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es

in

pecial

emphasis

is

given

to

cholesterol

oxidation

reactions,

but

also

substitution

of

the

3b-hydroxyl

g

–H

functionalization,

and

C–C

bond

forming

reactions

are

discussed.

Abbreviations: ACCN, 1,10-azobis(cyclohexane-1-carbonitrile; ADP, allyl diethyl phosphate; ampy, 2-aminomethylpyridine; 9-BBN, 9-borabicyclo[3.3.1]nonacaprolactamate; CDI, N,N0-carbonyldiimidazole; CDT, N,N0-carbonylditriazole; CTADC, cetyltrimethylammonium dichromate; DAST, diethylaminosulfur trifluoride; Ddicyanoanthracene; DDQ, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone; DEAD, diethyl azodicarboxylate; DIAD, diisopropyl azodicarboxylate; DMA, N,N-dimethylacetamid4-dimethylaminopyridine; DMF, dimethylformamide; DMSO, dimethyl sulfoxide; dppf, 1,10-bis(diphenylphosphino)ferrocene; ETDO, ethyl(trifluoromethyl)dioxirane; facfac-tris[2-phenylpyridinato-C2,N]iridium(III); Fe(acac)3, iron(III) acetylacetonate; LF, lumiflavin; L-Selectride, lithium tri-sec-butylborohydride; MCPBA, 3-chloroperoxacid; MMPP, magnesium bis(monoperoxyphthalate); NHPI, N-hydroxyphthalimide; pc, phthalocyaninato; PCC, pyridinium chlorochromate; piv (pivalate), trimethylacepyridine; Super-hydride, lithium triethylborohydride; TBA(F,Cl,Br,I), tetra-n-butylammonium halide; TBC, 4-tert-butylcatechol; TBHP, t-butyl hydroperoxide; TEA, triethTMS(F,Cl,Br,I), trimethylsilyl halide; tolane, diphenylacetylene; TsIm, N-(p-toluenesulfonyl)imidazole; TEMPO, (2,2,6,6-tetramethyl-piperidin-1-yl)oxyl; TFA, trifluoroacTFAA, trifluoroacetic anhydride; Tf2O, triflic (trifluoromethanesulfonic) anhydride.

Recent advances in cholesterol chemistry

roup,

a b s t r a c t

This

review

article

presents

advancetc.)

of

cholesterol

are

presented.

A

saddition to the C5–C6 double bond, C

cholesterol chemistry since 2000. Various transformations (chemical, enzymatic, electrochemical,

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 622. Substitution of a 3b-hydroxyl group. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 633. Oxidation to 3-ketone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674. Addition to the double bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 685. Reactions at allylic position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 696. Formation of new C–C bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 737. Polyhydroxylated cholestane derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 758. Side chain oxidation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 759. Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7610. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

1. Introduction

Cholesterol (cholest-5-en-3b-ol), from Ancient Greek chole-(bile) and stereos (solid) followed by the chemical suffix -ol for

alcohol, is an essential structural component of animal cell mem-branes that is required in order to establish proper membrane per-meability and fluidity. Cholesterol is thus considered to be a lipidmolecule. In addition to its importance within cells, cholesterol

ne; cap,CA, 9,10-e; DMAP,-Ir(ppy)3,ybenzoictate; py,ylamine;etic acid;

F

1DCM, 0 oC, 16 h, 76%HCF2CF2N(CH3)2

Scheme 2. Substitution by fluoride with retention of configuration.

Cl

CHCl3, DMSO (cat)

reflux, 1.5 h, 90%

NPh

Cl

Ph

Scheme 3. Substitution by chloride.

X

SOCl2 or SOBr2

C6F14, 3 days

X = Cl, yield 94%X = Br, yield 68%

Scheme 4. Substitution by chloride or bromide.

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also serves as a precursor for the biosynthesis of steroid hormones,bile acids, and vitamin D. Cholesterol is by far the most abundantmember of a family of polycyclic compounds known as sterols.Cholesterol is synthesized by animals; in vertebrates it is formedpredominantly in the liver. Cholesterol is almost completely absentin prokaryotes (e.g., bacteria) and is rare in plant sources, wheresterols of different side chains predominate.

The structure of cholesterol, its biosynthetic pathway andmetabolic regulation have fascinated both scientists and lay peoplealike for over 100 years. Thirteen Nobel Prizes have been awardedto scientists who devoted major parts of their careers to its study.

Cholesterol, the first steroid to ever be isolated, was discoveredby M.E. Chevreul in 1815. Cholesterol was isolated from the non-saponifiable portion of animal lipids. The chemical structure ofcholesterol was elucidated over the years, beginning in 1859. Thecompound was shown to contain a secondary hydroxyl groupand a double bond. The exact empirical formula (C27H46O) wasestablished in 1888 by F. Reinitzer. Proof for its structure wasobtained chiefly through the brilliant work of A. Windaus andH.O. Wieland. The structure of cholesterol as suggested byWindaus and Wieland in the 1920s was incorrect, but that doesnot detract in any way from their contribution. The true structurewas established in the 1930s based on X-ray diffraction data. Atotal synthesis of cholesterol was reported by Woodward et al. in1951 [1].

During the second half of the last century, chemical studies onsteroids, including cholesterol and other sterol transformations,were intensively conducted. Cholesterol is a readily available andrelatively cheap raw material that can be used in various chemicalsyntheses, such as steroid hormones, vitamin D derivatives, ecdys-teroids, brassinosteroids, etc. Cholesterol is frequently used as amodel system for testing new organic reactions. Many usefulchemical and enzymatic reactions have been elaborated that arenow widely used for multi-step steroid transformations leadingto products of practical importance. These are not only simplemanipulations of functional groups but also C–H activationreactions at allylic positions as well as remote functionalization,random or controlled, of chemically non-activated carbon atoms.Various methods of cleavage of C–C bonds (e.g. side chain cleav-age) and of forming new C–C bonds with organometallic reagentshave been elaborated.

At the close of the previous century special attention startedbeing paid to cholesterol oxidation products [2]. Since then thesecompounds constantly draw attention of biochemist and medicinalchemists (see e.g. March issue of Biochimie 2013). Cholesterol thatis present in foods of animal origin undergoes autoxidation duringprocessing as well as during storage, thus yielding toxic products.Cholesterol oxidation products are a family of cholesterol deriva-tives containing 27-carbon atoms. They are formed due to oxida-tion reactions caused by contact with oxygen, exposure tosunlight, heating treatments, etc. Furthermore, they can be gener-ated in the human organism through different oxidation processes,some of which require enzymes. These cholesterol metabolites arenow considered to be potentially involved in the initiation andprogression of major chronic diseases, including atherosclerosis,

1

Et2NSO

C8H17

HO

Scheme 1. Substitution by fluoride

neurodegenerative processes, diabetes, kidney failure, and ethanolintoxication. Cholesterol oxidation products have shown cytotoxic-ity as well as apoptotic and pro-inflammatory effects.

This review article presents advances in cholesterol chemistrysince 2000. It covers various cholesterol-based syntheses. How-ever, simple derivatization reactions of cholesterol (e.g. prepara-tion of carboxylic and inorganic acid esters, aliphatic andaromatic ethers, simple acetals or glycosides) have been excluded.These reactions are covered by other reviews [3,4].

2. Substitution of a 3b-hydroxyl group

It has been known that diethylaminosulfur trifluoride (DAST)generally reacts with alcohols via an SN2 or SNi mechanism [5].Also, the treatment of cholesterol (1) with DAST in furan(Scheme 1) provides only the fluorinated SN2 product (no attackto the electron-rich furan ring was observed) [6].

Another reagent that was recently introduced for the conver-sion of alcohols into alkyl fluorides was 1,1,2,2-tetrafluoroethyl-N,N-dimethylamine. With this reagent cholesterol was convertedto fluoride with retention of configuration (Scheme 2) [7].

N-phenylbenzimidoyl chloride has been demonstrated(Scheme 3) to be an efficient chlorinating reagent in catalyzed bydimethyl sulfoxide (DMSO) conversion of alcohols to correspond-ing chlorides (e.g. cholesterol was converted to cholesteryl chloridein a 90% yield) [8].

An interesting protocol for bromination and chlorination ofalcohols (e.g. cholesterol) in which the perfluorohexane layerregulates the rate of reagent transport was reported (Scheme 4)

F

F3, r.t. 5 min

, 56%

with inversion of configuration.

C8H17

MsO X

a) FeCl3, DCM, 5 min, 87% (X = Cl)

b) Fe(acac)3, TMSCl, DCM, 16 h, 75% (X = Cl)c) Fe(acac)3, TMSBr, DCM, 16 h, 56% (X = Br)d) Fe(acac)3, TMSI, DCM, 10 min, 63% (X = I)

C8H17

MsO H Cl H Cl H H H

+ + +

Cl

a) FeCl3, DCM, 5 min

b) Fe(acac)3, TMSCl, DCM, 12 h

a) 79% 5% 16% 0%

b) 5% 74% 4% 17%

Scheme 5. Synthesis of halides from cholesteryl mesylate.

BrMeCN, reflux, 16 h, 72%

BrCH(COOEt)2, Ph2SiH2

PPh (10 mol%)

Scheme 6. Appel bromination.

HO

1. p-NO2PhCOOH, DEAD, PPh3

2. NaOH, H2O, 0 oC, 24%

Scheme 7. Mitsunobu esterification followed by hydrolysis.

C8H17

NC

1NaCN, Tsim, TBAI, TEA

DMF, reflux, 6h, 40%

Scheme 8. Substitution by cyanide.

N3

1

DIAD, PPh3(PhO)2PON3THF, 67%

LiAlH4Et2O, 93%

HO H I H

CH3IDIAD, PPh3

THF, 86%

Scheme 9. Synthesis of

H2N

NaN3, PPh3

CCl4-DMF, 7 h, 85%

Scheme 10. Synthesis of cholesterylamine.

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[9]. The fluorous triphasic U-tube method is effective for lighterreagents; the thionyl chloride layer (yellow) vanishes and thechlorides are obtained from the right top organic layer in thechlorination of alcohols.

A novel halogenation reaction from sulfonates catalyzed byiron(III) was described [10]. The reaction can be performed as astoichiometric or catalytic version. This reaction provides a conve-nient strategy for efficient access to structurally diverse secondarychlorides, bromides and iodides. The stereochemical course of thereaction is governed by the substrate and the experimental condi-tions. Thus cholesterol mesylate was converted into correspondinghalides in good yields with overall retention of the configuration(Scheme 5). On the other hand, when the stoichiometric versionwas applied to 5a-cholestan-3b-ol mesylate, chloride with reten-tion of configuration was obtained as the major product along withsome of the 2a-chloride isomer and a small amount of 3-chloridewith inversion of configuration. The latter compound was themajor product of the catalytic reaction.

H2N

H2N H

1. NaN3, DMSO,90 oC, 91%

2. LiAlH4, Et2O,reflux, 61%

3-amino steroids.

C8H17

NH

OHN

OO

OOHOH

OH

P OHO

HO

Fig. 1. A mannose-6-phosphonate–cholesterylamine conjugate.

NN

Δ, 3 h, 95%

CDI, CH3CN

Scheme 13. Reaction of cholesterol with N,N’-carbonyldiimidazole.

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A catalytic system for Appel bromination was also worked out(Scheme 6) [11]. First, the ease of silane-mediated reduction of arange of cyclic phosphine oxides was explored. In addition, thecompatibility of silanes with electrophilic halogen donors wasdetermined for application in a catalytic Appel reaction. Alcoholswere effectively converted to bromides under optimized condi-tions using dibenzophosphole and diethyl bromomalonate. Choles-terol was converted to cholesteryl bromide with retention ofconfiguration due to the participation of the neighboring doublebond.

A two-step procedure for inversion of configuration at C-3 incholesterol via 3a-p-nitrobenzoate, though not very efficient, wasdescribed (Scheme 7) [12].

A convenient and efficient one-pot preparation of nitriles fromalcohols using N-(p-toluenesulfonyl)imidazole (TsIm) has beendescribed [13]. In this method the treatment of alcohols with amixture of NaCN, TsIm and triethylamine (TEA) in the presenceof catalytic amounts of tetra-n-butylammonium iodide (TBAI) inrefluxing DMF furnishes the corresponding alkyl nitriles inmoderate to good yields. The reaction of cholesterol afforded thecorresponding 3b-nitrile in a 40% yield (Scheme 8).

Cholesterol was transformed into a 3a-amino derivative usingthe two-step Mitsunobu procedure involving substitution withazide followed by LiAlH4 reduction (Scheme 9). The 3b-aminoanalog was obtained by double inversion of configuration via3a-iodide and 3b-azide [14].

A convenient and efficient one-pot sequence has been devel-oped for the transformation of alcohols to amines using sodiumazide, triphenylphosphine in CCl4-DMF (Scheme 10) [15].

A mannose-6-phosphonate–cholesterylamine conjugate wasprepared as a specific molecular adhesive linking cancer cells with

NH

1Ph

OPhCN, TiF4

DCM, 0 oC, 2 h, 75%

Scheme 11. Ritter reaction.

C8H17

N3

R

Cu(OAc)2, sodium asTHF - H2O, r.t., 12 h,

Scheme 12. Copper-catalyzed a

vesicles (Fig. 1) [16]. This hydrolytically stable sugar phosphonatecoupled to a steroid via a long and semi-rigid spacer binds both tothe mannose-6-phosphonate receptors of certain cancer cells andto the lipid bilayer of vesicles.

An improved method of synthesis of cholesteryl and 5a-choles-tanyl azides was described [17]. The method involves oxyphospho-nium-type activation and is based on the use of nicotynoyl azide asa convenient azide ion source.

An interesting variation of the Ritter reaction by using an inex-pensive Ti(IV)/nitrile reagent to prepare amides directly fromcyclic secondary alcohols was reported (Scheme 11) [18]. The useof chlorosulfites as in situ formed chelating leaving groups formedby the well-known reaction of alcohols and thionyl chloride, is crit-ical to the design of this new reaction. Amides are obtained fromcyclic alcohols with stereoretention (e.g. cholesterol was convertedto N-3b-cholesterylbenzamide in a 75% yield).

Copper-catalyzed azide-alkyne cycloaddition has been used forconstruction of steroids containing the 1,2,3-triazole ring in goodto excellent yields (Scheme 12). A combination of propargylicglycosides and steroidal azides as reaction partners allowed forthe synthesis of a large number of natural product analogs [19].

A variety of alcohols, including cholesterol, were converted intotheir corresponding imidazoles and triazoles in high yields by afacile reaction with N,N0-carbonyldiimidazole (CDI) or N,N0-carbon-ylditriazole (CDT) in acetonitrile (Scheme 13) [20].

The Mitsunobu coupling of 6-chloropurine with cholesterol toobtain nucleosteroids was investigated. In addition to the SN2 ma-jor product, two isomeric products were formed via the mesomerichomoallylic carbocation (Scheme 14) [21].

Thioether derivatives of cholesterol and other sterols wereprepared and subjected to anodic oxidation in the presence of a

NN

N

R

H

corbate70-100%

R = alkyl, aryl, acyl, glycosyl

zide-alkyne cycloaddition.

R

N

N N

N

Cl

H

+ DEAD, PPh3dioxane, r.t., 48 h

R

+ +

RN

N N

N

Cl

R =

22.8% 10.6% 12.6%

Scheme 14. Mitsunobu coupling of 6-chloropurine with cholesterol.

1. p-TsCl, Py

S

anodic oxidation, 52%

sugar-OH

SugarO

Scheme 15. Electrochemical glycosylation of i-cholesteryl thioether.

O

Bu3MH, ACCNtoluene, reflux, 3 hthiocarbonyldiimidazole

DMAP, CH3CNreflux, 150 min, 84%

N

N

S

RM = Ge, R = H (67%)M = Sn, R = H (5%)+ R = OH (51%) + R = OMe (11%)

Scheme 16. Radical deoxygenation of cholesterol thiocarbonylimidazolide with Bu3GeH.

C8H17

1I2, PPh3, imidazole, MeCN, 2 h

fac-Ir(ppy)3, i-Pr2NEt, MeOH, flow LED

Scheme 17. A one-pot dehydrogenation of cholesterol.

Pd(PPh3)4 + HO-(CH2CH2O)4-H + (sec-BuO)3Al/BuOH120 oC, 10 hin the air 120 oC, 30 min

H2O cat

HO O99%

Scheme 18. Catalytic hydrogenation/air oxidation.

OMeCN, 43%

1.9 V vs. Ag/AgCl

Scheme 19. Electrochemical oxidation of cholesterol to cholesta-4,6-dien-3-one.

ONa2CO3, DMF, 91%

Pd/C, ADP

Scheme 20. Pd/C-mediated dehydrogenation of cholesterol.

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sugar alcohol affording glyconjugates [22]. Isomeric 6b-3a,5a-cyclo-steroidal thioethers proved to be better sterol donorsthan the normal 3b-D5-steroidal thioethers (Scheme 15).

Electrochemical glycosylation with non-activated sterols, e.g.cholesterol, afforded products in lower yields. However, good cho-lesteryl donors (e.g. cholesteryl diphenylphosphate or trichloro-acetimidate) provided glycoconjugates in satisfactory yields [23].

A straightforward and practical method has been elaborated fordiscriminating the absolute stereogenic center at the C-3 positionsof sterols based on an induced CD. Various types of unsaturatedsterols were converted into their 2,20-binaphthyl esters, followed

by complete hydrogenation, to give saturated sterols. The CD spec-tra of the resulting derivatives showed bisignate curves centered at240 nm diagnostic for the configuration at C-3 [24].

Tributylgermanium hydride can be used as an alternative totributyltin hydride as a radical generating reagent. It was foundto be a more reliable reducing agent than tributyltin hydride inBarton–McCombie reactions. Thus radical deoxygenation of thecholesterol thiocarbonylimidazolide with Bu3GeH gave a 67% yieldof cholestane, while with Bu3SnH a mixture of products wasobtained. In addition to the cholestane, cholesterol and cholesterolmethyl ether were also formed (Scheme 16) [25].

A one-pot deoxygenation protocol for primary and secondaryalcohols was developed via a combination of the Garegg–Samuelsson reaction, visible light-photoredox catalysis, andflow chemistry. This procedure is characterized by mild reaction

HO HO O HOH

NaBH4: 93%

L-Selectride: 26% (74% when used in excess) 22% 6%

LiAlH4: 11% (46% when extended reaction time) 21% 21%

9-BBN: 79%

Super-hydride: 52%

BH3·(CH3)2S): 44%

Scheme 21. Reduction of 1,4,6-cholestatrien-3-one with different hydrides.

Otolane, p-xylene,150 oC, 12 h, 94%

Ru3(CO)12, PPh3

Scheme 22. Dehydrogenation of cholesterol with triruthenium dodecacarbonyltriphenylphosphine.

RuP

PFe

Ph2

Ph2

Cl

N

N

HH

Cl

OsP

PFe

Ph2

Ph2

Cl

Cl

N

N

H H

Scheme 23. Ruthenium and osmium catalysts for dehydrogenation.

ODCM, r.t., 2.5 h, 72%

PhIO, TEMPO, Yb(OTf)3

Scheme 24. Catalytic method of oxidation to cholest-5-en-3-one.

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conditions, easy-to-handle reactants and reagents, excellent func-tional group tolerance, and good yields. With this procedure, choles-terol was converted to 5-cholestene in a 72% yield (Scheme 17) [26].

3. Oxidation to 3-ketone

An aluminum hydroxide-supported palladium catalyst made bya one-pot synthesis through nanoparticle generation and gelationshows dual catalytic activity for olefinic hydrogenation and aerobicalcohol oxidation (Scheme 18) [27]. Cholestan-3-one was obtainedfrom cholesterol in a 99% yield but no information was given aboutthe configuration at C-5 in the product.

Simple, fast, and environmentally friendly electrochemical oxi-dation of cholesterol to cholesta-4,6-dien-3-one was described(Scheme 19) [28].

Anodic oxidation of cholesterol in the presence of sugars affor-ded mostly glycosides and glycoconjugates in addition to minoramounts of cholesta-4,6-dien-3-one and cholest-4-en-3-one [29].

Or.t., 28 h, 84%

PCC, DCM

O

HO

NaBH4, NiCl2MeOH, 20 min, 81%

Scheme 25. Synthesis 3,6-di

Novel, recyclable Pd/C catalyst-mediated dehydrogenation ofsterols has been developed [30]. The conversion of sterols to1,4,6-trien-3-ones is best achieved with Pd/C as a catalyst (10%)in the presence of six equivalents of allyl diethyl phosphate(ADP) and an excess amount of sodium carbonate in DMF undervigorous reflux conditions (Scheme 20).

The same 1,4,6-trien-3-one has been obtained by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) oxidation of cholesterol.Extensive studies on reduction of this compound under variousconditions (NaBH4, lithium tri-sec-butylborohydride (L-Selectride),LiAlH4, 9-borabicyclo[3.3.1]nonane (9-BBN), lithium triethylboro-hydride (Super-hydride), and BH3�(CH3)2S) were performed [31].Depending on the reaction conditions, each of compounds shownin Scheme 21 can be the major reaction product.

Dehydrogenation of alcohols via triruthenium dodecacarbonyl-triphenylphosphine-based homogeneous catalysis has been inves-tigated as an alternative to the classical method of Oppenaueroxidation (Scheme 22) [32]. A systematic study of variouscombinations of ligands and hydride acceptors was conducted.Cholesterol was dehydrogenated to cholest-4-en-3-one with tri-phenylphosphine as a metal ligand and diphenylacetylene (tolane)as the H-acceptor in p-xylene at 150 �C in a 94% yield.

Acceptorless dehydrogenation of cholesterol with easily acces-sible complexes [MCl2(dppf)-(ampy)] (M = Ru (cis), Os (trans);dppf = 1,10-bis(diphenylphosphino)ferrocene; ampy = 2-amino-methylpyridine) in the presence of a base (NaOiPr, KOtBu) was car-ried out [33]. A reaction using the ruthenium catalyst leads tocholest-4-en-3-one quantitatively in 3 h. A faster reaction has beenobserved with the osmium catalyst, for which a formation of thesteroid product was achieved in 1 h (Scheme 23).

Rapid oxidation of alcohols using catalytic amounts of TEMPOand Yb(OTf)3 in combination with a stoichiometric amount of PhIOwas described (Scheme 24) [34]. The oxidation of 5a-cholestanolafforded 5a-cholestanone in 94%, while an analogous reaction ofcholesterol gave cholest-5-en-3-one (no shift of a double bond tookplace).

Oxidation of cholesterol with pyridinium chlorochromate (PCC)afforded cholest-4-en-3,6-dione (Scheme 25) [35,36]. Then thediketone was reduced to cholesta-3b,6b-diol, which was furtherconverted to the corresponding disulfate. The latter showed mod-erate cytotoxic activity.

In search for new insecticidal agents a series of cholest-4-en-3,6-dione hydrazones and other cholesterol based hydrozone

OH

H NaO3SO

OSO3Na

H

sulfate from cholesterol.

O

O

HO

O

H

NaBH4, CoCl2MeOH, 20 min, 88%

O H

NOH

a) NH2OH.HCl, AcONa,EtOH, 55 oC, 1.5 h, 70%b) Jones, acetone,r.t. 1 h, 61%

MCPBA,DCM, r.t. 48 h

HO

O

H OOHO H

O

+

20%60%

Scheme 26. Synthesis of cytotoxic compounds from cholest-4-en-3,6-dione.

C8H17

BzO

C8H17

BzO H

5α/5β 9:1

OHBO

(2 equiv)

DMA, ClCH2CH2Cl

1.

2. TBC (2 equiv), air, 93%

Scheme 27. Reduction with catecholborane.

HOMeCN, reflux, 10 min83% (α : β 78:22)

MMPP

OHO

O

+

Scheme 28. Epoxidation with magnesium bis(monoperoxyphtalate).

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derivatives were prepared and tested against the pre-third-instarlarvae of oriental armyworm [37].

Regioselective reduction of cholest-4-en-3,6-dione is also possi-ble. A reaction with NaBH4/CoCl2 afforded 3b-hydroxy-6-ketone,which was further transformed into 6-oxime (Scheme 26) [38].In the next step it was oxidized with Jones reagent to (6E)-hydroxi-minocholest-4-en-3-one. This product occurs in nature and showscytotoxic activity against certain cancer cell lines. The Baeyer–Villiger reaction of 3b-hydroxy-6-ketone with MCPBA was notregioselective and afforded a mixture of isomeric lactones in a ratioof 3:1 [39]. These compounds showed moderate cytotoxic activity.

An alternative approach to the synthesis of cholest-4-en-3,6-dione and other steroidal 4-en-3,6-diones is modified (high ace-tone volume, low temperature) Jones oxidation [40]. The reactionaffords high yields (77–89%) of the product in relatively shortreaction times (1–2 h).

4. Addition to the double bond

A one-pot sequence for hydrogenation of alkenes involving hyd-roboration and reduction has been developed [41]. A mild radicalprocedure for the transformation of organoboranes to alkanesemploys 4-tert-butylcatechol (TBC), a well-established radicalinhibitor and antioxidant, as a source of hydrogen atoms. Anefficient chain reaction occurs due to the exceptional reactivity ofphenoxyl radicals toward alkylboranes. A reaction of cholesterylbenzoate was attempted first. Hydroboration at room temperatureusing two equivalents of catecholborane and N,N-dimethylacet-amide (DMA) as a catalyst afforded an intermediate organoboranethat was directly treated with TBC in the presence of air(Scheme 27). This reaction afforded the reduced product in a 93%isolated yield as a 5a/5b (9:1) mixture of diastereomers. Non-protected cholesterol could also be reduced under similar condi-tions. In this case, however, the hydroboration was performed with3 equivalents of catecholborane under neat conditions at 100 �C.The reaction afforded 5a-cholestan-3b-ol in a 72% yield as a singlediastereomer.

A new type of immobilized palladium catalyst, polymer incar-cerated (PI Pd), has been developed [42]. It was synthesized fromPd(PPh3)4 and copolymer, prepared by radical copolymerizationof styrene, epoxide monomer, and tetraethyleneglycol monomer.Cholesterol hydrogenation with PI Pd (5 mol%) was carried outusing H2 (1 atm) in DCM solution at room temperature for 24 h.Cholestanol was obtained in a 85% yield, but no information wasgiven as to its stereochemical purity.

Rapid synthesis of epoxides from corresponding homoallylicand allylic steroidal olefins was developed by using magnesiumbis(monoperoxyphthalate) hexahydrate (MMPP) as an oxidant sus-pended in acetonitrile at reflux (Scheme 28). Cholesterol afforded a78:22 mixture of epoxides in a 83% yield. A better yield wasachieved with cholesteryl acetate. The reaction of B-norcholesterolwas almost completely a-stereoselective.

In contrast to cholesterol and its esters, steroidal enones anddienones react with 30% H2O2 in an alkaline medium. Furtherreduction of epoxides with NaBH4 was studied [43].

The mixture of epimeric cholesterol epoxides obtained byMCPBA oxidation of cholesteryl 3-ether was further oxidized withCrO3 in 2-butanone to 5-hydroxy-6-ketones (Scheme 29) [44]. Theepimeric mixture was subjected to dehydration to a single 4-en-6-one with thionyl chloride.

Interestingly, the biohydrolysis of isomeric cholesterol epoxideswith various strains of Aspergillus niger is a diastereoconvergentprocess [45]. This means that from both a and b epoxides the same3b,5a,6b-triol is formed, i.e. each isomer of an a and b epoxidemixture shows opposite regioselectivity toward hydrolysis in thesebiotransformations.

RO RO

CrO3, H2O

65%

OO

OHRO

O

2-butanoneMCPBA1 SOCl2, py

90%

R = propargyl

dimersvia crossmetathesis

Scheme 29. Epoxidation of cholesterol with MCPBA followed by further transformations.

C8H17

OH2n+1Cn OHO

O

Fig. 2. Cholesterol derived substrate for radical polymerization.

anodic oxidation

Cl

+ +

+

ClCl

ClCl

OHCl

HO HO

HO

14% 9%

6% 4%

Scheme 30. Electrochemical oxidation of cholesterol in dichloromethane.

HO

1

O

CTADC

DCM, HOAcCTADCDCM

Scheme 31. Oxidation of cholesterol with cetyltrimethylammonium dichromate.

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The epoxy ring opening in a series of 3-alkoxy-5,6-epoxy-cholestanes with acrylic acid resulted in 5a-hydroxy-6b-acrylates(Fig. 2), which were subjected to free radical polymerization. Theobtained polymers were studied with the use of different methods[46,47].

A rapid (reaction time 5–10 min) method was reported for di-rect dihydroxylation of various alkenes using immobilized lipasefrom Pseudomonas, 50% hydrogen peroxide and ethyl acetate in amicrowave in a single step [48]. The reactions proceeded throughformation of an epoxide, but in most cases no intermediate epox-ide could be isolated from the olefins. However, the sterol epoxidesproved stable and did not undergo ring opening to the diol underreaction conditions. Cholesterol a-epoxide was obtained as a singlestereoisomer in a 85% yield.

The products of the double bond chlorination were obtained byelectrolysis of cholesterol in DCM (Scheme 30) [49]. It was proventhat the solvent was a source of chlorine. The cathodic reduction ofDCM to chloride ions, their diffusion to the anodic compartmentand electrooxidation in the presence of cholesterol caused theformation of chlorocholestanes.

Similarly, when electrolysis of cholesterol was carried out in asolution of tetraethylammonium bromide in aprotic solvents(dichloromethane, acetonitrile or acetic anhydride), the additionof electrochemically-generated elemental bromine onto the doublebond of the cholesterol derivatives gave their corresponding 5a,6b-dibromocholestan-3b-ol in about a 60% yield [50]. However,isomeric bromo-methoxy derivatives were obtained during elec-trolysis of cholesterol in methanol with tetraethylammonium bro-mide as a supporting electrolyte in addition to 5a,6b-dibromide.

Palladium nanoparticles were generated from tetrakis(triphen-ylphosphine)palladium in a mixture of tetra(ethylene glycol) andtetramethoxysilane (or titanium(IV) isopropoxide), then encapsu-lated in the silica matrix (or the titania matrix) by treatment withwater [51]. The resulting heterogeneous material showed highcatalytic activity in the hydrogenations of various alkenes.However, the hydrogenation of cholesterol was distinctly slow;the conversion was only 16% even after 24 h.

Microwave-assisted hydrogenation reactions were studied atmoderate temperature and pressure [52]. Cholesterol washydrogenated using Pd/C, H2 (50 psi) in ethyl acetate at 80 �C.The microwave-assisted reaction with simultaneous coolingafforded an over 99% yield of the hydrogenation product in

5 min. However, when the same reaction was performed in apreheated oil bath at 80 �C, only a 3% yield was obtained.

5. Reactions at allylic position

The oxidation of cholesterol by cetyltrimethylammoniumdichromate (CTADC) in dichloromethane (DCM) yielded 7-dehy-drocholesterol, while with the addition of acetic acid in DCM theproduct was found to be 5-cholesten-3-one (Scheme 31) [53]. Noyields of products are given. The kinetics of oxidation of cholesterolsuggests that the reaction occurs in the reversed micellar system,akin to an enzymatic environment, and that the reaction pathinvolves the intermediate formation of an ester complex, whichundergoes decomposition to give the product.

7-Dehydrocholesterol, obtained by using the classical methodof bromination and dehydrobromination, was further transformedto the 5a,8a-peroxy derivative (an eosin-photosensitized reactionwith air) followed by Jones oxidation to the 3-ketone. The productcan be used for the prevention and treatment of tumors [54].

Efficient and completely stereoselective copper-catalyzedallylic benzoyloxylation of sterol derivatives has been developed(Scheme 32) [55]. A mechanistic rationale justifying the total stere-oselectivity that is encountered has been proposed. 7a-Hydroxy-cholesterol was obtained in a two-step synthesis in a 61% overallyield.

Cholesterol undergoes facile photooxygenation sensitized by9,10-dicyanoanthracene (DCA) and lumiflavin (LF) to give a mix-ture of isomeric 5a- and 7a/7b-hydroperoxides (Scheme 33) [56].The reaction was proposed to proceed via the ene reaction ofsinglet oxygen and the subsequent rearrangement of the initiallyformed 5a-hydroperoxide.

Direct electrochemical oxidation of cholesterol on a platinumelectrode in glacial acetic acid containing sodium perchlorate and

C8H17

AcO AcODCM, 40 oC, 12 h, 76%

CuBr, t-BuOOCOPh

OCOPh HO OH

LiAlH481%

Scheme 32. Copper-catalyzed allylic benzoyloxylation.

HO

DCA or LF, hν/O2MeCN, r.t. 4-6 h

HOOOH

HO OOH

+

PPh3

HOOH

HO OH

+

PPh3

Scheme 33. Photooxygenation of cholesterol.

C8H17

Z ZDCE, 40 oC, 20 hZ = OH, 63%Z = OTBDMS, 81%Z = OAc, 80%

O

TBHP, Rh2(cap)4

Scheme 35. Allylic oxidation with t-butyl hydroperoxide/dirhodiumcaprolactamate.

NN

CuO

(II)

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sodium acetate as the supporting electrolyte afforded two majorproducts: 7a-acetoxycholesterol and 7b-acetoxycholesterol in aratio of 10:3, in addition to several minor products (Scheme 34)[57].

Cholesterol was oxidized with t-butyl hydroperoxide (TBHP)/dirhodium caprolactamate to the 7-oxo derivative in a 63% yield(Scheme 35) [58,59]. Oxidation of oxygen-protected steroids (ace-tate, TBDMS ether) gives higher yields. This oxidizing system canalso be used for allylic oxidations of enones to enediones in mod-erate to high yields, e.g. cholest-4-en-3-one was oxidized to cho-lest-4-ene-3,6-dione in a 50% yield [60].

Another catalyst that was conceived for allylic oxidation of D5-steroids using TBHP as an oxidant is a 2-quinoxalinol salen Cu(II)complex (Fig. 3) [61]. A variety of D5-steroidal substrates wereselectively oxidized to the corresponding enones. The yields, underoptimized conditions, are generally high (up to 99%), while reac-tion times have significantly been reduced as compared to othermethods.

Also, sodium hypochlorite was used as a cooxidant in allylic oxi-dations with TBHP. Cholesteryl acetate oxidized with TBHP/NaOClin DCM at 0–5 �C for 10 h afforded the 7-oxo derivative in a 68%yield [62].

An effective method was reported for the oxidation of choleste-ryl acetate to 7-oxo-cholesteryl acetate with molecular oxygen inthe presence of catalytic amounts of N-hydroxyphthalimide (NHPI)under mild conditions (Scheme 36) [63]. It was found thatCo(OAc)2 could cooperate with Mn(OAc)2 to enhance the catalyticability of NHPI, thus resulting in better yields (86% underoptimized conditions).

HO

1

OAc HO OAc

anodic oxidationacetic acid, r.t. +

product ratio 10:3

Scheme 34. Electrochemical allylic acetoxylation of cholesterol.

7-Hydroperoxycholesterol is considered to be an intermediatecompound of the cholesterol oxidation path as the first productthat is formed when cholesterol is oxidized by triplet oxygen[64]. To verify the formation of hydroperoxides in cholesterolheated model systems, 7a-hydroperoxycholesterol was synthe-sized by cholesterol photooxidation, followed by rearrangement

NNO

OH

Fig. 3. 2-Quinoxalinol salen Cu(II) complex.

C8H17

AcO AcOacetone, 25 oC, 8 h, 86% O

O2 (1 Atm), NHPI (0.1 eq),Co(OAc)2 (0.005 eq),Mn(OAc)2 (0.005 eq)

Scheme 36. Allylic aerobic oxidation in the presence of N-hydroxyphtalimide.

HO

OH

HO

OH

OH

C8H17

+ +SeO2

3-6%10-75% 5-27%

Scheme 37. Selenium dioxide oxidation of cholesterol.

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at room temperature in chloroform. Attempts to crystallize thiscompound failed due to its decomposition to 7-oxocholesteroland 7a-hydroxycholesterol, but not to 7b-hydroxycholesterol, aswas reported earlier.

Selenium dioxide oxidation of cholesterol reveals solvent-dependent product selectivity and facile one-pot synthesis of threederivatives: 4b-hydroxy-, 4b,7a-dihydroxy-, and A,B-diaromaticsteroids. When cholesteryl acetate was oxidized with SeO2, in addi-tion to these products, the 6a-hydroxy-D4-steroid was obtained(Scheme 37) [65].

Since the above approach did not appear to be very reliable andsuitable for labeling purposes, a different synthesis of 4b-hydroxy-cholesterol was worked out [66]. The method consists in bromina-tion of cholesterol which is followed by treatment with silver

HO

OH

1. Br2, AgOAc, Py, CHCl32. KOH, MeOH, 15% (2 steps)

Scheme 38. Synthesis of 4b-hydroxycholesterol.

HO

OH

AcO

SeO2

AcOOH

OH

55%

Ac2O, P

98%

1. acetone, Al(O-i-Pr)32. NaBH4, CeCl33. Ac2O, Py, 54% (1-3)

RuO472%

1. Ac2O,2. SOCl290% (1

Scheme 39. Synthesis of 4-hyd

acetate to provide the acetate protected intermediate. The latterwas then hydrolyzed to afford, after purification, 4b-hydroxycho-lesterol in a 15% yield (Scheme 38).

Recent studies suggest that 7-dehydrocholesterol derived oxys-terols, mainly 4a- and 4b-hydroxylated derivatives, play importantroles in the pathophysiology of Smith–Lemli–Opitz syndrome, ametabolic disorder that is caused by defective 3b-hydroxysterol-D7-reductase. Efficient methods for the preparation of stereoiso-meric 4a- and 4b-hydroxy-7-dehydrocholesterol have beendeveloped (Scheme 39) [67,68]. The principal reactions involvedwere: (upper pathway) direct 4b-hydroxylation of cholesterol withselenium dioxide (for the 4b-OH derivative) and (lower pathway)cis-4a,5a-dihydroxylation of the allylic 3b-acetoxy-D4 intermedi-ate with in situ generated RuO4 and subsequent dehydration withSOCl2 (for the 4a-OH derivative). Both 4a- and 4b-hydroxycholes-terol 3b-acetates were subjected to bromination with 1,3-dibro-mo-5,5-dimethylhydantoin/azobisisobutyronitrile, followed bydehydrobromination with tetrabutyl ammonium bromide/tetrabu-tyl ammonium fluoride.

A synthesis of several squalamine-related polyaminosterolsfrom cholesterol have been reported [69]. In these compoundsspermidine is introduced in the B ring. In the key-step the azidogroup was introduced directly on the C-7 allylic position of cho-lesteryl acetate using trimethylsilyl azide in the presence of Pb(IV)

AcO

OAc

AcO

OAc

HO

OH

y

Py, Py-2)

1. 1,3-dibromo-5,5-dimethyl-hydantoin/AIBN

2. TBABr, TBAF3. NaOH, MeOH

α-OH, 68% (1-3)β-OH, 58% (1-3)

roxy-7-dehydrocholesterol.

C8H17

AcO AcODCM, r.t., 2 h, 68%

Pb(OAc)4, (CH3)3SiN3

N3 HO NH

7α/7β 77:23

NH

H2N

Scheme 40. Synthesis of squalamine-related polyaminosterols.

TFAOO

1

TFAO

Al(i-PrO)3/benzoquinone

benzene, reflux, 21 h, 81%TFAA

O HO

TFA100%

(2 steps)

H2O, H+ NaBH482%

3α/3β 2:9

Scheme 41. Preparation of steroidal 4,7-dien-3-ones and 7-dehydrosterols.

O

1

SO

O

NH2

1. ClSO2NCO, HCOOH, MeCN2. NaH, DMF,CHCl3, 65% [FePc]Cl, AgSbF6

O

S NH

O

O

PhI(OPiv)2, toluene, MeCN, 58%

Scheme 42. Iron-catalyzed intramolecular allylic C-H amination.

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acetate (Scheme 40). The epimeric mixture of azides (the ratio7a/7b = 77:23) was obtained in a 68% yield. Each of the epimerswas separately transformed into squalamine analogs which werethen subjected to biological activity evaluation.

A convenient method has been elaborated for the preparation ofsteroidal 4,7-dien-3-ones and 7-dehydrosterols (Scheme 41) [70].Wettstein-Oppenauer oxidation of cholesterol was followed bytrifluoroacetylation to afford the D2,4,6-enol ester. In the presenceof the trifluoroacetic acid generated during the esterification, theinitially formed enol ester was converted to the D3,5,7-isomer.The latter was acid hydrolyzed to cholesta-4,7-dien-3-one orreduced with NaBH4 to 7-dehydrocholesterol.

Non-enzymatic oxidation of cholesterol occurs mostly at theallylic position affording 7-hydroperoxycholesterol, which isfurther transformed to 7-oxo- or 7-hydroxycholesterol. The cyto-toxicity of oxysterols was systematically studied in tumor and nor-mal cells. A number of ring-B oxygenated steroids have beensynthesized, including 7-oxocholesterol, 7a-hydroxy-cholesterol,7b-hydroxycholesterol, 5a,6a-epoxycholesterol, 5b,6b-epoxycho-lesterol, cholestan-3b,5a,6b-triol, and 3b,5a-dihydroxycholestan-6-one. The structural requirements needed to induce selectivetoxicity were also discussed [71].

Generation of cholesterol hydroperoxides and their decomposi-tion products induces various types of cell damage. The decompo-sition of some organic hydroperoxides into peroxyl radicals isknown to be a potential source of singlet molecular oxygen in

biological systems. Generation of singlet O2 from cholesterolhydroperoxide isomers was evidenced by near-IR, 18O-labeledhydroperoxides and mass spectrometry [72].

Heat-induced cholesterol oxidation at 150 �C was kineticallystudied. The kinetic model developed in this study can be usedto predict the inhibition of cholesterol oxidation products (5,6-epoxides, 7-hydroxy and 7-hydroperoxy compounds) by quercetinor stearylamine during the heating of cholesterol [73,74].

A library of diastereomerically pure epoxysterols, prepared bycombining chemical and enzymatic methodologies, was evaluatedfor cytotoxicity toward human cancer and non-cancer celllines [75]. Unsaturated steroids were oxidized by magnesiumbis(monoperoxyphthalate) hexahydrate in acetonitrile, and theresulting epimeric epoxides were enzymatically separated usingNovozym 435 or lipase AY. Some of the synthesized epoxysterolsshow potent cytotoxicity and higher activity on selected cancer celllines.

p-Extended palladium porphyrins have been synthesized withextremely high absorbances in the 700–800 nm range [76]. Thepalladium complex was prepared and proved to be a good catalystfor the oxidation of cholesterol with singlet oxygen, which makesthis chromophore a potential candidate for photodynamic therapy.

Iron-catalyzed intramolecular allylic C�H amination hasrecently been reported (Scheme 42) [77]. This reaction employs[Fe(III)pc]Cl (pc = phthalocyaninato), which is an inexpensive com-mercial compound that is typically used as an industrial additive

C8H17

HO

HO

OMe

O

O

O

OMe

[(C6H6)(PCy3)(CO)RuH]+BF4-

[(C6H6)(PCy3)(CO)RuH]+BF4-

Scheme 43. Catalytic alkylation of alkenes with alcohols.

HO

[(C6H6)(PCy3)(CO)RuH]+BF4-

O

1 +cyclopentene (0.15 equiv),toluene/DMSO 9:1, 100 oC, 10 h, 67%

C8H17

OH

O

HN

OH

1 +HN

OH

C8H17

[(C6H6)(PCy3)(CO)RuH]+BF4-

cyclopentene (1.5 equiv),toluene/C6H5Cl 1:1, 100 oC, 8 h, 72%

Scheme 44. Ruthenium-catalyzed dehydrative C-H alkylation of phenols with cholesterol.

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for ink and rubber manufacturing. The system strongly favorsallylic C�H amination over aziridination and amination of all otherC�H bond types. Cholesteryl sulfamate subjected to a reactionwith catalytic amounts of [FePc]Cl and AgSbF6 and two equivalentsof PhI(OPiv)2 afforded a single diastereomer of the allylic amina-tion product in a 58% yield.

6. Formation of new C–C bonds

Selective catalytic alkylation of alkenes with alcohols that formsa carbon–carbon bond between vinyl carbon–hydrogen (C–H) andcarbon-hydroxy centers with the concomitant loss of water wasreported (Scheme 43) [78]. The cationic ruthenium complex½ðC6H6ÞðPCy3ÞðCOÞRuH�þBF�4 catalyzes the alkylation in a solutionwithin 2–8 h at temperatures ranging from 75 to 110 �C and toler-ates a broad range of substrate functionality, including amines andcarbonyls. The alkylation of both cholesterol and progesterone

with p-methoxybenzyl alcohol under standard conditions afforded6- or 4-p-methoxybenzylated products, respectively, withoutaffecting either the alcohol or carbonyl functional groups.

The same ruthenium complex was used for the dehydrative C–Halkylation reaction of phenols with alcohols to form ortho-substi-tuted phenol products (Scheme 44) [79]. C�H alkylation of estronewith cholesterol formed a 1:1 diastereomeric mixture of the cou-pling product, while the analogous C�H alkenylation of 2-hydroxy-carbazole with cholesterol led to a clean formation of the oxidativecoupling product.

Hydrogen abstraction from the C-7 position of cholesterol bytriplet excited benzophenone was studied [80]. The intramolecularversion of this reaction was proven to be highly diastereoselective(Scheme 45) [81,82].

Prepared from cholesterol by diketene condensation, diazotransfer, and deacetylation in 68% overall yield, cholest-5-en-3b-yl diazoacetate was subjected to standard conditions for diazo

C8H17

O

O

O

R1 R2

R1, R2 = H, Me

C8H17

O

O

R1 R2

OH

hν

Scheme 45. Intramolecular hydrogen abstraction by triplet excited benzophenone.

C8H17

O

O

N2

O

O O

O

+DCM, reflux, 5 h

Rh2L4

Rh

O

Rh

NH

COOMe

Rh2(5R-MEPY)4: yield 81%; product ratio = 94:6;

5R

Rh2(5S-MEPY)4: yield 74%; product ratio = 33:67

Scheme 46. Decomposition of cholesteryl diazoacetate with chiral dirhodium(II) carboxamidate catalysts.

1

OOH

OOH

OOH

HO

HO

HO

HO

NOH

O

R

R = H, α- or β-OHOOH

AcO

OsO4+

ratio 3:4

Scheme 47. Synthesis of analogs of cytotoxic 6-oximes isolated from sponges.

AcO

OH

AcO

OH

OH

H20(S)-Hydroxycholesterol Active analog

Fig. 4. 20(S)-Hydroxycholesterol and its active analog.

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decomposition with a series of chiral dirhodium(II) carboxamidatecatalysts (Scheme 46) [83]. The products from carbon–hydrogeninsertion were obtained in high yields and selectivities. The useof S-configured catalysts shows a distinctive preference forinsertion into the 3-position to form b-lactone products. TheR-configured catalysts direct insertion preferentially to the equato-rial C-H bond at the 2-position. Substituents or functional groupsat the 5/6-position prevent C–H insertion from taking place atthe 4-position.

7. Polyhydroxylated cholestane derivatives

Disodium 2b,3a-dihydroxy-5a-cholestan-6-one disulfate wassynthesized in seven steps from cholesterol [84]. The 2b,3a-diolsystem was obtained by opening of the 2a,3a-epoxide. A similarderivative of 5a-cholan-24-oic acid was also prepared [85].

Polyhydroxylated cholestanes, namely 2a,3a,5a-cholestane-triol-6-one and 2b,3b,5a-cholestane-triol-6-one, were obtained asintermediates in the synthesis of analogs of cytotoxic 6-oximes iso-lated from sponges (Scheme 47) [86]. The key-step of the synthesiswas the reaction of 5a-hydroxycholest-2-en-6-one with OsO4. Thereaction afforded a mixture of 2a,3a- and 2b,3b-diols in a ratio of3:4.

11

HO

O

NADPH, O2, H+

- NADP+, - H2O+ O

Scheme 48. Enzyme-catalyzed conversion of cholesterol to pregnenolone.

HO

O

HHO

O

HO H

1

introduction of Δ7

Scheme 49. Biosynthetic pathwa

C8H17

HOO

Mycobacterium sp.

Scheme 50. Microbial transformation

Further hydroximinosteroid analogs with different oxygenatedpositions in the A, B, or D ring were obtained and screened for theircytotoxic activity [87].

A modified four-step synthesis was reported of 5-hydroxy-5a-cholesta-2,7-dien-6-one from cholesterol, an intermediate in thesynthesis of insect hormones along with its dihydroxylation in ringA [88]. A latter step was carried out with stoichiometric OsO4 orwith the catalytic system OsO4/N-methylmorpholine N-oxide.However, in both cases the b-dihydroxylation prevailed.

8. Side chain oxidation

Cholesterol oxidation in the side chain is usually an enzymaticreaction. While sterols oxidized in the ring are implicated in toxiceffects, sterols oxidized enzymatically in the side chain play impor-tant biological roles and are intermediates in the transformation ofcholesterol into bile acids and steroid hormones.

The use of naturally occurring oxysterols, i.e. 22(R)-hydroxy-cholesterol, 20(S)-hydroxycholesterol, and 22(S)-hydroxycholes-terol, as potential osteogenic agents was studied [89] (Fig. 4).Also, synthesis and biological evaluation of novel oxysterol deriva-tives designed as anabolic bone growth agents were reported [90].

The cholesterol side-chain cleavage enzyme is commonly re-ferred to as P450scc, where ‘‘scc’’ is an acronym for side-chaincleavage. P450scc is a mitochondrial enzyme that catalyzes theconversion of cholesterol to pregnenolone (Scheme 48) [91]. Thisis the first reaction in the process of steroidogenesis in all mamma-lian tissues that specialize in the production of various steroid hor-mones. P450scc is a member of the cytochrome P450 superfamilyof enzymes.

It was proven by using deuterated intermediates that Ajugahairy roots are capable of introducing a double bond at the7-position at a late stage of 20-hydroxyecdysone biosynthesis, thussuggesting the possibility of an alternative biosynthetic pathwaywhich does not involve 7-dehydrocholesterol as an intermediate(Scheme 49) [92].

HO

HO

OHOH

OH

O

H

ys to 20-hydroxyecdysone.

O O

O

+

43% 6%

s of 5,6-cyclopropanocholestanes.

AcOBr

Br

AcOBr

Br

OH

ETDO45%

HO

OH

HO

O

24-oxocholesterol 25-hydroxycholesterol

Scheme 51. Regioselective hydroxylation of the tertiary C25–H bond.

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Microbial transformations of 5,6-cyclopropanocholestanes withMycobacterium sp. were described (Scheme 50) [93]. Cleavage ofthe cyclopropyl ring was observed in low extents, and the majorproducts were 17-keto steroids with an intact 5,6-cyclopropyl ring.

The regioselective hydroxylation of the tertiary C25–H bond in5a,6b-dibromocholestan-3b-yl acetate by ethyl(trifluoro-methyl)dioxirane (ETDO), generated in situ from 1,1,1-trifluoro-2-butanone and potassium peroxymonosulfate, was elaborated(Scheme 51) [94]. With this method a concise synthesis of natu-rally occurring oxysterols, i.e. 25-hydroxycholesterol, as well asits 3-sulfate, and 24-oxocholesterol, starting from cholesterol,was carried out.

R

1

O

H

R = OAc, Cl, or H

R

C

S

NHArH2NHN

Scheme 53. Novel 6(R)-spiro-1,3,4-

HO O

O

HO

OOH

HOOOH

HO

O

O3

singlet O2

H+ Hock reaction

Aldolreaction

Dehydration

5(6 7)abeo-sterol

Scheme 52. Alternative pathways to cholesterol-derived aldehydes.

9. Miscellaneous

Cholesterol oxidation gives rise to a mixture of oxidized prod-ucts. Different types of products are generated according to thereactive species that is involved. Recently, attention has beenfocused on two cholesterol aldehydes, 3b-hydroxy-5b-hydroxy-B-norcholestane-6b-carboxyaldehyde and 3b-hydroxy-5-oxo-5,6-secocholestan-6-al. These aldehydes can be generated by ozoneas well as by singlet molecular oxygen-mediated cholesterol oxida-tion (Scheme 52). It has been suggested that the B-seco-steroid ispreferentially formed by ozone and that the B-nor-aldehyde ispreferentially formed by singlet molecular oxygen.

In earlier studies both of these aldehydes were considered to becholesterol ozonation products that are generated by atheroscle-rotic lesions [95].

However, further studies have shown that B-nor-ketoaldehydeand its aldolization products can also arise from Hock cleavage ofthe cholesterol 5a-hydroperoxide, which has been shown to occurdue to singlet oxygen oxidation of cholesterol [96].

However, these findings did not unequivocally dismiss the pos-sibility that ozone is produced endogenously, or that ozone isresponsible for the cleavage of the D5 bond in cholesterol in vivo.Various methods for detection of cholesterol-derived aldehydeswere elaborated [97,98]. The precise analysis of the ratio of alde-hydes (in favor of the B-seco compound) strongly supported themechanism involving ozone oxidation of cholesterol [99].However, Hock cleavage of 5a-hydroperoxide may also contributeto the B-seco oxysterol formation.

Naturally occurring marine 5(6 ? 7)abeo-sterols and synthe-sized ring B abeo-sterols showed good inhibitory activity againstMycobacterium tuberculosis [100].

N

H

NH

NH

SAr

R HN

N

SAc

N

Ac

Ar

Ac2O, Py

Ar = p-tolyl

thiadiazolines from cholesterol.

, 0 oC, 1.5 h, 46%

Tf2O, DMAP

Scheme 54. Dehydration of cholesterol with triflic anhydride.

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Novel 6(R)-spiro-1,3,4-thiadiazoline 5a-cholestane derivativeswere synthesized from cholesterol (Scheme 53) [101]. Thekey-step in this synthesis was the cyclization of 6-ketone thiosem-icarbazone with acetic anhydride. The obtained thiadiazolinederivatives exhibited antibacterial activity.

The addition of triflic anhydride (Tf2O) to some hydroxy steroidsin the presence of an excess base directly leads to steroidal olefins[102]. Thus, 3a-hydroxy- and 3b-hydroxy-5a-steroids afford D2-olefin products, while 3a-hydroxy-5b-steroids yield D3-olefinswhen treated with Tf2O/DMAP. In contrast to that, 3b-hydroxy-D5-steroids, e.g. cholesterol, give 3a,5a-cyclo-D6-steroids (Scheme 54).

10. Conclusions

Cholesterol, as an allylic alcohol with a large hydrophobic por-tion, undergoes various transformations. These transformationsare intensively studied due to cholesterol’s biological importance.Most cholesterol chemical reactions concern the region of rings Aand B bearing the following functional groups: the 3b-hydroxylgroup and the C5–C6 double bond. Reactions of the 3b-hydroxylgroup concern mainly its substitution, elimination or oxidation.These reactions are frequently specific for cholesterol due to theparticipation of the neighboring double bond. Many substitutionreactions give products of stereoretention in contrast to analogicalreactions of 5a-cholestan-3b-ol. The addition to the double bond ofcholesterol, its oxidation, including cleavage, and various reactionsat the allylic position are also frequently performed transforma-tions. Chemical oxidation of cholesterol provides different prod-ucts, depending on the structure of the oxidizing agent that isinvolved. However, the chemical transformations are intended tobe regio- and stereoselective and give rise to a single, stereochem-ically pure product. In contrast to that, cholesterol autoxidationprocesses occurring during food processing and storage are verycomplex in nature and lead to a variety of different products,named oxysterols, which are potentially toxic. The reactions ofthe cholesterol side chain are mostly enzymatic transformations,and oxysterols formed by hydroxylation of the hydrocarbon moi-ety of cholesterol are less harmful. Cholesterol, due to its rigidstructure, is often used as a model compound to test various newreagents. There are many examples of C–C bond-forming reactionsand processes leading to steroids with additional heterocyclicrings. A comprehensive review of recent advances in cholesterolchemistry is given in this article.

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Source: Steroids