C and 400 °C if compressed to pressures above 40 bar as shown in Table 2.1.163 These temperatures and pressures are within the range of reactions carried out by CF synthesis at high temperature and pres-sures. Transformations in these common organic solvents at supercritical state therefore offers a good proof of concept for newly designed CF reactors for high-temperature and high-pressure synthesis.

35

3. Aims of Present Study

The main aims of this work are presented below:

• To design and evaluate a resistively heated CF system by investi-gating the application of the reactor to transformations involving supercritical fluids and synthetic transformations driven by high temperature and high pressure.

• To synthesize new spirooxindoles in a regio- and stereoselective manner using palladium catalyzed Mizoroki-Heck reaction of aryl iodide substrates. The work will also demonstrate the application of the synthesized spirooxindole in making a new rigid UAA.

• Develop new methods for the regio- and stereoselective synthesis of spirobenzofuranes via an intramolecular Mizoroki-Heck reac-tion, and demonstrate the effects of bulky protecting group on ste-reoselectivity of the intramolecular Mizoroki-Heck reaction. An example of a rigidified UAA with spirobenzofurane skeleton will also be synthesized.

36

4. CF Synthesis under High-Temperature/ High-Pressure Conditions Using a Resistively Heated Flow Reactor (Paper I)

Objectives of the Project 1. To investigate a newly designed CF reactor that can be heated di-

rectly by electric resistance and capable of delivering temperatures up to 400 °C and pressure up to 200 bar.

2. To evaluate the reactor using reactions that involve supercritical con-ditions.

3. To evaluate the reactor against popular high-temperature organic re-actions.

1. Resistively Heated Flow Reactor - Design With the evolving nature of CF synthesis, there needed to be a move towards robustness with simplification, without compromising on effective-ness.109,122,123,135 It was therefore desirable to design a CF reactor that was ca-pable of delivering high temperature and high pressure in a greener, safer man-ner, with relatively inexpensive components. It was also important for the newly designed CF reactor to be easy to assemble, operate and replaced.

As against previously reported options for delivering high-temperature and high-pressure CF flow synthesis,137,138,145,164 electric resistance from direct cur-rent was chosen as the source of heat for the newly designed CF. This was to provide a relatively cheaper, greener, efficient and safer source of heat energy. In designing the reactor, materials were chosen to withstand the high temper-ature and pressure it was intended for. More so, the reactor was validated with synthetic chemistry transformations that only occur under high temperature and high pressure.

37

Figure 4.1. A Schematic representation of the resistively heated CF reactor system.

The system is designed to utilize a typical LC pump in pushing the reactants into the reactor (depicted as coil in Figure 4.1). Pressure control and thermal quenching was done with the BPR and cooling water respectively. The oper-ational system is pictured in Figure 4.2.

Figure 4.2. An operational resistively heated CF reactor showing the major compo-nents

2. Reactor material The material of choice for this reactor had to be capable of withstanding high temperature (up to 400 °C) and high pressure (up to 200 bar). Moreso, it should be chemically inert, resistant to corrosion to a high degree, readily available and inexpensive to source. Steel became an obvious choice: possessing the aforementioned properties, and easier to modify. Stainless steel was preferred, having a relatively low electrical conductivity. Coiled steel of different diam-eter with open ends was used. The steel reactor was made in different sizes (length, volume and internal diameter) as can be seen in Figure 4.3. These reactors are simple and can be easily manufactured to the desired dimensions.

38

Figure 4.3. Different reactors were made with different volumes. On the left is the 1 mL reactor, in the middle is a reactor with volume of 1.2 mL, and on the right, a re-actor with volume of 6 mL.

By passing direct current through steel, a potential difference can be created between two ends of the steel material. As can be seen in Figure 4.3, the po-tential difference was created between the two ends (red and black wires showing the two ends of the reactors). This potential difference as well as the resistance to the current passed is then converted to heat energy. This was then followed by passing a direct current through the coil using a direct current (DC) power source. The DC power source was capable of delivering current up to 20 A and voltage up to 42 V, while also generating an effective maxi-mum power output of about 320 W.

Pressure demands - LC pump and Back Pressure Regulator

The reactants were pumped into the into the CF reactor with an LC pump through one of the open ends of the reactor. The other end was connected to an outlet leading to the BPR and passing the cooling water to the collection point. This created a two-way control of the pressure within the reactor by controlling the flow rate from the pump and controlling the output rate from the BPR. Increasing the flow rate from the LC pump will increase the pressure. Tightening the BPR will reduce the outflow of the processed reactants from the reactor, thus increasing the pressure independent of the flow rate. Increas-ing the reaction pressure typically increases the reaction rate, even independ-ent of other reaction variables.146 The components of the BPR are shown in Figure 4.4.

39

Figure 4.4. The backpressure regulator used in this study is shown. The inlet and outlet can be seen in pictures 1 and 3. Picture 4 shows the makeup of the BPR in-cluding the spring which can be compressed to increase the pressure within the re-actor.

3. Temperature control The temperature of the reactor was measured using a thermostat con-nected to the top end (output end) of the reactor. The thermostat was also coupled with the DC power source so that the temperature can be directly controlled by the voltage input (the DC power supply and thermostat are stacked on each other in rightmost image in Figure 4.6). This was to allow for further precise temperature control directly from both current and voltage input. This is an added safety measure, as well as key for a rapid heating (and cooling) of the reactor. Heat loss in the CF reactor is minimized by placing it in an insulated enclosure as shown in Figure 4.5. When in use, an insulated lid was placed above the insulated enclosure shown in Figure 4.5.

Figure 4.5. The 6 mL reactor (on the left) is placed in an insulated enclosure (shown to the right of the picture) to minimize heat loss. When the reactor is in operation, an insulated lid covers the enclosure to the right.

When a reactor is heated to a temperature as high as 400 °C, there will be concerns relating to effective distribution of the heat across the length of the reactor coil. To investigate this, an infrared image of an operational CF reactor

40

was taken at 280 °C and 132 °C (Figure 4.6). From the IR image, it can be seen that there was a more even distribution of the heat at higher temperature than at lower temperature. The ‘cold spots’ are visible at 132 °C, while no such was observed at 280 °C. The thermostat in Figure 4.6 was recorded when the reac-tor reached 400 °C. This temperature was achieved by a current and voltage input of 9.30 A and 11.90 V respectively when heating up the 6 mL reactor.

Figure 4.6. An infrared camera image of the 6 mL reactor when in operation. To the left is the reactor at 280 °C and to the right of the picture is the reactor at 132 °C. The thermostat recorded the temperature at 400 °C when 9.30 A and 11.90 V was applied to the 6 mL reactor.

Evaluation of the CF reactor 4. Heating rate and temperature fluctuation The reactor was first tested to establish the heating rates in three different or-ganic solvents: methanol, ethanol, and acetonitrile. This was done by passing the solvents through the CF reactor at a steady flow rate and measuring the time to reach set temperatures. The results obtained are shown in Figure 4.7. Using a flow rate of 0.5 mL/min, the 6 mL reactor was heated from room temperature to the maximum expected temperature of 400 °C. It was observed that the heating rate was about 100 °C /min across all solvents. Thus, it took 5 minutes to heat the reactor to 400 °C from room temperature (Fig 4.7).

41

Figure 4.7. The heating rate with acetonitrile passing through the system.

On the other hand, cooling rates were also measured after disconnecting the power supply and removing the insulators. It was observed that cooling the CF reactor from 400 °C to 100 °C took 4 minutes. However, going from 100 °C to room temperature took another 6-10 minutes. It was thus established that heating and cooling with the CF reactor was done effectively in about 5 minutes (Figure 4.7). This relatively fast heating and cooling is very important in CF chemistry, as one of the main advantages is to achieve faster reactions.

5. Residence time (Reaction Time) The residence time of a reactant inside the CF reactor is a function of the flow rate and volume of the reactor.142 However, at high temperature and pressure, other factors affect the residence time. Higher diffusion rates and solvent ex-pansion are expected at high temperature and pressure. 136,142,145 In order to factor all these variables into the estimation of reaction times, an experiment was carried out.

Benzoic acid 4.1a (in Table 4.2) was dissolved in each of methanol, ethanol and acetonitrile. The solution was then pumped through the CF reactor at dif-ferent temperatures. The processed output was collected and analyzed by TLC. The residence time (time spent in the reactor) was determined by spot-ting the processed liquid on a TLC plate and recording the time between en-trance into the reactor and the time for the first spot of processed benzoic acid to appear, to the nearest 0.5 minute. The reactor volume was 6 mL, and the flow rate was maintained at 0.5 mL/min. From the results shown in Table 4.1, it was discovered that relative to room temperature, there was a two-fold de-crease in residence time at 300 °C. This residence time was further decreased four-folds when the temperature was increased to 400 °C. So for example, a reaction in methanol, with a flow rate of 0.5 mL/min in a 6 mL reactor was estimated to have residence times of 6 minutes and 3 minutes if carried out at 300 °C and 400 °C respectively. No such solvent expansion was however no-ticed when toluene was used.

0200400600

0 2 4 6

Tem

pera

ture

(° C)

Time

Heating rate

0

200

400

600

0 5 10 15

Cooling rate

42

Table 4.1. Investigation of solvent expansion in the resistively heated CF reactor.

Temperature (°C)

Pressure (bar)

Flow rate (mL/min)

Residence time (min)

20 150 0.5 12

100 150 0.5 12

250 140 0.5 8

300 140 0.5 6

350 138 0.5 4

400 138 0.5 3

6. Validation of the Resistively Heated CF Reactor Reactions involving common organic solvents at supercritical states best high-light the advantages of high temperature CF synthesis. Ethanol, methanol, and acetonitrile are common organic solvents that reach supercritical states at tem-peratures between 250 °C and 400 °C, and pressures above 70 bar.163 At super-critical states, these solvents exhibit properties that are not obtainable at am-bient temperatures and low pressures, without the addition of catalysts or ad-ditives.

Apart from these supercritical solvent transformations, another popular chem-ical reaction that is driven by high temperature was also used to validate the CF reactor. The Diels-Alder cyclization was carried out at high temperature and pressures, without catalysts or additives.

7. Supercritical Solvent Reactions a. ScEtOH- Esterification. Supercritical state of ethanol is reached when ethanol is heated to temperatures above 243 °C and pressure above 64 bar.163 ScEtOH is popular for its direct esterification of carboxylic acid (Scheme 4.1) in a catalyst-free transformation to its corresponding ethyl ester in good yield. This class of reactions has been extensively applied to the production of biodiesel on an industrial scale. The reaction has also been reported at 300 °C and 120 bar by Kappe and co-work-ers.142 It was therefore a good starting point for the resistively heated CF reac-tor validation to investigate the transformation of carboxylic acids to corre-sponding ethyl ester in ScEtOH (Scheme 4.1).

43

Scheme 4.1. Direct etherification of carboxylic acid using ScEtOH.

Absolute ethanol was pumped through the 6 mL CF reactor at the desired tem-perature and pressure for about 10 minutes until steady stage was reached. Benzoic acid 4.1a, dissolved in absolute ethanol to a concentration of 40 mg/mL, was then pumped into the reactor under different conditions reported in Table 4.2. The processed products were collected and analyzed by 1H-NMR. As the concentration of the processed liquid was kept constant at 40 mg/mL, the yield was estimated from the volume of collected processed prod-uct.

Table 4.2. Direct conversion of benzoic acid to its ethyl ester using sScEtOH in a re-sistively heated CF reactor.

Entry Temperature (°C)

Flow rate (mL/min)

Residence Timea (min)

Conversionb (%)

Yieldc

(%)

1 300 1.0 3 55 51

2 300 0.5 6 85 82

3 300 0.25 12 84 74

4 325 0.5 6 79 71

5 350 0.5 4 90 76

6 375 0.5 4 100 40

aResidence time is reported after adjusting for solvent expansion. bConversion is re-ported as the percentage of carboxylic acid that was transformed and estimated by 1H NMR. cIsolated yield. The purity of isolated product in all instances was more than 95% as estimated by NMR.

44

The pressure was kept constant at 120 bar, and the concentration kept constant throughout the experiment. It was observed that the highest yield was achieved at 300 °C, with a flow rate of 0.5 mL/min, which translate to 6 minutes resi-dence time (entry 2, Table 4.2). Increasing the temperature or residence time did not increase the isolated yield of the ester. It was also observed that in-creasing the temperature to 375 °C gave very low yield (40%), despite the full conversion of the benzoic acid to its corresponding ethyl ester. This suggests extensive decomposition of the benzoic acid. This decomposition has previ-ously been observed and reported by Kappe and coworkers using different CF systems.136,142

Different carboxylic acids were further subjected to esterification with ScEtOH in the resistively heated CF reactor. Derivatives of benzoic acids 4.1b, 4.1c and 4.1d were all converted to their corresponding ethyl esters in good yields (70%, 86% and 91% respectively) at 300 °C and 6 minutes resi-dence time (entries 1-3, Table 4.3). Heterocyclic carboxylic acids 4.1f and 4.1g, and naphthoic acid 4.1e were also converted, albeit at higher temperature of 350 °C and lower residence time of 4 minutes.

Table 4.3. Esterification of different carboxylic acids using resistively heated CF re-actor was performed using ScEtOH.

Conditions: 6 mL reactor was used. Flow rate was 0.5 mL/min. Residence time is re-ported after adjusting for solvent expansion at supercritical conditions. a. Isolated yield is recorded in which purity was over 95% as determined by 1H-NMR, with res-idence time of 6 min, and temperature of 300 °C. b. Residence time of 4 min, and temperature of 350 °C.

45

b. ScMeOH - Transesterification. Methanol is another common organic solvent which reaches supercritical state at temperatures above 300 °C when compressed to about 140 bar.165 At this supercritical state, the nucleophilic properties of methanol are enhanced, thus favoring transesterification (Scheme 4.2).142,166

Scheme 4.2. Transesterification of methanol using supercritical methanol, with no catalyst nor additives.

Ethyl ester 4.3a was chosen as a test compound, having been previously re-ported in similar transesterification reaction in methanol.142 Using the resis-tively heated CF reactor, ScMeOH state was achieved at about 350 oC and 180 bar. Different esters in methanol were passed through the 6 mL resistively heated CF reactor under different conditions shown in Table 4.4. This reaction afforded the chance to go as high as 400 °C and 180 bars, further testing the robustness of the CF reactor, while pushing it to its maximum working condi-tions.

Tabl

e 4.

4. S

cope

of t

rans

este

rific

atio

n to

form

met

hyl e

sters

dire

ctly

usi

ng a

resis

tivel

y he

ated

CF

reac

tor.

Entry

Re

acta

nt

Prod

uct

Tem

pera

ture

(° C)

Tim

ea (m

in)

Conv

ersio

nb (%

) Y

ield

c (%

)

1 35

0 4

62

-

375

4 81

-

400

3 99

92

2 37

5 4

87

-

400

3 99

90

3 37

5 4

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48

Different esters were converted to their methyl esters in good yields. From the table, it can be seen that improved conversion was achieved by improving the reactor temperature. However, unlike the formation of ethyl esters in ScEtOH, decomposition of the esters was not observed in ScMeOH. The obtained yields were high, and no decomposition products were observed from the 1H-NMR spectra. Thus, possible decomposition due to the CF reactor material or design can be assumed insignificant.

The flow rate was kept constant, while increasing the temperature. This re-sulted in decreased residence time as earlier discussed in Table 4.1. This re-duced residence time did not affect the conversion or the isolated yield. Me-thyl esters 4.4a, 4.4b and 4.4d were formed from the transformation of ethyl esters 4.3a, 4.3b and 4.3d respectively with isolated yields of/over 90% at 400 oC. Benzoic anhydride 4.3c and allylic ester 4.3e were both converted to their methyl esters in 86% and 82% yield respectively after 3 minutes residence in the CF reactor.

c. ScMeCN - Direct Carboxylic Acid to Nitrile Conversion Acetonitrile reaches supercritical state at temperatures above 275 °C and pres-sures above 48 bar. ScMeCN has been reported in some direct synthetic trans-formations, such as direct addition to allylic carbons, and direct conversion of carboxylic acids to nitrile. The direct conversion of carboxylic acid to nitrile has been particularly accomplished using a CF reactor that was heated using GC ovens.136

The transformation was earlier reported by Cantillo and Kappe to go through the mechanism shown in Scheme 4.3.136 As with other solvents earlier dis-cussed, solvent expansion at supercritical state becomes significant. This was also pointed out by Kappe and co-workers in their investigation of the direct conversion of carboxylic acids to nitriles using CF synthesis.136

Scheme 4.3. Suggested mechanism for the direct conversion of carboxylic acid to corresponding nitrile by ScMeCN.

49

Solvent expansion in acetonitrile was investigated and it was discovered that acetonitrile expansion was between 1.7 and 3 folds when run at 250 °C and 350 °C respectively, at 100 bar pressure. This was factored into the reported residence times reported in Table 4.5. The performance of the reaction at var-ious conditions was investigated while keeping the pressure constant at 100 bar. As there were no distinguishable protons between the starting carboxylic acid and the processed nitrile, 1H-NMR was not an effective tool for analysis. It was then decided to use GC-MS as the choice analytical tool for analyzing this transformation. In order to do this, an internal standard was used for em-pirical quantification. Naphthalene was chosen due to its stability at tempera-tures up to 400 °C, and structural dissimilarity to carboxylic acids or nitriles to ensure peak separation on the GC-MS spectra.

The direct conversion of 2-methoxybenzoic acid 4.5a to its corresponding ni-trile was investigated at different conditions. From the results in Table 4.5, it was discovered that the most productive condition was achieved with a flow rate of 0.25 mL/min (adjusted to 8 minutes residence time) at 350 °C. These results were similar to those reported when a GC-oven heated CF reactor was used.136 Although, conversion with the resistively heated CF reactor was slightly better (100 % vs 98%), the isolated yield was lower (82% vs 98%). It must be highlighted however, that the residence time with the resistively heated reactor was markedly lower than that needed when using the GC-oven heated reactor (8 minutes vs 25 minutes).

Table 4.5. Direct conversion of carboxylic acids to nitrile was achieved with super-critical acetonitrile.

Entry Temperature

(°C) Flow rate (mL/min)

Residence time (min)

Conversion (%)

[Isolated Yield]

1 250 0.5 8 49

2 300 0.5 6 76

3 325 0.5 6 74

4 350 0.5 4 82

50

5 350 1.0 2 66

6 350 0.25 8 100 [82]

7 375 0.5 4 100

The transformation was further improved by increasing the temperature; how-ever, this did not correspond to an increase in the isolated yield (entry 7, Table 4.5). Temperature increase was also discouraging due to the likelihood of de-composition of the carboxylic acid and/or the nitrile.

One common drawback of the application of CF to synthetic chemistry is the issue of solubility. The system is not very tolerant of solids and/or precipitates. In the course of this set of experiments, the issue of solubility came up a few times, especially with N-heterocyclic carboxylic acids which were sparingly soluble in acetonitrile at room temperature. The production of N-heterocyclic nitriles was an exciting prospect, due to the immense value of these nitriles in medicinal chemistry, especially in cyclization reactions to form heterocycles. It was therefore a worthy challenge to find a way round the solubility issues.

One way of addressing the solubility problem was to dissolve the N-heterocy-clic carboxylic acids in a cosolvent. Thus, N-methyl-2-pyrrolidione (NMP) was used as a cosolvent (up to 20% v/v), as it is capable of dissolving a range of N-heterocyclic carboxylic acids. Acetylpiperidyl carboxylic acid 4.5c was selected to investigate the effect of cosolvent in the transformation of carbox-ylic acid to nitrile in ScMeCN. The results are presented in Table 4.6 and the outcomes were quantified as a ratio of the nitrile formed to the internal stand-ard.

The reaction was clean as only the peaks of the cosolvent, product, and/or starting material were observed by GC-MS in most instances. The conversion of the carboxylic acid to nitrile was high in most cases (above 85% in all in-stances). Full conversion was obtained at 350 °C, even in as low as 4 minutes using the resistively heated CF reactor (entry 7, Table 4.6). The isolated yield at this condition was 63%. Increasing the temperature above 350 °C, or in-creasing the residence time above 4 minutes did not result in increase in the ratio of product to internal standard (entries 8-10, Table 4.6).

51

Table 4.6. Direct conversion of acetylpiperidyl carboxylic acid to its corresponding nitrile using supercritical acetonitrile and NMP as cosolvent.

Entry Temperature (°C)

Flow rate (mL/min)

Resi-dence timea (min)

Conver-sionb (%)

Product/ Internal Standardc

1 250 0.5 8 86 0.24

2 250 0.25 16 91 2.23

3 300 0.5 6 92 2.16

4 300 0.25 12 98 2.81

5 325 0.5 6 98 3.02

6 325 0.25 12 99 3.24

7 350 0.5 4 100 3.38 {63%}

8 350 0.25 8 100 3.36

9 375 0.5 4 100 3.36

10 375 0.25 8 100 3.32

aResidence time is reported after adjusting for solvent expansion at supercritical con-ditions. bConversion is determined by GC-MS, with naphthalene as internal stand-ard. cThe relative yield was quantified as a ratio of the product to internal standard.

Having identified the optimal conditions for the direct carboxylic acid to ni-trile conversion using ScMeCN with or without cosolvent, different carbox-ylic acids were investigated (Table 4.7). Aromatic and aliphatic carboxylic acids were all converted to their corresponding nitriles in over 90% conversion and isolated yields in and above 55%. The yield obtained with N-heterocyclic

52

carboxylic acid conversion was lower than the other carboxylic acids. The conversion of some N-heterocyclic carboxylic acids showed peaks that corre-spond to decarboxylative products. These examples are not included in Table 4.7. Pyrrole based acids preferentially gave exclusively decarboxylative elim-ination products (not included in results).

Table 4.7. Conversion of carboxylic acids to corresponding nitriles using supercriti-cal acetonitrile

aResidence time of 8 min, with reactor temperature at 350 °C. bResidence time of 4 min, at 350 °C. Isolated yield was reported with purity over 95% according to NMR.

8. High Temperature Diels - Alder Reactions Having established the effectiveness of the resistively heated CF reactor with reactions in supercritical solvents, attention turned to one of the most useful transformations in organic chemistry. The Diels-Alder reaction is one that in-volves a diene and dienophile in a [4+2] addition. The reaction usually re-quires high temperature, or a catalyst or additive to push to completion. In some cases, the reaction may take days to complete, even with heating. A pre-viously reported application of Diels-Alder reaction involving diene 4.7a and dienophile 4.8a was investigated. The reaction takes 5 days to complete, even

53

with heating at 100 oC.138 The reaction time had also been taken down to 20 minutes by heating a 250 °C in a microwave reactor. The reaction (residence) time was further lowered to 2 minutes using a CF system at 280 °C and 130 bar.138 As these CF conditions are within the working capability of the resis-tively heated CF reactor, the reaction was investigated. The temperature and flow rates were optimized with the pressure kept constant at 90 bar, see Table 4.8.

Table 4.8. Effect of temperature and residence time on the Diels-Alder reaction be-tween diene 4.7a and dienophile 4.8a in a 6 mL resistively heated CF reactor.

Entry Temperature (°C)

Flow rate (mL/min)

Residence Time (min)

Conversion

(%)

1 100 1 6 0.3

2 200 3 2 10

3 200 2 3 85

4 200 1 6 100 [87]

5 300 3 2 100 [84]

The reaction was investigated with no catalyst or additives. As there was no solvent expansion experienced with toluene, the residence time was derived directly from the flow rate and reactor volume. The results obtained indicated that there was increased conversion as the temperature or residence time was increased. Full conversion was obtained in 6 minutes when the temperature was 200 °C, giving an isolated yield of 87% (entry 4, Table 4.8). The time for full conversion was further reduced to 2 minutes when heated to 300 °C, giving an isolated yield of 84% (entry 5, Table 4.8).

The Diels-Alder reaction was also repeated with dienophile 4.8b, and similar results were obtained. The reaction was a little sluggish, with full conversion obtained in 6 minutes when heated at 300 °C and 90 bar. The isolated yield was however slightly higher at 89% (Scheme 4.4).

54

Scheme 4.4. Diels-Alder reaction with butyl acrylate was carried out in 6 min using the resistively heated CF reactor.

Conclusion and Projections A highly effective, yet simple CF system was designed and evaluated. The resistively heated CF system offers a relatively cheaper, simple and easy-to-assemble CF reactor that handles high-temperature and high-pressure chem-istry up to 400 °C and 200 bar. The developed reactor was easy to operate without elaborate training. The material for the reactor can be cheaply sought and modified to deliver different volumes and dimensions of the reactor. Maintaining the reactor was also easy, and cleaning was done by simply flush-ing the reactor with solvents.

Specific safety concerns regarding the use of the developed CF reactor were addressed in the project. The connection of the heat source with a thermostat enables high process control, even at high temperatures. The applied current and voltage required to heat the reactor are unlikely to trigger electrochemical reactions, and no such reactions were observed in this project.

Rapid heating rate was demonstrated with the CF reactor reaching 400 °C in less than 5 minutes. The cooling rate was also rapid, thus proving to be an impressive reactor for rapid heating and cooling. Organic reactions have been safely performed at temperatures up to 400 °C with pressure up to 200 bar. The reactor has been used in both academia and industry and meeting the op-erational safety standards in both environments.

Efficiency in translating direct current to heat energy on the resistively heated CF reactor was highlighted by heating the reactor to 400 °C with just 9.30 A and 11.90 V (total power output about 110 W heated the 6 mL reactor to 400 °C).

Although the temperature of the reactants was not directly measured, the re-actions at supercritical states served as a proof that, at the very least, the tem-perature and pressure of the reacting solvents had reached the supercritical

55

states of the solvent. Moreover, the results obtained from the Diels-Alder re-action are similar to those obtained by other groups using different reactor systems at high temperatures.

As with other CF reactors, the need for homogeneity is important, and solu-bility of reactants at room temperature could create hindrance to the applica-bility of this reactor. Although this was touched on with the use of cosolvents in the direct carboxylic acid to nitrile transformations, perhaps this reactor could offer more. As solubility generally increases with increasing tempera-ture, a preheating and premixing chamber could be connected before the reac-tants are pumped into the CF reactor.

Finally, the simplicity to assemble and the user friendliness of the CF reactor makes it a good prototype for demonstration of CF reactions in educational and research establishments.

56

4. Regio- and Stereoselective Synthesis of Spirooxindoles via Mizoroki–Heck Coupling of Aryl Iodides (Paper II)

a. Aims and Objectives The aim of this project was to synthesize new cyclopentene-based spirooxin-doles via a palladium(0)-catalyzed spirocyclization using aryl iodides. The formation of a new carbon-carbon bond in making the spirooxindole was to be investigated, with special attention on the regioselectivity of the reaction.

The experiments in the project were selected to investigate the reactions of enantiopure cyclopentene-tethered aryl iodides when subjected to conditions for intramolecular Mizoroki-Heck cyclization. The selectivity in the formation of conformationally restricted and stereopure spirooxindoles via a 5-exo trig cyclisation was also investigated. This project builds on the selective inter and intramolecular arylation of optically pure amino cyclopentene derivatives made from enantiopure 2-azabicyclo[2.2.1]hept-5-en-3-one (Vince lac-tam).37,38 To further emphasize the importance of this class of compounds in peptidomimetics, the conversion of the synthesized spirooxindoles to unnatu-ral amino acids was also to be demonstrated.

b. Results and Discussions Building on the methods developed for efficient synthesis of cyclization precur-sors,37,38 enantiopure Vince lactam 5.1 was subjected to solvonolysis with thio-nyl chloride in methanol according to Scheme 5.1. The conversion of Vince lactam 5.1 to ester 5.2a gave a 100% yield, with the hydrochloride salt 5.2a isolated by simply evaporating the solvent (methanol) and thionyl chloride.

Scheme 5.1. Preparation of double protected amino acid derivative 5.2b from Vince lactam via a solvonolysis reaction to 5.2a.

57

Next, the free amine in 5.2a was protected with 2,5-dimethylpyrrole. The 2,5-dimethylpyrrole was selected as a protecting group due to the ease of protec-tion and deprotection. More so, it has been reported that the 2,5-dimethylpyr-role has a major directing effect on the Mizoroki-Heck reaction.37 The pro-tection of 5.2a was immediately followed by double bond isomerization to form the more stable 5.2b, with an overall yield of 81%.

Protected amine 5.2b was coupled with a range of iodoanilines in the presence of trimethylaluminium to form the amide precursors 5.3a-5.3j in high yields (69%-85%).167 The results as presented in Scheme 5.2 showed that electron-rich and electron-poor anilines were coupled in good yields. Products that sug-gest side reactions were not detected, despite the aryl iodide functionality been prone to possible dehalogenation in certain conditions.

Scheme 5. 2. Preparation of Mizoroki-Heck cyclization precursors 5.3 by using tri-methylalumium in toluene.

Following the synthesis of amide precursors, precursor 5.3a was subjected to different conditions for the Mizoroki-Heck reaction as presented in Scheme 5.3. The conditions had different phosphine ligands, catalyst concentration,

58

reaction temperature and time. The palladium source was kept as Palladium acetate, Pd(OAc)2 as it was considered to be robust and cheap.

Scheme 5.3. Spirocyclization of 5.3a gave 4 distinct peaks corresponding to spi-rooxindoles 5.4a and 5.5a, dehalogenation product 5.6a and unreacted precursor 5.3a.

The Mizoroki-Heck cyclization has been reported to involve possible regiose-lectivity arising from the palladium hydride elimination-reinsertion process, which can lead to double bond isomerization to different carbocyclic regioi-somers.37 With this possibility in mind, the initial results from the ring-closing reaction indicated four distinct vinyl signals according to 1H NMR analysis. The signals corresponded to the starting material 5.3a, spirooxindoles 5.4a and 5.5a, while the dehalogenated starting material 5.6a was observed in min-imal amounts.

The effect of ligands on the cyclization was then investigated. Conditions such as catalyst loading, temperature, and reaction times were kept constant, while different ligands were used in the reaction. In the first test, no ligands were used, and little control of regioselectivity was observed (entry 1, Table 5.1). Some of the favored spirooxindole 5.4a were observed, with a lot of the other spirooxindole 5.5a (22:78 of 5.4a/5.5a). Other side products were also ob-served in the reaction. It has been reported that the Mizoroki-Heck reaction involving aryl iodides can proceed in certain instances where no ligand is used.35 Six different phosphine ligands were also investigated and near full conversion of the substrate 5.3a was observed in all cases, although with sig-nificant differences in regioselectivities. The ligands investigated are an as-sortment of monodentate, bidentate, bulky, and electron rich. The selectivity for spirooxindole 5.4a for each ligand is represented in Figure 5.1 (adapted from entries 1-7, Table 5.1).

59

Figure 5.1. A representation of the regioselectivty of 5.4a from reaction in Scheme 5.3 with different ligands used. A=PPh3, B= tBu3PH.BF4, C= P(o-Tol)3.

From the results, it can be seen that monodentate triphenylphosphine, PPh3 and bidentate 1,1’-ferrocenediyl-bis(di-phenylphosphine), DPPF, were the most selective for spirooxindole 5.4a. However, bulky, electron-rich ligands 2-di-cyclohexylphosphino-2’,4’,6’-triisopropylbiphenyl (Xphos) and (2-bi-phenyl)di-tert-butylphosphine (JohnPhos) were less selective for the desired spirooxindole 5.4a. Apart from PPh3 and DPPF which were most selective for spirooxindole 5.4a, tri (o-tolyl)phosphine, P(o-Tol)3 gave the highest selectiv-ity of 56:44 (5.4a/5.5a), while the tri-tert-butylphosphine obtained from its pre-ligand (tBu3PH.BF4) showed the least selectivity 9:75 for 5.4a/5.5a. Hav-ing identified the most promising ligands for the reaction, further screening were done using the two-standout ligands PPh3 and DPPF.

Table 5.1. Optimization and reaction conditions to obtain a highly regioselective Mizoroki-Heck Spirocyclization of 5.3a.a

22

83

9

56

94

35

20

None A B C DPPF XPhos JohnPhos

% 5

.4a

Ligands

Ligands vs % 5.4a

Entry Phos-phine

Ligand

Temper-ature (°C )

Pd(OAc)2 (%)

Time (h)

5.4a (%)

5.5a (%)

5.6a (%)

1 None 80 20 16 22 78 0

60

a General Method: A reaction vial was charged with Pd(OAc)2 and the ligand (2 times palladium loading), followed by 0.52 mmol (2.1 equiv) of Et3N with 1.5 mL DMF, and the mixture was allowed to stir under nitrogen for 10 min at room temperature. Next, 0.25 mmol precursor 5.3a (in 0.5 mL DMF) was slowly added, before temper-ature was increased. *Reaction was carried out in sealed vials with MW heating. ** Reaction was carried out in a TPGS-750-M/Water (2/98) surfactant solution. Yield (%) was determined from 1H NMR, full conversion of 5.3a was observed in most entries, except entry 3, which had 6% unreacted 5.3a.

The catalyst loading was reduced in both ligands up to 2.5%. The results were however contrasting, as can be seen in Figure 5.2. The selectivity for desired spirooxindole increased with reducing catalyst loading when the ligand was PPh3. On the other hand, the selectivity increased with increasing catalyst loading with DPPF. In line with our objective of improving reaction effi-ciency, PPh3 was chosen as the most suitable ligand.

2 PPh3 80 20 16 83 11 5

3 tBu3PH.BF4

80 20 16 9 75 10

4 P(o-Tol)3 80 20 16 56 43 0

5 DPPF 80 20 16 94 3 3

6 XPhos 80 20 16 35 65 0

7 JohnPhos 80 20 16 20 80 0

8 PPh3 80 5 16 97 2 1

9 DPPF 80 5 16 83 17 0

10 PPh3 80 2.5 16 96 3 1

11 DPPF 80 2.5 16 79 20 1

12 PPh3 100 5 16 93 5 2

13 DPPF 100 5 16 97 1 2

14 PPh3 100 2.5 16 91 8 1

15 DPPF 100 2.5 16 90 10 0

16 PPh3 120* 5 1 88 10 2

17 PPh3 150* 5 0.5 82 17 1

18 PPh3 80** 5 16 60 35 5

61

Figure 5.2. Contrasting results were obtained for the 5.4a/5.5a ratio when using PPh3 and DPPF as ligands.

The results obtained from screening the catalyst loading showed that it was possible to achieve regioselectivity up to 97% without needing to suppress the regioselectivity by other means. Thus, silver salts were excluded from the re-action to remove bias in the suppression of isomerization.

Next, attention was turned to reaction conditions such as temperature. Increas-ing temperature has been found to increase conversions and regioselectivity in Mizoroki-Heck reactions.168 In this project, it was discovered that increas-ing the temperature by 20 °C did not significantly increase the regioselecitivy or conversion of the amide to the spirooxindoles. The reaction temperature was thus kept at 80 °C.

Reaction time was lowered to 1 h when the reaction was heated at 120 °C in a MW reactor. Under these conditions, the regioselectivity (5.4a/5.5a ratio) was reduced (88:10) as against 97:2 that was obtained with conventional heating overnight (entry 8 vs entry 16, Table 5.1). The reaction time was further low-ered to 30 minutes when heated to 150 °C, albeit with slightly unfavorable regiochemical outcome, at 82:17.

In the course of experimentation, it was observed that in instances where dry solvents were not used, the observed regioselectivity as 5.4a/5.5a ratio was low. The presence of water in the reaction can have different effects on the Mizoroki-Heck reaction.These effects were investigated in the synthesis of spirooxindoles by using a solvent that contained a lot of water. The reaction was investigated using a surfactant solution made of 98% water and 2% TPGS-750-M (a surfactant reported by Lipshutz and co-workers as useful in palladium-catalyzed reactions).169 In this project however, the regioselectivity was reduced (about 60:35 of 5.4a/5.5a) when this surfactant solution was used as the solvent. The reaction conditions that best afforded spirooxindoles in

70

80

90

100

2,5 5 20

%

5.4a

Catalyst Loading (%)

PPh3 as Ligand

70

80

90

100

2,5 5 20

DPPF as Ligand

62

high regioselectivity and allow for robust application was chosen (entry 8, Ta-ble 5.1).

The selected conditions were next applied to the synthesis of a range of spi-rooxindoles as shown in Scheme 5.4. Full conversion was observed in all pre-cursors cyclized. Moreover, regioselectivity was above 95% in 9 of the 10 entries. The outlier was the cyclization of electron-deficient 5.3h, which showed a regioselectivity of 87:13 for 5.4h/5.5h.

Scheme 5.4. Spirooxindoles 5.4 were synthesized using conditions in entry 8, Table 5.1. Regioisomeric purities in bracket (5.4:5.5).

During analysis of the products, no other diastereoisomer of the spirooxin-doles was observed, thus the diastereoselectivity was reported as d.e.> 98%. As the regioselectivity was above 95% in most instances, and the diastereose-lectivity was more than 98% d.e., the need to separate the two regioisomers

63

from each other was reduced. Therefore, only in instances where the ratio of 5.4/5.5 was less than 95%, were the compounds separated. The relative stere-ochemistry of the synthesized spirooxindoles was in agreement with those previously published where bromides were used substrates for the Mizoroki-Heck reaction.37

As a further example of chemoselectivity of the reaction, precursors bearing halide groups were successfully cyclized, with single products observed. Spi-rooxindole 5.4e was obtained with the inactivated bromide remaining intact. This chemoselectivity is important for subsequent chemical reactions involv-ing the inactivated groups. This chemoselectivity advantage was taken in fur-ther functionalizing the bromide in spirooxindole 5.4e.

Scheme 5.5. Carbonylation of 5.4e afforded protected unnatural amino acid 5.7.

Spirooxindole 5.4e was subjected to conditions previously reported for palla-dium-catalyzed carbonylation as shown in Scheme 5.5.170 The reaction in-volves a non-gaseous CO source, Mo(CO)6. The Herrmann-Beller palladacy-cle was used as the palladium source, with tri-tert-butylphosphine tetrafluo-ride borate as pre-ligand. Benzyl alcohol was chosen as the nucleophile in other to obtain a benzylated ester that could be easily hydrolyzed to get the free acid. The carbonylation reaction was carried out under MW heating in a sealed reaction vial. Full conversion was observed after 1 hour of heating at 150 oC, affording the benzyl ester 5.7 in 66% isolated yield (Scheme 5.5).

Having successfully synthesized di-protected unnatural amino acid 5.7 on a spirooxindole scaffold, we next performed orthogonal deprotection to afford the mono protected amino acids. Firstly, the rigidified amino acid 5.7 was hy-drolyzed using nickel chloride hexahydrate (3 equiv) and sodium borohydride (9 equiv). The chemoselective reaction afforded the free carboxylic acid 5.8 in 74% isolated yield after stirring at room temperature for 2 h (Scheme 5.6).171

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Scheme 5.6. Deprotection of benzyl carboxylate 5.7 to provide free acid 8.

Selective N-deprotection of benzyl ester 7 was also carried out using excess N-hydroxylamine hydrochloride (6 equiv) and potassium hydroxide (4 equiv).172 The reaction was carried out in aqueous methanol (methanol/water 3:1), and heated to 90 °C as shown in Scheme 5.7.

Scheme 5.7. Selective N-deprotection of 5.7 to give free amine 5.9.

c. Conclusion The selective synthesis of ten cyclopentene-functionalized aryl iodide cycliza-tion substrates was successfully synthesized in good yields. The cyclization substrates were then subjected to the optimal conditions that allowed a smooth regioselective and stereoselective intramolecular Mizoroki-Heck spirocy-clization to ten enantiopure spirooxindoles.

One of the synthesized spirooxindoles was transformed into rigidified unnat-ural amino acids by a palladium-catalyzed alkoxycarbonylation involving a gas-free CO source and subsequent chemoselective deprotections. This further highlights the potentials of spirooxindoles functionalization in peptidomimet-ics.

65

5. Regio- and Stereo-Selective Synthesis of Allylic Spiroethers (Spirobenzofuranes) via Intramolecular Mizoroki-Heck Reaction (Paper III)

a. Aims and Objectives Building on from the regioselective synthesis of spirooxindoles (paper II), at-tention was shifted to synthesis of another spirocycle with a different heteroa-tom. This project set out to apply the intramolecular Mizoroki-Heck reaction in the synthesis of spirobenzofuranes, a form of spiroethers. As highlighted in introductory chapters, spiroethers are rare, but important scaffolds in medici-nal chemistry research. This synthesis of spiroethers will be investigated to effect a high regio- and stereoselectivity on the synthesis. The project will thus allow for further insight into the effect of the 2,5-dimethylpyrrole in imposing stereoselectivity on the reaction through steric hindrance. The new spiroben-zofuranes will again be used as a scaffold in making mono protected rigidified unnatural amino acids.

b. Results and Discussions As with the synthesis of spirooxindoles, the synthesis of spiroethers was started from enantiopure Vince lactam 5.1. The Vince lactam was converted to its isomerized ester 5.2b according to the method in Scheme 5.1. The ester 5.2b was then reduced to its alcohol 6.3 using excess diisobutylaluminium hydride, DIBAL-H (3 equiv) as shown in Scheme 6.1. The synthesized alco-hol 6.3 was easily purified by column chromatography to afford the pure al-cohol in 83% yield.

66

Scheme 6.1. Synthesis of alcohol 6.3 by the reduction of ester 5.2b using excess DIBAL-H.

Next, the alcohol was etherified using modified Mitsunobu conditions. The Mitsunobu reaction has proven a clean reaction for etherification of alco-hols.173. A range of ortho-iodophenols were coupled with alcohol 6.3 in the presence of two equivalents of diisopropyl azodicarboxylate (DIAD) and PPh3, to afford the (R)-1-(3-((2-iodophenoxy)methyl)cyclopent-3-en-1-yl)-2,5-dimethyl-1H-pyrrole substrates 6.4a-6.4l as depicted in Scheme 6.2. The alcohol used was in slight excess (1.2 equiv) in order to facilitate easier sepa-ration of the starting iodophenols from the target ether products.

The reaction was well accommodating of electron-withdrawing and electron-donating groups, albeit with varying yields. It was observed that slightly higher yields were recorded for etherification of the more electron-poor iodo-phenols (6.4g, 61%, 6.4e, 79%). The reaction also worked well with substrates wherein the hydroxyl groups in the iodophenol was hindered. The presence of the bulky iodide and methoxy groups at the ortho positions to the hydroxyl groups in 6.4i, 6.4j and 6.4l did not affect the full conversion of the iodophe-nols. However, the isolated yields varied, with 6.4i giving an isolated yield of 71% as against 39% with 6.4j. This further highlights the difference in yield that could arise from having different substituents; in this case a nitro group against a nitrile group, both in para positions. The etherification product of iodovanillin, 6.4l, had the lowest yield of all phenols investigated. This can be attributed to the reactive aldehyde present in the para position, as multiple side products were observed during analysis.

67

Scheme 6.2. Mitsunobu etherification of different iodophenols.

Following the successful synthesis of substrates, attention was then turned to the intramolecular Mizoroki-Heck reaction for possible cyclization of the sub-strates to spiroethers. The reaction screening was done by subjecting substrate 6.4a to conditions previously reported for Mizoroki-Heck reaction in the for-mation of spirooxindoles discussed in the previous chapter.39 Accordingly, Pd(OAc)2 and PPh3/DPPF were chosen as the catalyst source and ligands re-spectively. Triethylamine was used as the organic base to complete the cata-lytic system. Dry DMF was used as a solvent and the reactions were carried out under inert atmosphere. As stated in the project objectives, the regioselec-tivity of the reaction was also to be investigated. In order to do this without bias, silver salts and tetraethylammonium halides were excluded from the re-action. These reagents have been well reported to reduce double bond migra-tions in palladium catalyzed Mizoroki-Heck reactions.174,175

68

The intramolecular Mizoroki-Heck reaction was set up and the crude products analyzed by 1H-NMR. Analysis of the crude product showed four distinct set of vinyl signals. The peaks were found to correspond to either the unreacted precursor 6.4a, cyclized spiroethers 6.5a or 6.6a, and the dehalogenated ether 6.7a (Table 6.1). This provided the different options to optimize the reaction.

The unreacted precursor could be driven to completion by improving the re-action conditions such as temperature and catalyst loading. The regioselectiv-ity for 6.5a or 6.6a was improved by also fine-tuning the reaction conditions. More so, one key step in the catalytic cycle for the Mizoroki-Heck reaction is the oxidative addition of the aryl halide to the palladium (0) species. It is pos-sible that there will be some instances where there is a rate determining inser-tion of the palladium-aryl halide complex into the double bond. This may lead to dehalogenation becoming a competing reaction, thus generating the dehalo-genated side product 6.7a. Dehalogenation is more likely with iodides than other halides in these palladium catalyzed reactions.43,175

Table 6.1. Optimization of reaction conditions for intramolecular Mizoroki-Heck re-action using 6.4a.a

Entry

Pd

(OA

c)2

(%)

Liga

nd (%

) Te

mp

(o C)

Tim

e (h

) 6.

4a

6.5a

6.

6a

6.7a

6.

5a/6

.6a

Isol

ated

Y

ield

of

5a (%

)

1 5

PPh 3

(10)

80

16

73

27

-

- -

2 5

DPP

F (1

0)

80

16

60

33

7 -

83:1

7

3 10

PP

h 3 (2

0)

80

16

55

34

11

- 76

:24

4 10

D

PPF

(20)

80

16

50

44

6

- 88

:12

5 10

PP

h 3 (2

0)

100

16

15

74

11

- 87

:13

6 10

D

PPF

(20)

10

0 16

13

77

10

-

89:1

1

7 10

PP

h 3 (2

0)

120

16

0 58

35

7

62:2

8

8 10

D

PPF

(20)

12

0 16

0

66

20

14

77:2

3

9 10

PP

h 3 (2

0)

140

16

0 86

6

8 93

:7

13

10

20

PPh 3

(40)

14

0 16

0

87

5 8

95:5

21

11

20

PPh 3

(40)

12

0 16

0

98

2 0

98:2

38

12

20

DPP

F (4

0)

120

16

0 83

3

14

98:2

29

13

20

PPh 3

(40)

10

0 16

0

96

2 2

98:2

83

14

20

PPh 3

(40)

12

0 (M

W)

1 0

94

2 4

98:2

57

15

10

PPh 3

.3SO

3- Na+

(20)

14

0 16

0

55

45

0 55

:45

16

10

Pd (P

Ph3) 4

14

0 16

0

37

63

0 37

:63

a Pre

curs

or 6

.4a

(50

mg,

0.1

3 m

mol

) in

1.0

mL

dry

DM

F, li

gand

2 ti

mes

cat

alys

t loa

ding

(sho

wn

in b

rack

et in

liga

nd c

olum

n) a

nd E

t 3N (0

.26

mm

ol).

The

regi

osel

ectiv

ity (r

atio

of 6

.5a/

6.6a

) in

the

crud

e pr

oduc

t was

det

erm

ined

from

1 H-N

MR

anal

ysis.

No

dias

tere

oiso

mer

s of 6

.5a

wer

e ob

serv

ed b

y LC

-MS

or 1 H

-NM

R, h

ence

repo

rted

as d

.e. >

98%

.

71

Analysis of the products obtained when the reaction was heated at 80 oC showed that the conversion was incomplete, even after 16 h, irrespective of the ligand used or the catalyst loading (entries 1-4, Table 6.1). However, it was clear that the amount of unreacted precursor 6.4a, was reduced by increas-ing catalyst loading (entries 3 and 4 vs entries 1 and 2 in Table 6.1). The dehalogenated side product 6.7a was not observed when the reaction was car-ried out at 80 °C (entries 1-4, Table 6.1). Increasing the temperature by 20 °C led to increased conversion, but still not full consumption of 6.4a when the palladium catalyst loading was 10% (entries 5 and 6, Table 6.1). A further increase of the reaction temperature to 120 °C however afforded full conver-sion of the starting ether 6.4a, although with a substantially lower ratio of 6.5a/6.6a when compared to the reaction at 100 °C. These results lead to a reasoning that there were two possible means of improving the reaction selec-tivity towards desired product 5a: increasing the reaction temperature or in-creasing the catalyst loading.

Taking the first option, the reaction temperature was increased by 20 °C again to 140 °C. At this temperature, full conversion was observed, and 1H-NMR analysis of the crude product showed only the spiroether 6.5a, traces of its regioisomer 6.6a, and <10% of the dehalogenated by-product 6.7a. However, when the reaction was worked up and isolated, the isolated yield was very small, at 13%. This poor mass balance was attributed to the decomposition of 6.5a and 6.6a at elevated reaction temperatures. Perhaps, the decomposition of enamine 6.6a was faster than that of 6.5a, hence the high ratio of 6.5a/6.6a observed.

Accordingly, the focus was shifted to increasing the palladium catalyst load-ing. Pleasingly, it was observed that increased catalyst loading led to increased 6.5a/6.6a ratio, even at temperatures as low as 100 °C (entries 12 and 13, Table 6.1). Although a catalyst loading of 20% is usually classified as high for a Mizoroki-Heck reaction, it was concluded from these results that the high cat-alyst loading was essential to achieve regioselectivity in these cyclopentyl-based spiroethers.

Other catalytic systems were also investigated by changing the ligands (entries 15 and 16 Table 6.1). In these instances, the regioselectivity did not improve as observed in the 6.5a/6.6a ratios. The optimal conditions was identified as a 20% Pd(OAc)2 loading, with 40% PPh3 at 100 oC, as seen in entry 13, Table 6.1. These conditions gave a high yield of 83% with the double bond well controlled. The analysis of the product showed only one set of signals corre-sponding to spirooxindole 6.5a. No other stereoisomers were observed either on the LCMS or NMR. Thus, the diastereomeric excess (d.e) was estimated as >98%. This excellent diastereoselectivity is attributed to the face selective approach which was imposed on the reactive system by the bulky 2,5-dime-thylpyrrole protected amine in the 4-position of the cyclopentene core.

72

The optimal reaction conditions were also repeated using MW heating, albeit at a higher temperature (120 °C). This resulted in similar regioselectivity, but lower isolated yield (entry 14, Table 6.1). This further buttress the improved yield of the reaction at lower temperature. The conditions in entry 13 of Table 6.1 were thus selected to further investigate the scope and limitations of the reaction. The Mizoroki-Heck reaction of the remaining precursors 6.4b-6.4l earlier synthesized in Scheme 6.2 was thus carried out at 100 °C, with a 20% catalyst loading. The relatively high catalyst loading was deemed necessary to provide high isolated yield, high regioselectivity and a robust applicability to a diverse set of precursors.

Pleasingly, full conversion was achieved in the spirocyclization reactions for all substrates 6.4b-6.4l. The regioselectivity was above 80:20 in all annulation reactions. Only one diastereoisomer was observed in all the isolated spi-roethers 6.5a-6.5l. It was observed that the regioselectivity was lower when using electron-poor precursors, as seen with the low 6.5/6.6 ratio in the reac-tion of 6.4g and 6.4i. Importantly, the desired spirobenzofuranes 6.5a-6.5l were readily purified by column chromatography. The isolated yields as well as the ratio of 6.5/6.6 in the crude (shown in brackets) are presented in Table 6.2.

The reaction was also chemoselective as the chloro- and bromo- groups in 6.5c and 6.5d remained intact after spirocyclization. The reaction also af-forded good yields of 75% and 62% for 6.5c and 6.5d respectively, albeit with a much lower regioselectivty for 6.5d (ratio of 6.5/6.6 of 89:11 vs 98:2). Elec-tron withdrawing and donating groups were also fully converted to spiroethers in varying yields. Precursors containing nitro (6.4e and 6.4i), methoxy (6.4f), methyl (6.4b), nitrile (6.4j), and carbonyl groups (6.4h and 6.4k) were all fully converted to spirobenzofuranes in isolated yields between 40% and 68%. Lower yields were however observed in the spirocyclization to 6.5g and 6.5l despite full consumption of the starting precursors. This further suggest pos-sible decomposition of the precursors or the products.

In another investigation of chemoselectivity of the reaction process, precur-sors bearing iodide groups in ortho positions were also cyclized. This also allows for a further test of the chemoselectivity of the transformation. Dehalo-genation was prominent in the spirocyclization of 6.4i to 6.5i, with the dehalo-genated product 6.7i isolated in 26% yield. In contrast, dehalogenation was not observed in the spirocyclization of the nitrile bearing 6.4j to 6.5j. This could point to an influence of the ring substituents on the dehalogenation pro-cess. Interestingly, the yield of 6.5i (54%) was higher than that of 6.5j (40%), despite the dehalogenation process. This could be due to possible greater de-composition of 6.4j/6.5j relative to that of 6.4i/6.5i/6.6i.

73

Table 6.2. Intramolecular Mizoroki-Heck Reaction using Aryl Iodides 6.4a-6.4l.

Spirocyclization was scaled up for 6.4k to 6.5k, from 0.25 mmol to 1.6 mmol scale. The isolated yield (62%) was slightly higher at 1.6 mmol scale than at 0.25 mmol scale (55%). The ratio of regioisomers 6.5k/6.6k was also im-proved to 93:7 from 91:9, and no other diastereomer of 6.5k was observed. Spiroether 6.5k was later crystallized and examined by X-ray crystallography. The absolute configuration of 6.5k was established, and the 3R, 4’S stereo-structure was confirmed. This highlights the powerful anti-directing effect of the 2,5-dimethylpyrrole group placed on the 4’ position of the cyclopentene. Stereoselectivity is thus controlled by this protecting group.

74

Figure 6.1. An ORTEP representation of the X-ray crystallography structure of spi-roether 6.5k (underlaid). The 3R, 4’S-configuration of the stereocentres can be seen from the C8-N1 bond and C10-C20 bond directions.

The spirobenzofuranes 6.5a-6.5l represent an excellent platform to access ri-gidified spirocyclic amino acids with an oxygen containing spirocyclic scaf-fold. Thus, the possibility of late stage functionalization of the synthesized spiroethers was further demonstrated by selectively deprotecting spiroether 6.5k. Orthogonal deprotection was carried out by using simple methods for selective deprotection of the ester and N-deprotection. Firstly, selective N-deprotection was performed with hydroxylamine hydrochloride and triethyla-mine at 100 °C, in aqueous methanol for 30 h. This afforded the free amine in spirocycle 6.9 in 67% yield. On the other hand, the methyl ester in 6.5k was also selectively hydrolysed using LiCl in DMF in a reaction carried out at 150 °C for 16 h. The isolated yield was 91% of the free carboxylic acid 6.8 (Scheme 6.3).

75

Scheme 6.3. Selective deprotection of spiroether 6.5k to give the free acid and free base.

c. Conclusion The results obtained from this project have indicated that the intramolecular Mizoroki-Heck reaction is a powerful tool in the synthesis of spiroethers. Spi-robenzofuranes were obtained in high selectivity by Mizoroki-Heck ring-clo-sure from a series of aryl iodide tethered cyclopentenes that were derived from (+)-vince lactam bearing a dimethylpyrrole protected chiral amino substitu-ents as the stereo controlling moiety. The regioselectivity of the reaction was carefully controlled by fine-tuning the reaction conditions leading to double bond position selectivity (81:19 up to 98:2). The intramolecular spirocycliza-tion was also highly stereoselective (d.e. >98), with the bulky 2,5-dime-thylpyrrole group acting as a directing group for the face selective reaction. The regioselectivity and stereoselectivity were confirmed by X-ray crystallog-raphy of one of the spirobenzofuranes while the 1H-NMR was used to confirm regioselectivity in all the spirobenzofuranes synthesized.

Rigidified amino acid derivatives were made using the methoxy ester pro-tected spirobenzofurane 6.5k which was prepared with high stereoselectivity. These results further indicate that allylic spirocycles can be formed in stere-oselective and regioselective manner using the Mizoroki-Heck reaction.

76

6. Acknowledgements

A popular Yoruba proverb says, ‘one who’s shown favor and is not grateful is no different to a thief who’s robbed the benefactor’. Considering this, I would like to express my extreme gratitude to the following people, without whom this PhD journey would have been impossible.

My main supervisor, Professor Mats Larhed, for the guidance and immeasur-able support throughout the entire period. You were very understanding and most helpful in all situations.

My co-supervisor, Associate Professor Luke Odell; thank you for all your guidance and support. I am also very grateful for the welcome you gave me on arrival in Sweden and for finding time to be present at ARIADME meetings and workshop. I am also grateful to Professor Anders Karlen and Pär Matsson for guidance on the antibiotics project.

My industrial supervisors Joakim Bergman and Jonas Brånalt at Astrazeneca, Mölndal; Thank you for the supervision, insightful scientific discussions and making me feel welcomed at Mölndal. I am grateful for the time I spent at your labs. The experience was truly wonderful. In particular, I am particularly grateful for the encouraging words of Joakim at the crucial halfway point of my doctoral studies. I am also grateful to Alexander Wetzel for his support and scientific discussions, especially around the spirooxindole projects.

Magnus Fagrell; thank you for the providing different reactors for the CF pro-ject. I am grateful for the support, especially in modifications requested while I was at Mölndal.

I am also grateful to Prof. Anders Karlen and Pär Matsson for lead and support on project in the first 2 years of my PhD.

Jonas Sävmarker; thank you for assisting a lot with the flow project, and most importantly, supporting me while I was at Mölndal. I am also very apprecia-tive for the discussions we had at the many project meetings while you were here.

Tamal, Carlos and Jens, thank you for the many discussions around the Heck projects.

77

Colleagues, past and present at ILK, most especially, the ones who were most welcoming at the start of my PhD. Marc Stevens, thank you for the very broth-erly welcome to the department. I am also very grateful for your advice and guidance in the early days of my PhD.

To the pleasant colleagues currently at ILK, I am happy to have met you all and thank you for making this place a nice working environment.

To my fellow ‘ESRs’ under the ARIADME consortium, thank you for being wonderful friends. I enjoyed every one of the meetings and workshops.

Anja and Ulrika; for trusting me with teaching duties and for guidance and support while undertaking these duties. I am also grateful to other members of ILK I have participated in teaching duties with.

I have been lucky to have office mates who have helped with scientific and personal guidance. Thank you Prasad for giving me plenty tips at the start of my PhD. I am also highly grateful to Edouard for the advice, support and frankly, for putting up with me. Thank you for the ‘gift’ so cheap I could not pay for. I look forward to inviting you to my farm in Nigeria one day.

Administrative staffs have been very kind and helpful in handling my many troublesome administrative issues. As I do not know how most things work, and with my rather unimpressive grasp of Swedish, the administrative staffs have been very kind and helpful. My sincere gratitude to Birgitta, Karin and the team.

Other friends and colleagues at the BMC and Uppsala, not mentioning names here is not willful omission, neither does it diminish my gratitude to you. You have all been wonderful.

To my parents and siblings (Egbon Biola, Dotun, Lanre and Yinka), thank you for the moral support and standing by me. You have been a major pillar on which this success was built.

To Kudi, thank you for holding the home together, and being supportive even at the most trying moments. I am still your daddy!

Thank you Jemie too (avo oo).

Lastly, my lovely daughters Rahmah and Mariam, you have been my main motivators since you came into my life. No day is a bad day as long as I can get home to play with you. Therefore, while experiments may fail and un-pleasant experiences may becloud my heart, the rush to hug me when I get home completely rejuvenates me almost every day.

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7. References

(1) Basavaiah, D.; Dharma Rao, P.; Hyma, R. S. The Baylis-Hillman Reaction: A Novel Carbon-Carbon Bond Forming Reaction. Tetrahedron. 1996, 8001–8062. https://doi.org/10.1016/0040-4020(96)00154-8.

(2) Hart, D. J. Free-Radical Carbon-Carbon Bond Formation in Organic Synthe-sis. Science (80-. ). 1984, 223 (4639), 883–887. https://doi.org/10.1126/sci-ence.223.4639.883.

(3) Trost, B. M. Selectivity: A Key to Synthetic Efficiency. Science. 1983, pp 245–250. https://doi.org/10.1126/science.219.4582.245.

(4) Lindström, B.; Pettersson, L. J. A Brief History of Catalysis. Cattech 2003, 7 (4), 130–138. https://doi.org/10.1023/A:1025001809516.

(5) Zhou, Q.-L. Transition-Metal Catalysis and Organocatalysis: Where Can Progress Be Expected? Angew. Chemie Int. Ed. 2016, 55 (18), 5352–5353. https://doi.org/10.1002/anie.201509164.

(6) Matyjaszewski, K. Transition Metal Catalysis in Controlled Radical Polymerization: Atom Transfer Radical Polymerization. Chem. - A Eur. J. 1999, 5 (11), 3095–3102. https://doi.org/10.1002/(SICI)1521-3765(19991105)5:11<3095::AID-CHEM3095>3.0.CO;2-#.

(7) Hollingworth, C.; Ronique Gouverneur, V. 2929 Cite This. Chem. Commun 2012, 48, 2929–2942. https://doi.org/10.1039/c2cc16158c.

(8) Zhong, C.; Shi, X. When Organocatalysis Meets Transition-Metal Catalysis. European J. Org. Chem. 2010, 2010 (16), 2999–3025. https://doi.org/10.1002/ejoc.201000004.

(9) Du, Z.; Shao, Z. Combining Transition Metal Catalysis and Organocatalysis-an Update. Chem. Soc. Rev 2013, 42, 1337. https://doi.org/10.1039/c2cs35258c.

(10) Heck, K. F.; Nolley, J. P. Palladium-Catalyzed Vinylic Hydrogen Substitu-tion Reactions with Aryl, Benzyl, and Styryl Halides. J. Org. Chem. 1972, 37 (14), 2320–2322. https://doi.org/10.1021/jo00979a024.

(11) Mizoroki, T.; Mori, K.; Ozaki, A. Arylation of Olefin with Aryl Iodide Cata-lyzed by Palladium. Bull. Chem. Soc. Jpn. 1971, 44 (2), 581–581. https://doi.org/10.1246/bcsj.44.581.

79

(12) Torborg, C.; Beller, M. Recent Applications of Palladium-Catalyzed Cou-pling Reactions in the Pharmaceutical, Agrochemical, and Fine Chemical In-dustries. Adv. Synth. Catal. 2009, 351 (18), 3027–3043. https://doi.org/10.1002/adsc.200900587.

(13) Li, H.; Neumann, H.; Beller, M. Palladium-Catalyzed Aminocarbonylation of Allylic Alcohols. Chem. - A Eur. J. 2016, 22 (29), 10050–10056. https://doi.org/10.1002/chem.201601260.

(14) Drent, E.; Van Dijk, R.; Van Ginkel, R.; Van Oort, B.; Pugh, R. I. Palladium Catalysed Copolymerisation of Ethene with Alkylacrylates: Polar Comono-mer Built into the Linear Polymer Chain. Chem. Commun. 2002, 2 (7), 744–745. https://doi.org/10.1039/b111252j.

(15) Dumas, A.; Couvreur, P. Palladium: A Future Key Player in the Nanomedi-cal Field? Chem. Sci. 2015, 6 (4), 2153–2157. https://doi.org/10.1039/c5sc00070j.

(16) Sehnal, P.; Taylor, R. J. K.; Fairlamb, L. J. S. Emergence of Palladium(IV) Chemistry in Synthesis and Catalysis. Chem. Rev. 2010, 110 (2), 824–889. https://doi.org/10.1021/cr9003242.

(17) Coeffard, V.; Guiry, P. Recent Advances in Ligand Design for the Intermo-lecular Asymmetric Mizoroki- Heck Reaction. Curr. Org. Chem. 2010, 14 (3), 212–229. https://doi.org/10.2174/138527210790231928.

(18) Hickman, A. J.; Sanford, M. S. High-Valent Organometallic Copper and Palladium in Catalysis. Nature 2012, 484 (7393), 177–185. https://doi.org/10.1038/nature11008.

(19) Corbet, J. P.; Mignani, G. Selected Patented Cross-Coupling Reaction Tech-nologies. Chem. Rev. 2006, 106 (7), 2651–2710. https://doi.org/10.1021/cr0505268.

(20) Giannerini, M.; Fañanás-Mastral, M.; Feringa, B. L. Direct Catalytic Cross-Coupling of Organolithium Compounds. Nat. Chem. 2013, 5 (8), 667–672. https://doi.org/10.1038/nchem.1678.

(21) Hornillos, V.; Giannerini, M.; Vila, C.; Fañanás-Mastral, M.; Feringa, B. L. Catalytic Direct Cross-Coupling of Organolithium Compounds with Aryl Chlorides. Org. Lett. 2013, 15 (19), 5114–5117. https://doi.org/10.1021/ol402408v.

(22) Pinxterhuis, E. B.; Giannerini, M.; Hornillos, V.; Feringa, B. L. Fast, Greener and Scalable Direct Coupling of Organolithium Compounds with No Additional Solvents. Nat. Commun. 2016, 7. https://doi.org/10.1038/ncomms11698.

(23) Biffis, A.; Centomo, P.; Del Zotto, A.; Zecca, M. Pd Metal Catalysts for Cross-Couplings and Related Reactions in the 21st Century: A Critical Re-view. Chemical Reviews. 2018, 2249–2295. https://doi.org/10.1021/acs.chemrev.7b00443.

80

(24) Barnard, C. F. J. Palladium-Catalyzed Carbonylation - A Reaction Come of Age. Organometallics. 2008, 5402–5422. https://doi.org/10.1021/om800549q.

(25) Wu, X. F.; Neumann, H.; Beller, M. Synthesis of Heterocycles via Palla-dium-Catalyzed Carbonylations. Chemical Reviews. 2013, 1–35. https://doi.org/10.1021/cr300100s.

(26) Anbarasan, P.; Schareina, T.; Beller, M. Recent Developments and Perspec-tives in Palladium-Catalyzed Cyanation of Aryl Halides: Synthesis of Ben-zonitriles. Chemical Society Reviews. 2011, 5049–5067. https://doi.org/10.1039/c1cs15004a.

(27) Dorel, R.; Grugel, C. P.; Haydl, A. M. The Buchwald–Hartwig Amination After 25 Years. Angew. Chemie - Int. Ed. 2019, 58 (48), 17118–17129. https://doi.org/10.1002/anie.201904795.

(28) Sonogashira, K. Palladium-Catalyzed Alkynylation: Sonogashira Alkyne Synthesis. In Handbook of Organopalladium Chemistry for Organic Synthe-sis; John Wiley & Sons, Inc.: New York, USA, 2003; pp 493–529. https://doi.org/10.1002/0471212466.ch22.

(29) Jutand, A. Mechanisms of the Mizoroki–Heck Reaction. In The Mizoroki–Heck Reaction; John Wiley & Sons, Ltd: Chichester, UK; pp 1–50. https://doi.org/10.1002/9780470716076.ch1.

(30) Phan, N. T. S.; Van Der Sluys, M.; Jones, C. W. On the Nature of the Active Species in Palladium Catalyzed Mizoroki–Heck and Suzuki–Miyaura Cou-plings – Homogeneous or Heterogeneous Catalysis, A Critical Review. Adv. Synth. Catal. 2006, 348 (6), 609–679. https://doi.org/10.1002/adsc.200505473.

(31) Heck, R. F. Palladium-Catalyzed Vinylation of Organic Halides. In Organic Reactions; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 1982; pp 345–390. https://doi.org/10.1002/0471264180.or027.02.

(32) Oestreich, M. Directed Mizoroki-Heck Reactions. Top. Organomet. Chem. 2008, 24, 169–192. https://doi.org/10.1007/3418_2007_063.

(33) Weck, M.; Jones, C. W. Forum Mizoroki−Heck Coupling Using Immobi-lized Molecular Precatalysts: Leaching Active Species from Pd Pincers, En-trapped Pd Salts, and Pd NHC Complexes. Inorg. Chem. 2007, 46, 6, 1865-1875. https://doi.org/10.1021/ic061898h.

(34) Heck, R. F. The Mechanism of Arylation and Carbomethoxylation of Ole-fins with Organopalladium Compounds. J. Am. Chem. Soc. 1969, 91 (24), 6707–6714. https://doi.org/10.1021/ja01052a029.

(35) Cabri, W.; Candiani, I. Recent Developments and New Perspectives in the Heck Reaction. Acc. Chem. Res. 1995, 28 (1), 2–7. https://doi.org/10.1021/ar00049a001.

81

(36) Overman, L. E.; Poon, D. J. Asymmetric Heck Reactions via Neutral Inter-mediates: Enhanced Enantioselectivity with Halide Additives Gives Mecha-nistic Insights. Angew. Chemie (International Ed. English) 1997, 36 (5), 518–521. https://doi.org/10.1002/anie.199705181.

(37) Roy, T.; Brandt, P.; Wetzel, A.; Bergman, J.; Brånalt, J.; Sävmarker, J.; Lar-hed, M. Selective Synthesis of Spirooxindoles by an Intramolecular Heck−Mizoroki Reaction. Org. Lett. 2017, 19, 10, 2738-2741 https://doi.org/10.1021/acs.orglett.7b01094.

(38) Wetzel, A.; Bergman, J.; Brandt, P.; Larhed, M.; Brånalt, J. Regio-and Ste-reoselective Synthesis of Functionalized Cyclopentene Derivatives via Mi-zoroki−Heck Reactions. Org. Lett. 2017, 19, 7, 1602-1605 https://doi.org/10.1021/acs.orglett.7b00325.

(39) Adeyemi, A.; Wetzel, A.; Bergman, J.; Brånalt, J.; Larhed, M. Regio- and Stereoselective Synthesis of Spirooxindoles via Mizoroki-Heck Coupling of Aryl Iodides. Synlett 2019, 30 (1) 82-88. https://doi.org/10.1055/s-0037-1611360.

(40) Qin, L.; Ren, X.; Lu, Y.; Li, Y.; Zhou, J. Intermolecular Mizoroki-Heck Re-action of Aliphatic Olefins with High Selectivity for Substitution at the In-ternal Position. Angew. Chemie - Int. Ed. 2012, 51 (24), 5915–5919. https://doi.org/10.1002/anie.201201806.

(41) Wu, C.; Zhou, J. Asymmetric Intermolecular Heck Reaction of Aryl Hal-ides. J. Am. Chem. Soc. 2014, 136 (2), 650–652. https://doi.org/10.1021/ja412277z.

(42) Oestreich, M. Neighbouring-Group Effects in Heck Reactions. European J. Org. Chem. 2005, 2005 (5), 783–792. https://doi.org/10.1002/ejoc.200400711.

(43) Mc Cartney, D.; Guiry, P. J. The Asymmetric Heck and Related Reactions. Chemical Society Reviews. October 2011, pp 5122–5150. https://doi.org/10.1039/c1cs15101k.

(44) Geoghegan, K. Regioselectivity in the Heck (Mizoroki-Heck) Reaction; 2014; pp 17–41. https://doi.org/10.1007/978-3-319-10338-9_2.

(45) Link, J. T.; Wada, C. K. Intramolecular Enantioselective Mizoroki-Heck Re-actions. In The Mizoroki-Heck Reaction; John Wiley & Sons, Ltd: Chiches-ter, UK, 2009; pp 433–462. https://doi.org/10.1002/9780470716076.ch12.

(46) Ashimori, A.; Matsuura, T.; Overman, L. E.; Poon, D. J. Catalytic Asym-metric Synthesis of Either Enantiomer of Physostigmine. Formation of Qua-ternary Carbon Centers with High Enantioselection by Intramolecular Heck Reactions of (Z)-2-Butenanilides. J. Org. Chem. 1993, 58 (25), 6949–6951. https://doi.org/10.1021/jo00077a005.

82

(47) Grigg, R.; Sridharan, V. Transition Metal Alkyl Complexes: Multiple Inser-tion Cascades. In Comprehensive Organometallic Chemistry II; Elsevier, 1995; pp 299–321. https://doi.org/10.1016/b978-008046519-7.00113-1.

(48) Grigg, R.; Santhakumar, V.; Sridharan, V.; Stevenson, P.; Teasdale, A.; Thornton-Pett, M.; Worakun, T. The Synthesis of Bridged-Ring Carbo- and Hetero-Cycles via Palladium Catalysed Regiospecific Cyclisation Reactions. Tetrahedron 1991, 47 (46), 9703–9720. https://doi.org/10.1016/S0040-4020(01)91034-8.

(49) Dounay, A. B.; Overman, L. E. The Asymmetric Intramolecular Mizoroki-Heck Reaction in Natural Product Total Synthesis. In The Mizoroki-Heck Reaction; John Wiley & Sons, Ltd: Chichester, UK, 2009; pp 533–568. https://doi.org/10.1002/9780470716076.ch16.

(50) Dankwardt, J. W.; Flippin, L. A. Palladium-Mediated 6-Endo-Trig Intramo-lecular Cyclization of N-Acryloyl-7-Bromoindolines. A Regiochemical Var-iant of the Intramolecular Heck Reaction. J. Org. Chem. 1995, 60 (8), 2312–2313. https://doi.org/10.1021/jo00113a001.

(51) Nilsson, P.; Larhed, M.; Hallberg, A. A New Highly Asymmetric Chelation-Controlled Heck Arylation. J. Am. Chem. Soc. 2003, 125 (12), 3430–3431. https://doi.org/10.1021/ja029646c.

(52) Ashimori, A.; Overman, L. E. Catalytic Asymmetric Synthesis of Quarter-nary Carbon Centers. Palladium-Catalyzed Formation of Either Enantiomer of Spirooxindoles and Related Spirocyclics Using a Single Enantiomer of a Chiral Diphosphine Ligand. Journal of Organic Chemistry. August 1, 1992, pp 4571–4572. https://doi.org/10.1021/jo00043a005.

(53) Imbos, R.; Minnaard, A. J.; Feringa, B. L. A Highly Enantioselective Intra-molecular Heck Reaction with a Monodentate Ligand. J. Am. Chem. Soc. 2002, 124 (2), 184–185. https://doi.org/10.1021/ja017200a.

(54) Von Schenck, H.; Åkermark, B.; Svensson, M. Electronic Control of the Re-giochemistry in the Heck Reaction. J. Am. Chem. Soc. 2003, 125 (12), 3503–3508. https://doi.org/10.1021/ja028755o.

(55) Zheng, Y. J.; Tice, C. M. The Utilization of Spirocyclic Scaffolds in Novel Drug Discovery. Expert Opin. Drug Discov. 2016, 11 (9), 831–834. https://doi.org/10.1080/17460441.2016.1195367.

(56) Rosenberg, S.; Leino, R. Synthesis of Spirocyclic Ethers. 2009. Synthesis 2009, 16, 2651-2673 https://doi.org/10.1055/s-0029-1216892.

(57) Müller, G.; Berkenbosch, T.; Benningshof, J. C. J.; Stumpfe, D.; Bajorath, J. Charting Biologically Relevant Spirocyclic Compound Space. Chem. - A Eur. J. 2017, 23 (3), 703–710. https://doi.org/10.1002/chem.201604714.

(58) Gober, C. M.; Carroll, P. J.; Joullié, M. M. Triazaspirocycles: Occurrence, Synthesis, and Applications. https://doi.org/10.2174/1570193X13666160225001001.

83

(59) Carreira, E. M.; Fessard, T. C. Four-Membered Ring-Containing Spirocy-cles: Synthetic Strategies and Opportunities. Chem. Rev. 2014, 114, 16, 8257-8322. https://doi.org/10.1021/cr500127b.

(60) Pavlovska, T. L.; Redkin, R. G.; Lipson, V. V.; Atamanuk, D. V. Molecular Diversity of Spirooxindoles. Synthesis and Biological Activity. Mol. Divers. 2016, 20 (1), 299–344. https://doi.org/10.1007/s11030-015-9629-8.

(61) Melnykov, K. P.; Artemenko, A. N.; Ivanenko, B. O.; Sokolenko, Y. M.; Nosik, P. S.; Ostapchuk, E. N.; Grygorenko, O. O.; Volochnyuk, D. M.; Ryabukhin, S. V. Scalable Synthesis of Biologically Relevant Spirocyclic Pyrrolidines. ACS Omega 2019, 4, 4, 7498-7515. https://doi.org/10.1021/acsomega.9b00896.

(62) Fleischhacker, W.; Lauritz, S.; Urban, E.; Baumann, P.; Bittiger, H. Synthe-sis and Biochemical Studies of Spirocyclic Amino Acids. II. Activity of 2-Azaspiro[5.5]Undecane-7-Carboxylates as GABA-Uptake Inhibitors. Eur. J. Med. Chem. 1995, 30 (9), 707–713. https://doi.org/10.1016/0223-5234(96)88288-2.

(63) Müller, G.; Berkenbosch, T.; Benningshof, J. C. J.; Stumpfe, D.; Bajorath, J. Charting Biologically Relevant Spirocyclic Compound Space. Chem. - A Eur. J. 2017, 23 (3), 703–710. https://doi.org/10.1002/chem.201604714.

(64) Genin, M. J.; Mishra, R. K.; Johnson, R. L. Dopamine Receptor Modulation by a Highly Rigid Spiro Bicyclic Peptidomimetic of Pro-Leu-Gly-NH2. J. Med. Chem. 1993, 36, 3481-3483

(65) Wang, Y.; Liu, J.; Dransfield, P. J.; Zhu, L.; Wang, Z.; Du, X.; Jiao, X.; Su, Y.; Li, A.; Brown, S. P.; et al. Discovery and Optimization of Potent GPR40 Full Agonists Containing Tricyclic Spirocycles. ACS Med. Chem. Lett. 2013, 4, 6, 551-555. https://doi.org/10.1021/ml300427u.

(66) Zannad, F.; McMurray, J. J. V.; Krum, H.; van Veldhuisen, D. J.; Swedberg, K.; Shi, H.; Vincent, J.; Pocock, S. J.; Pitt, B. Eplerenone in Patients with Systolic Heart Failure and Mild Symptoms. N. Engl. J. Med. 2011, 364 (1), 11–21. https://doi.org/10.1056/NEJMoa1009492.

(67) Pitt, B.; Pfeffer, M. A.; Assmann, S. F.; Boineau, R.; Anand, I. S.; Claggett, B.; Clausell, N.; Desai, A. S.; Diaz, R.; Fleg, J. L.; et al. Spironolactone for Heart Failure with Preserved Ejection Fraction. N. Engl. J. Med. 2014, 370 (15), 1383–1392. https://doi.org/10.1056/NEJMoa1313731.

(68) Kotha, S.; Panguluri, N. R.; Ali, R. Design and Synthesis of Spirocycles. European J. Org. Chem. 2017, 2017 (36), 5316–5342. https://doi.org/10.1002/ejoc.201700439.

(69) Trost, B. M.; Adams, B. R. A Cyclocontraction-Spiroannulation: A Stere-oselective Approach to Spirocycles. J. Am. Chem. Soc. 1983, 105 (14), 4849–4850. https://doi.org/10.1021/ja00352a062.

84

(70) Bassindale, M. J.; Hamley, P.; Leitner, A.; Harrity, J. P. A. Spirocycle As-sembly through Selective Tandem Ring Closing Metathesis Reactions. Tet-rahedron Lett. 1999, 3247-3250. https://doi.org/10.1016/S0040-4039(99)00375-5.

(71) Yu, B.; Zheng, Y.-C.; Shi, X.-J.; Qi, P.-P.; Liu, H.-M. Natural Product-De-rived Spirooxindole Fragments Serve as Privileged Substructures for Dis-covery of New Anticancer Agents. Anticancer. Agents Med. Chem. 2016, 16 (10), 1315–1324. https://doi.org/10.2174/1871520615666151102093825.

(72) Yu, B.; Yu, D. Q.; Liu, H. M. Spirooxindoles: Promising Scaffolds for Anti-cancer Agents. European Journal of Medicinal Chemistry. 2015. https://doi.org/10.1016/j.ejmech.2014.06.056.

(73) Rottmann, M.; McNamara, C.; Yeung, B. K. S.; Lee, M. C. S.; Zou, B.; Rus-sell, B.; Seitz, P.; Plouffe, D. M.; Dharia, N. V.; Tan, J.; et al. Spiroin-dolones, a Potent Compound Class for the Treatment of Malaria. Science (80-. ). 2010, 329 (5996), 1175–1180. https://doi.org/10.1126/sci-ence.1193225.

(74) Turner, H. Spiroindolone NITD609 Is a Novel Antimalarial Drug That Tar-gets the P-Type ATPase PfATP4. Future Medicinal Chemistry. 2016, 227–238. https://doi.org/10.4155/fmc.15.177.

(75) Hunter, A. C.; Schlitzer, S. C.; Stevens, J. C.; Almutwalli, B.; Sharma, I. A Convergent Approach to Diverse Spiroethers through Stereoselective Trap-ping of Rhodium Carbenoids with Gold-Activated Alkynols. J. Org. Chem. 2018, 83 (5), 2744–2752. https://doi.org/10.1021/acs.joc.7b03196.

(76) Badillo, J. J.; Hanhan, N. V.; Franz, A. K. Enantioselective Synthesis of Substituted Oxindoles and Spirooxindoles with Applications in Drug Dis-covery. Curr. Opin. Drug Discov. Dev. 2010, 13 (6), 758–776.

(77) Ball-Jones, N. R.; Badillo, J. J.; Franz, A. K. Strategies for the Enantioselec-tive Synthesis of Spirooxindoles. Org. Biomol. Chem. 2012, 10 (27), 5165–5181. https://doi.org/10.1039/c2ob25184a.

(78) Xia, M.; Ma, R.-Z. Recent Progress on Routes to Spirooxindole Systems Derived from Isatin. J. Heterocycl. Chem. 2014, 51 (3), 539–554. https://doi.org/10.1002/jhet.1114.

(79) Mei, G.-J.; Shi, F. Catalytic Asymmetric Synthesis of Spirooxindoles: Re-cent Developments. Chem. Commun 2018, 54, 6607. https://doi.org/10.1039/c8cc02364f.

(80) Bariwal, J.; Voskressensky, L. G.; Van Der Eycken, E. V. Recent Advances in Spirocyclization of Indole Derivatives. Chem. Soc. Rev 2018, 47, 3831. https://doi.org/10.1039/c7cs00508c.

85

(81) Kamisaki, H.; Nanjo, T.; Tsukano, C.; Takemoto, Y. Domino Pd-Catalyzed Heck Cyclization and Bismuth-Catalyzed Hydroamination: Formal Synthe-sis of Elacomine and Isoelacomine. Chem. - A Eur. J. 2011, 17 (2), 626–633. https://doi.org/10.1002/chem.201002287.

(82) Kamisaki, H.; Yasui, Y.; Takemoto, Y. Pd-Catalyzed Intramolecular Ami-dation of 2-(Buta-1,3-Dienyl)Phenylcarbamoyl Chloride: A Concise Synthe-sis of Spiro[Indoline-3,3′-Pyrrolidine]. Tetrahedron Lett. 2009. 50(21), 2589-2592. https://doi.org/10.1016/j.tetlet.2009.03.100.

(83) Jaegli, S.; Dufour, J.; Wei, H.; Piou, T.; Duan, X.-H.; Vors, J.-P.; Neuville, L.; Zhu, J. Palladium-Catalyzed Carbo-Heterofunctionalization of Alkenes for the Synthesis of Oxindoles and Spirooxindoles. Angew. Chem., Int. Ed 2003, 36 (2), 45. https://doi.org/10.1021/ol101778c.

(84) Deppermann, N.; Thomanek, H.; G P Prenzel, A. H.; Maison, W. Pd-Cata-lyzed Assembly of Spirooxindole Natural Products: A Short Synthesis of Horsfiline. J. Org. Chem 2010, 75, 5994–6000. https://doi.org/10.1021/jo101401z.

(85) Liu, R.-R.; Xu, Y.; Liang, R.-X.; Xiang, B.; Xie, H.-J.; Gao, J.-R.; Jia, Y.-X. Organic & Biomolecular Chemistry Spirooxindole Synthesis via Palladium-Catalyzed Dearomative Reductive-Heck Reaction . Org. Biomol. Chem., 2017,15, 2711-2715. https://doi.org/10.1039/c7ob00146k.

(86) Liu, J.; Peng, H.; Yang, Y.; Jiang, H.; Yin, B. Regioselective and Stereose-lective Pd-Catalyzed Intramolecular Arylation of Furans: Access to Spiroox-indoles and 5H-Furo[2,3-c]Quinolin-4-Ones. J. Org. Chem. 2016, 81, 20, 9695-9706. https://doi.org/10.1021/acs.joc.6b01774.

(87) Katsoulis, I. A.; Kythreoti, G.; Papakyriakou, A.; Koltsida, K.; Anasta-sopoulou, P.; Stathakis, C. I.; Mavridis, I.; Cottin, T.; Saridakis, E.; Vour-loumis, D. Synthesis of 5,6-Spiroethers and Evaluation of Their Affinities for the Bacterial A Site. ChemBioChem 2011, 12 (8), 1188–1192. https://doi.org/10.1002/cbic.201100076.

(88) Mavridis, I.; Kythreoti, G.; Koltsida, K.; Vourloumis, D. Rigid Spiroethers Targeting the Decoding Center of the Bacterial Ribosome. Bioorganic Med. Chem. 2014, 22 (4), 1329–1341. https://doi.org/10.1016/j.bmc.2013.12.068.

(89) Rosenberg, S.; Leino, R. Synthesis of Spirocyclic Ethers. Synthesis. 2009, 16, 2651–2673. https://doi.org/10.1055/s-0029-1216892.

(90) Lankri, D.; Mostinski, Y.; Tsvelikhovsky, D. Palladium-Catalyzed Cascade Assembly of Tricyclic Spiroethers from Diene-Alcohol Precursors. 2017. J. Org. Chem. 2017, 82, 18, 9452-9463. https://doi.org/10.1021/acs.joc.7b01481.

86

(91) Blaskovich, M. A. T. Unusual Amino Acids in Medicinal Chemistry. J. Med. Chem. 2016, 59, 24, 10807-10836 https://doi.org/10.1021/acs.jmed-chem.6b00319.

(92) Neumann-Staubitz, P.; Neumann, H. The Use of Unnatural Amino Acids to Study and Engineer Protein Function. Current Opinion in Structural Biol-ogy. 2016, 38, 119–128. https://doi.org/10.1016/j.sbi.2016.06.006.

(93) Sung Lee, B.; Shin, S.; Yeob Jeon, J.; Jang, K.-S.; Yeol Lee, B.; Choi, S.; Hyeon Yoo, T. Incorporation of Unnatural Amino Acids in Response to the AGG Codon. ACS Chem. Biol. 2015, 10, 7, 1648-1653. https://doi.org/10.1021/acschembio.5b00230.

(94) N, A. R.; Spear, L. A.; Williams, T. L.; Luk, L. Y.; Tsai, Y.-H. Using Genet-ically Incorporated Unnatural Amino Acids to Control Protein Functions in Mammalian Cells. Essays Biochem. 2019, 63, 237–266. https://doi.org/10.1042/EBC20180042.

(95) Ryu, Y.; Schultz, P. G. Efficient Incorporation of Unnatural Amino Acids into Proteins in Escherichia Coli. Nat Methods 2006,3, 263–265. https://doi.org/10.1038/NMETH864.

(96) Young, T. S.; Schultz, P. G. Beyond the Canonical 20 Amino Acids: Ex-panding the Genetic Lexicon *. 2010. doi: 10.1074/jbc.R109.091306 https://doi.org/10.1074/jbc.R109.091306.

(97) Minnihan, E. C.; Yokoyama, K.; Stubbe, J. Unnatural Amino Acids: Better than the Real Things? Introduction and Context. F1000 Biology Reports 2009, 1:88 (doi:10.3410/B1-88). https://doi.org/10.3410/B1-88.

(98) Dougherty, D. A. Unnatural Amino Acids as Probes of Protein Structure and Function. Current Opinion in Chemical Biology. 2000, 4, 645–652 https://doi.org/10.1016/S1367-5931(00)00148-4.

(99) Maluch, I.; Czarna, J.; Drag, M. Applications of Unnatural Amino Acids in Protease Probes. Chem. – An Asian J. 2019, 14 (23), 4103–4113. https://doi.org/10.1002/asia.201901152.

(100) Huber, R. Flexibility and Rigidity, Requirements for the Function of Pro-teins and Protein Pigment Complexes. Eleventh Keilin Memorial Lecture. Biochem. Soc. Trans. 1987, 15 (6), 1009–1020. https://doi.org/10.1042/bst0151009.

(101) Pavone, V.; Lombardi, A.; Maggi, C. A.; Quartara, L.; Pedone, C. Confor-mational Rigidity versus Flexibility in a Novel Peptidic Neurokinin A Re-ceptor Antagonist. J. Pept. Sci. 1995, 1 (4), 236–240. https://doi.org/10.1002/psc.310010404.

(102) Lim, D.; Burgess, K. Spirocyclic Peptidomimetics Featuring 2,3-Meth-anoamino Acids; J. Am. Chem. Soc. 1997, 119, 41, 9632-9640.

87

(103) Synthesis of Rigidified Cyclic Peptides by Ring-Closing Metathesis. Syn-facts 2019, 15(08), 0949. https://doi.org/10.1055/s-0039-1690475.

(104) Miller, S. J.; Blackwell, H. E.; Grubbs, R. H. Application of Ring-Closing Metathesis to the Synthesis of Rigidified Amino Acids and Peptides; J. Am. Chem. Soc. 1996, 118, 40, 9606-9614.

(105) Alastair G Lawson, M. D.; MacCoss, M.; Heer, J. P. Importance of Rigidity in Designing Small Molecule Drugs To Tackle Protein−Protein Interactions (PPIs) through Stabilization of Desired Conformers. J. Med. Chem. 2018, 61, 10, 4283-4289. https://doi.org/10.1021/acs.jmedchem.7b01120.

(106) Hanessian, S.; Auzzas, L. The Practice of Ring Constraint in Peptidomimet-ics Using Bicyclic and Polycyclic Amino Acids. Acc. Chem. Res. 2008, 41, 10, 1241-1251. https://doi.org/10.1021/ar8000052.

(107) Mottola, H. A. Flow Analyses Revisited. Anal. Chem. 1981, 53 (12), 1313–1316. https://doi.org/10.1021/ac00235a730.

(108) Porta, R.; Benaglia, M.; Puglisi, A. Flow Chemistry: Recent Developments in the Synthesis of Pharmaceutical Products. Org. Process Res. Dev. 2016, 20 (1), 2–25. https://doi.org/10.1021/acs.oprd.5b00325.

(109) Movsisyan, M.; Delbeke, E. I. P.; Berton, J. K. E. T.; Battilocchio, C.; Ley, S. V.; Stevens, C. V. Taming Hazardous Chemistry by Continuous Flow Technology. Chem. Soc. Rev. 2016, 45 (18), 4892–4928. https://doi.org/10.1039/c5cs00902b.

(110) Pastre, J. C.; Browne, D. L.; Ley, S. V. Flow Chemistry Syntheses of Natu-ral Products. Chemical Society Reviews. Royal Society of Chemistry 2013, 8849–8869. https://doi.org/10.1039/c3cs60246j.

(111) Jas, G.; Kirschning, A. Continuous Flow Techniques in Organic Synthesis. Chem. - A Eur. J. 2003, 9 (23), 5708–5723. https://doi.org/10.1002/chem.200305212.

(112) Gutmann, B.; Cantillo, D.; Kappe, C. O. Continuous-Flow Technology - A Tool for the Safe Manufacturing of Active Pharmaceutical Ingredients. An-gew. Chemie - Int. Ed. 2015, 54 (23), 6688–6728. https://doi.org/10.1002/anie.201409318.

(113) Lukas, T. J.; Prystowsky, M. B.; Erickson, B. W. Solid-Phase Peptide Syn-thesis under Continuous-Flow Conditions. Proc. Natl. Acad. Sci. U. S. A. 1981, 78 (5), 2791–2795. https://doi.org/10.1073/pnas.78.5.2791.

(114) Wegner, J.; Ceylan, S.; Kirschning, A. Flow Chemistry - A Key Enabling Technology for (Multistep) Organic Synthesis. Adv. Synth. Catal. 2012, 354 (1), 17–57. https://doi.org/10.1002/adsc.201100584.

(115) Britton, J.; Raston, C. L. Multi-Step Continuous-Flow Synthesis. Chem. Soc. Rev. 2017, 46 (5), 1250–1271. https://doi.org/10.1039/c6cs00830e.

88

(116) Mata, A.; Hone, C. A.; Gutmann, B.; Moens, L.; Kappe, C. O. Continuous-Flow Pd-Catalyzed Carbonylation of Aryl Chlorides with Carbon Monoxide at Elevated Temperature and Pressure. ChemCatChem 2019, 11 (3), 997–1001. https://doi.org/10.1002/cctc.201801974.

(117) Applications of Flow Chemistry in Drug Development: Highlights of Recent Patent Literature. Org. Process Res. Dev. 2018, 22, 1, 13-20. https://doi.org/10.1021/acs.oprd.7b00363.

(118) Bogdan, A. R.; Poe, S. L.; Kubis, D. C.; Broadwater, S. J.; McQuade, D. T. The Continuous-Flow Synthesis of Ibuprofen. Angew. Chemie - Int. Ed. 2009, 48 (45), 8547–8550. https://doi.org/10.1002/anie.200903055.

(119) Bogdan, A. R.; Dombrowski, A. W. Emerging Trends in Flow Chemistry and Applications to the Pharmaceutical Industry. J. Med. Chem. 2019, 62, 14, 6422-6468. https://doi.org/10.1021/acs.jmedchem.8b01760.

(120) Christopher McWilliams, J.; Allian, A. D.; Opalka, S. M.; May, S. A.; Jour-net, M.; Braden, T. M. The Evolving State of Continuous Processing in Pharmaceutical API Manufacturing: A Survey of Pharmaceutical Companies and Contract Manufacturing Organizations. Org. Process Res. Dev. 2018, 22, 9, 1143-1166. https://doi.org/10.1021/acs.oprd.8b00160.

(121) Malet-Sanz, L.; Susanne, F. Continuous Flow Synthesis. A Pharma Perspec-tive. J. Med. Chem. 2012, 55, 9, 4062-4098. https://doi.org/10.1021/jm2006029.

(122) Wiles, C.; Watts, P. Continuous Flow Reactors: A Perspective. Green Chem., 2012,14, 38-54. https://doi.org/10.1039/c1gc16022b.

(123) Mason, B. P.; Price, K. E.; Steinbacher, J. L.; Bogdan, A. R.; McQuade, T. D. Greener Approaches to Organic Synthesis Using Microreactor Technol-ogy. Chem. Rev. 2007, 107, 6, 2300-2318. https://doi.org/10.1021/cr050944c.

(124) Herath, A.; Cosford, N. D. P. Continuous-Flow Synthesis of Highly Func-tionalized Imidazo-Oxadiazoles Facilitated by Microfluidic Extraction. Beilstein J. Org. Chem. 2017, 13, 239–246. https://doi.org/10.3762/bjoc.13.26.

(125) Browne, D. L.; Wright, S.; Deadman, B. J.; Dunnage, S.; Baxendale, I. R.; Turner, R. M.; Ley, S. V. Continuous Flow Reaction Monitoring Using an On-Line Miniature Mass Spectrometer. Rapid Commun. Mass Spectrom. 2012, 26 (17), 1999–2010. https://doi.org/10.1002/rcm.6312.

(126) Sans, V.; Cronin, L. Towards Dial-a-Molecule by Integrating Continuous Flow, Analytics and Self-Optimisation. Chem. Soc. Rev., 2016,45, 2032-2043. https://doi.org/10.1039/c5cs00793c.

89

(127) McMullen, J. P.; Stone, M. T.; Buchwald, S. L.; Jensen, K. F. An Integrated Microreactor System for Self-Optimization of a Heck Reaction: From Mi-cro- to Mesoscale Flow Systems. Angew. Chemie Int. Ed. 2010, 49 (39), 7076–7080. https://doi.org/10.1002/anie.201002590.

(128) Loren, B. P.; Ewan, H. S.; Avramova, L.; Ferreira, C. R.; Sobreira, T. J. P.; Yammine, K.; Liao, H.; Cooks, R. G.; Thompson, D. H. High Throughput Experimentation Using DESI-MS to Guide Continuous-Flow Synthesis. Sci. Rep. 2019, 9 (1), 1–8. https://doi.org/10.1038/s41598-019-50638-7.

(129) Wegner, J.; Ceylan, S.; Kirschning, A. Ten Key Issues in Modern Flow Chemistry. Chem. Commun 2011, 47, 4583–4592. https://doi.org/10.1039/c0cc05060a.

(130) Fardost, A.; Russo, F.; Schanche, J.-S.; Fagrell, M.; Larhed, M. Evaluation of a Nonresonant Microwave Applicator for Continuous-Flow Chemistry Applications. Org. Process Res. Dev. 2012, 16, 5, 1053-1063. https://doi.org/10.1021/op300003b.

(131) Britton, J.; Jamison, T. F. The Assembly and Use of Continuous Flow Sys-tems for Chemical Synthesis. Nat. Protoc. 2017, 12 (11), 2423–2446. https://doi.org/10.1038/nprot.2017.102.

(132) Polyzos, A.; O’Brien, M.; Petersen, T. P.; Baxendale, I. R.; Ley, S. V. The Continuous-Flow Synthesis of Carboxylic Acids Using CO2 in a Tube-In-Tube Gas Permeable Membrane Reactor. Angew. Chemie Int. Ed. 2011, 50 (5), 1190–1193. https://doi.org/10.1002/anie.201006618.

(133) Konda, V.; Rydfjord, J.; Sä, J.; Larhed, M. Safe Palladium-Catalyzed Cross-Couplings with Microwave Heating Using Continuous-Flow Silicon Carbide Reactors. Org. Process Res. Dev 2014, 18, 10. https://doi.org/10.1021/op5001989.

(134) Adeyemi, A.; Bergman, J.; Brånalt, J.; Sävmarker, J.; Larhed, M. Continu-ous Flow Synthesis under High-Temperature/High-Pressure Conditions Us-ing a Resistively Heated Flow Reactor. Org. Process Res. Dev. 2017, 21, 7, 947-955. https://doi.org/10.1021/acs.oprd.7b00063.

(135) Newby, J. A.; Blaylock, D. W.; Witt, P. M.; Pastre, J. C.; Zacharova, M. K.; Ley, S. V.; Browne, D. L. Design and Application of a Low-Temperature Continuous Flow Chemistry Platform. Org. Process Res. Dev. 2014, 18 (10), 1211–1220. https://doi.org/10.1021/op500213j.

(136) Cantillo, D.; Kappe, C. O. Direct Preparation of Nitriles from Carboxylic Acids in Continuous Flow. J. Org. Chem. 2013, 78, 20, 10567-10571. https://doi.org/10.1021/jo401945r.

(137) Glasnov, T. N.; Kappe, C. O. The Microwave-to-Flow Paradigm: Translat-ing High-Temperature Batch Microwave Chemistry to Scalable Continuous-Flow Processes. Chem. - A Eur. J. 2011, 17 (43), 11956–11968. https://doi.org/10.1002/chem.201102065.

90

(138) Damm, M.; Glasnov, T. N.; Kappe, C. O. Translating High-Temperature Microwave Chemistry to Scalable Continuous Flow Processes. Org. Process Res. Dev. 2010, 14, 1, 215-224 https://doi.org/10.1021/op900297e.

(139) Glasnov, T. N.; Kappe, C. O. Microwave-Assisted Synthesis under Continu-ous-Flow Conditions. Macromol. Rapid Commun. 2007, 28 (4), 395–410. https://doi.org/10.1002/marc.200600665.

(140) Kunz, U.; Turek, T. Flow through Reactors for Organic Chemistry: Directly Electrically Heated Tubular Mini Reactors as an Enabling Technology for Organic Synthesis. Beilstein J. Org. Chem. 2009, 5. https://doi.org/10.3762/bjoc.5.70.

(141) Singh, S.; Köhler, J. M.; Schober, A.; Groß, G. A. The Eschenmoser Cou-pling Reaction under Continuous-Flow Conditions. Beilstein J. Org. Chem. 2011, 7, 1164–1172. https://doi.org/10.3762/bjoc.7.135.

(142) Razzaq, T.; Glasnov, T. N.; Kappe, C. O. Continuous-Flow Microreactor Chemistry under High-Temperature/Pressure. European J. Org. Chem. 2009, 3 (9), 1321–1325. https://doi.org/10.1002/ejoc.200900077.

(143) Razzaq, T.; Kappe, C. O. Continuous Flow Organic Synthesis under High-Temperature/Pressure :Conditions. Chem. - An Asian J. 2010, 5 (6), 1274–1289. https://doi.org/10.1002/asia.201000010.

(144) Sullivan R.J., Newman S.G. (2018) Flow-Assisted Synthesis of Heterocy-cles at High Temperatures. In: Sharma U., Van der Eycken E. (eds) Flow Chemistry for the Synthesis of Heterocycles. Topics in Heterocyclic Chem-istry, vol 56. Springer, Cham. https://doi.org/10.1007/7081_2018_18.

(145) Razzaq, T.; Kappe, C. O. Continuous Flow Organic Synthesis under High-Temperature/Pressure Conditions. Chem. - An Asian J. 2010, 5, 1274-1289 https://doi.org/10.1002/asia.201000010.

(146) Baiker, A. Supercritical Fluids in Heterogeneous Catalysis. Chem. Rev. 1999, 99 (2–3), 453–473. https://doi.org/10.1021/cr970090z.

(147) Oakes, R. S.; Clifford, A. A.; Rayner, C. M. The Use of Supercritical Fluids in Synthetic Organic Chemistry. J. Chem. Soc. Perkin 1 2001, No. 9, 917–941. https://doi.org/10.1039/b101219n.

(148) Centi, G.; Perathoner, S. Catalysis and Sustainable (Green) Chemistry. In Catalysis Today; 2003, 77, 287–297. https://doi.org/10.1016/S0920-5861(02)00374-7.

(149) Hauthal, W. H. Advances with Supercritical Fluids [Review]. In Chemo-sphere; 2001, 43, 123–135. https://doi.org/10.1016/S0045-6535(00)00332-5.

(150) Eckert, C. A.; Knutson, B. L.; Debenedetti, P. G. Supercritical Fluids as Sol-vents for Chemical and Materials Processing; Nature 1996, 383, 313–318. https://doi.org/10.1038/383313a0.

91

(151) Squires, T. G.; Venier, C. G.; Aida, T. Supercritical Fluid Solvents in Or-ganic Chemistry. Fluid Phase Equilib. 1983, 10 (2–3), 261–268. https://doi.org/10.1016/0378-3812(83)80039-9.

(152) Kamat, S. V.; Iwaskewycz, B.; Beckman, E. J.; Russell, A. J. Biocatalytic Synthesis of Acrylates in Supercritical Fluids: Tuning Enzyme Activity by Changing Pressure. Proc. Natl. Acad. Sci. U. S. A. 1993, 90 (7), 2940–2944. https://doi.org/10.1073/pnas.90.7.2940.

(153) Fischer, A.; Mallat, T.; Baiker, A. Continuous Amination of Propanediols in Supercritical Ammonia. Angew. Chemie - Int. Ed. 1999, 38 (3), 351–354. https://doi.org/10.1002/(SICI)1521-3773(19990201)38:3<351::AID-ANIE351>3.0.CO;2-0.

(154) Poliakoff, M. Friedel-Crafts Alkylation in Supercritical Fluids: Continuous, Selective and Clean. Chem. Commun. 1998, c (3), 359–360. https://doi.org/10.1039/a707149c.

(155) Sears, T.; Miller, T. A.; Bondybey, V. E. Jahn–Teller Distortions in C 6 H 3 F 3 + and C 6 H 3 Cl 3 + ; J. Chem. Phys. 1980, 72, 6070. https://doi.org/10.1063/1.439063.

(156) Amandi, R.; Licence, P.; Ross, S. K.; Aaltonen, O.; Poliakoff, M. Friedel-Crafts Alkylation of Anisole in Supercritical Carbon Dioxide: A Compara-tive Study of Catalysts. Org. Process Res. Dev. 2005, 9 (4), 451–456. https://doi.org/10.1021/op050044y.

(157) Narayan, R. C.; Madras, G. Alkylation of Fatty Acids in Supercritical Alco-hols. Energy and Fuels 2016, 30 (5), 4104–4111. https://doi.org/10.1021/acs.energyfuels.6b00266.

(158) Tangestanifard, M.; Ghaziaskar, H. S. Methylation of Toluene with Metha-nol in Sub/Supercritical Toluene Using H-Beta Zeolite as Catalyst. J. Super-crit. Fluids 2016, 113, 80–88. https://doi.org/10.1016/j.supflu.2016.03.013.

(159) Sato, T.; Sekiguchi, G.; Adschiri, T.; Arai, K. Ortho-Selective Alkylation of Phenol with 2-Propanol without Catalyst in Supercritical Water. Ind. Eng. Chem. Res. 2002, 41 (13), 3064–3070. https://doi.org/10.1021/ie0200712.

(160) Black, H. Supercritical Carbon Dioxide: The “Greener” Solvent. Environ. Sci. Technol. 1996, 30 (3), 124A–127A. https://doi.org/10.1021/es962136w.

(161) Qiao, Y. X.; Theyssen, N.; Eifert, T.; Liauw, M. A.; Franciò, G.; Schenk, K.; Leitner, W.; Reetz, M. T. Concerning the Role of Supercritical Carbon Di-oxide in SN1 Reactions. Chem. - A Eur. J. 2017, 23 (16), 3898–3902. https://doi.org/10.1002/chem.201604151.

(162) Adschiri, T.; Yoko, A. Supercritical Fluids for Nanotechnology. J. Super-crit. Fluids 2018, 134, 167–175. https://doi.org/10.1016/j.sup-flu.2017.12.033.

92

(163) Bondesgaard, M.; Becker, J.; Xavier, J.; Hellstern, H.; Mamakhel, A.; Iversen, B. B. Guide to By-Products Formed in Organic Solvents under Sol-vothermal Conditions. J. Supercrit. Fluids 2016, 113, 166–197. https://doi.org/10.1016/j.supflu.2016.02.012.

(164) Razzaq, T.; Glasnov, T. N.; Kappe, C. O. Continuous-Flow Microreactor Chemistry under High-Temperature/Pressure Conditions. European J. Org. Chem. 2009, 2009 (9), 1321–1325. https://doi.org/10.1002/ejoc.200900077.

(165) Saka, S.; Kusdiana, D. Biodiesel Fuel from Rapeseed Oil as Prepared in Su-percritical Methanol. Fuel 2001, 80 (2), 225–231. https://doi.org/10.1016/S0016-2361(00)00083-1.

(166) Li, S.; Zong, Z. M.; Li, Z. K.; Wang, S. K.; Yang, Z.; Xu, M. L.; Shi, C.; Wei, X. Y.; Wang, Y. G. Sequential Thermal Dissolution and Alkanolyses of Extraction Residue from Xinghe Lignite. Fuel Process. Technol. 2017. https://doi.org/10.1016/j.fuproc.2017.07.025.

(167) Basha, A.; Lipton, M.; Weinreb, S. M. A Mild, General Method for Conver-sion of Esters to Amides. Tetrahedron Lett. 1977, 48, 4171 - 4174. https://doi.org/10.1016/S0040-4039(01)83457-2.

(168) Aggarwal, V. K.; Staubitz, A. C.; Owen, M. Optimization of the Mizoroki-Heck Reaction Using Design of Experiment (DoE). Org. Process Res. Dev. 2006, 10, 1, 64-69. https://doi.org/10.1021/op058013q.

(169) Lipshutz, B. H.; Ghorai, S.; Abela, A. R.; Moser, R.; Nishikata, T.; Duplais, C.; Krasovskiy, A.; Gaston, R. D.; Gadwood, R. C. TPGS-750-M: A Sec-ond-Generation Amphiphile for Metal-Catalyzed Cross-Couplings in Water at Room Temperature. J. Org. Chem 2011, 76, 4379–4391. https://doi.org/10.1021/jo101974u.

(170) Lagerlund, O.; Larhed, M. Microwave-Promoted Aminocarbonylations of Aryl Chlorides Using Mo(CO) 6 as a Solid Carbon Monoxide Source. J. Comb. Chem. 2006, 8, 1, 4-6. https://doi.org/10.1021/cc050102r.

(171) Khurana, J. M.; Arora, R. Rapid Chemoselective Deprotection of Benzyl Es-ters by Nickel Boride. Synthesis. 2009, 7, 1127–1130. https://doi.org/10.1055/s-0028-1087982.

(172) Bruekelman, S. P.; Leach, S. E.; Meakins, G. D.; Tirel, M. D. Protection of Primary Amines as N-Substituted 2,5-Dimethylpyrroles. J. Chem. Soc. Per-kin Trans. 1 1984, 2801–2807. https://doi.org/10.1039/p19840002801.

(173) Mitsunobu, O. The Use of Diethyl Azodicarboxylate and Tri-phenylphosphine in Synthesis and Transformation of Natural Products. Syn-thesis (Stuttg). 1981, 1981 (01), 1–28. https://doi.org/10.1055/s-1981-29317.

93

(174) Grigg, R.; Sridharan, V.; Stevenson, P.; Sukirthalingam, S. The Synthesis of Spirocyclic Compounds by Regiospecific Palladium Catalysed Cyclisation Reactions. Tetrahedron 1989, 45 (11), 3557–3568. https://doi.org/10.1016/S0040-4020(01)81034-6.

(175) Jagtap, S. Heck Reaction—State of the Art. Catalysts 2017, 7(9), 267 https://doi.org/10.3390/catal7090267.

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