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Asymmetric synthesis ofb0-amino-a,b-enones via addition

ofa,b-unsaturated ketone-derived enolates to chiral

N-phosphonyl imines

Yiwen Xiong a, Haibo Mei a,b, Jianlin Han a,b,c,, Guigen Li b,d, Yi Pan a,

a School of Chemistry and Chemical Engineering, State of Key Laboratory of Coordination, Nanjing University, Nanjing 210093, Chinab Institute for Chemistry & BioMedical Sciences, Nanjing University, Nanjing 210093, Chinac High-Tech Research Institute of Nanjing University, Changzhou 213164, Chinad Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409-1061, USA

a r t i c l e i n f o

Article history:

Received 10 December 2013

Revised 25 February 2014

Accepted 2 March 2014

Available online 12 March 2014

Keywords:

N-Phosphonyl imine

a,b-Unsaturated ketoneAsymmetric Mannich reaction

b0-Amino-a,b-enonesDiastereoselectivity

a b s t r a c t

Efficient asymmetric Mannich addition reactions betweena,b-unsaturated ketone derived enolates andN-phosphonyl imines were reported. These reactions could proceed smoothly for a variety of imine sub-

strates with good (up to 96%) chemical yields and excellent diastereoselectivities (up to 98:2 dr). The

method reported in this work provides an easy access to chiral b0-amino-a,b-enones. 2014 Elsevier Ltd. All rights reserved.

Amino enone derivatives are one of the most important classes

of nitrogen-containing biological compounds.1 Their chemistry has

attracted a lot of attentions from organic chemists leading to dis-

covery and development of numerous useful compounds in the

fields of biological and medicinal chemistry.2 Among this type of

compounds, b0-amino-a,b-enones take a particular importantplace.3 They belong to important organic synthetic intermediates

as they can be easily converted into the corresponding amino acid,4

pyrrolidine,5 thiazole,6 oxazole7 and many other multifunctional

compounds. They have also been served as ligand for the stereose-

lective synthesis of Mannich-type products.8 Furthermore, these

functional moieties were discovered as drug precursors, such as

antimicrobial molecules,9 Parkinsons/Alzheimers disease inhibi-tor10 and MTB (mycobacterium tuberculosis bacteria) inhibitor.11

In recent years, several representative methods have been doc-

umented by chemists for the synthesis of b0-amino-a,b-enones.These methods included radical coupling with nitrones and nitrile

oxides,12 HornerWadsworthEmmons olefination,13 DielsAlder

reaction with various dienophiles,14 BaylisHillman reaction of

2-cycloocten-1-one,15 aziridine opening by aldehyde16 and so

on.17 However, most of these methods focused on symmetric

synthesis or some special examples. To the best of our knowledge,

only two asymmetric examples were reported until now, which

were zinc-promotes Mannich-type reaction of chiral aldimines

with 2-silyloxybutadienes18 and five-step conversion of cinna-

mates.19 So, the development of facile and efficient methodology

for asymmetric preparation ofb0-amino-a,b-enones is still urgentand challenging. Phosphonyl imine chemistry has been studied in

our group for several years.20 This chiral phosphonyl auxiliary,

developed by our group has been successfully implied into many

asymmetric syntheses.21 Taking into account the importance of

b0-amino-a,b-enones and our research interests in the chemistryof phosphonyl imine, we, herein, reported a concise asymmetric

Mannich addition reaction of aryl-substituted enone derived eno-lates with N-phosphonyl imine, givingb0-amino-a,b-enones withgood yields and excellent diastereoselectivities (Scheme 1).

This asymmetric Mannich addition reaction was initially exam-

ined with (E)-4-phenylbut-3-en-2-one derived enolate and isopro-

pyl substituted chiralN-phosphonyl imine1a (Table 1). According

to our previous reports,21 several common bases were examined

for the generation of ketone enolate and the following addition

reaction. LiHMDS performed best among these bases, achieving

61% yield and 92:8 diastereoselectivity after 24 h (entry 3, Table 1).

The other organic bases, such as n-BuLi, LDA, NaHMDS and KHMDS,

did not work at all, giving almost no desired product (entries 1, 2, 4

http://dx.doi.org/10.1016/j.tetlet.2014.03.005

0040-4039/ 2014 Elsevier Ltd. All rights reserved.

Corresponding authors. Tel.: +86 25 83686133; fax: +86 25 83593153.

E-mail addresses: hanjl@nju.edu.cn(J. Han), yipan@nju.edu.cn(Y. Pan).

Tetrahedron Letters 55 (2014) 24762479

Contents lists available at ScienceDirect

Tetrahedron Letters

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / t e t l e t

http://dx.doi.org/10.1016/j.tetlet.2014.03.005mailto:hanjl@nju.edu.cnmailto:yipan@nju.edu.cnhttp://dx.doi.org/10.1016/j.tetlet.2014.03.005http://www.sciencedirect.com/science/journal/00404039http://www.elsevier.com/locate/tetlethttp://www.elsevier.com/locate/tetlethttp://www.sciencedirect.com/science/journal/00404039http://dx.doi.org/10.1016/j.tetlet.2014.03.005mailto:yipan@nju.edu.cnmailto:hanjl@nju.edu.cnhttp://dx.doi.org/10.1016/j.tetlet.2014.03.005http://crossmark.crossref.org/dialog/?doi=10.1016/j.tetlet.2014.03.005&domain=pdf

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Table 2

Scope of chiral N-phosphonyl imines and enonesa

NP

N

Ph Ph

O N

Ar

NP

N

Ph Ph

O NH

Ar

O

1) LiHMDS, THF, -78 o C

30 h

2) aq NH4Cl+

O

213

R

R

Entry Ar R Product Yieldb (%) Drc

1 C6H5 H 3a 70 92:8

2 4-ClC6H4 H 3b 68 96:4

3 2-FC6H4 H 3c 84 94:6

4 3-FC6H4 H 3d 93 98:2

5 4-FC6H4 H 3e 90 96:4

6 1-Naphthyl H 3f 96 96:4

7 2-Naphthyl H 3g 70 89:11

8 4-CH3C6H4 H 3h 77 96:4

9 4-CH3OC6H4 H 3i 82 96:4

10 3-CF3C6H4 H 3j 93 98:2

11 2-Furyl H 3k 94 91:9

12 C6H5 4-Cl 3l 74 93:7

13 C6H5 3-Cl 3m 71 97:3

a Reaction conditions: enone 2 (0.4 mmol), LiHMDS (0.3 mmol), imine1 (0.15 mmol), THF (7.0 mL) at 78 C.b

Isolated yields.c Determined by 31P NMR.

Table 1

Optimization of reaction conditionsa

O

NP

N

Ph Ph

O N 1) base

2) aq NH4Cl

NP

N

Ph Ph

O NH

Ph

O

Ph

1a

+

2a 3a

Entry Base T(C) Solvent Time (h) Yieldb (%) Drc

1 n-BuLi 78 THF 24 NR

2 LDA 78 THF 24

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and 5). DBU, NaOMe and Cs2CO3 were also not good choices for this

system (entries 68). Then, the solvent for this reaction was

investigated. THF was found to be the best choice, and the other

two solvents led to both decreased chemical yields (14% and 37%

for entries 10 and 11, respectively) and diastereoselectivities

(dr = 91:9 for entry 11). When the reaction was conducted in tolu-

ene, almost no desired product formed (entry 9). Both yields and

diastereoselectivities decreased dramatically when the reaction

temperature was increased (entries 12 and 13). Finally, the reac-

tion time was examined, and it was found that 30 h was needed

for the reaction (entries 14 and 15).

With optimized reaction condition in hand, the scope of

N-phosphonyl imines was then tested. As shown in Table 2, the

asymmetric reaction of lithium enolates with the chiralN-phos-

phonyl imines could proceed well, providing b0-amino-a,b-enonesin good to excellent chemical yields (6896%) and up to 98:2 dia-

stereoselectivities. Variation of the substituents on the aromatic

rings of imines did not show any obvious effect on reaction

efficiency. Both electron-withdrawing (entries 25 and 10) and

electron-donating group (entries 8 and 9) substituted aromatic

phosphonyl imine were found to be suitable substrates for this

reaction. Good to excellent diastereoselectivities were obtained

for all the examined cases, and the highest diastereoselectivity

was found in the cases of the imine substrates containing a

meta-substituted aromatic ring (98:2 dr, entries 3 and 10). Notably,

imines with a naphthyl ring 1fand1gwere also well tolerated in

this reaction and gave the target product in good yields and high

diastereoselectivities (entries 6 and 7). Especially for 1f, higher

chemical yield and better diastereoselectivity were found (96%

yield and 96:4 dr, entry 6). It was also found that the imine with

furyl group1kworked well in the current reaction and gave struc-

turally interesting product3kin 94% yield and 91:9 dr (entry 11).

Finally, the different aryl-substituted a,b-enones were examined inthis reaction. The results showed that they worked well and gave

the desired products with good yields and excellent diastereoselec-

tivities (entries 12 and 13).

Finally, the newly formed chirality of the b 0-carbon center was

investigated by removing the chiral auxiliary of compound 3a to

known compound. The chiral phosphonyl auxiliary on 3a couldbe easily removed when it was treated with 36% HCl aqueous

solution in methanol and stirred at room temperature for 24 h

(Scheme 2). The corresponding free b0-amino-a,b-enone was thenconverted into its N-ethoxycarbonyl derivative by reaction with

diethyl carbonate to give compound4a in an overall yield of 81%.

Then, the absolute configuration of the newly generated chiral cen-

ter was unambiguously assigned asSby comparison of the optical

rotation with those of known sample.19 The stereochemical assign-

ments of other products were made by analogy, based on similarity

of their chiroptical properties and NMR data.

On the basis of the above result, a six-membered chairlike tran-

sition state was suggested to explain the resulting stereoselectivity

(Scheme 3). This transition state model is similar to that of our pre-

vious chiral phosphonyl imine system.21a The lithium metal cation

connects with two reaction partners through oxygen atom and

nitrogen atom. This manner would be responsible for the excellent

control of diastereoselectivity of the system and resulting in S

configuration.

In summary, we have developed a facile asymmetric Mannich

reaction between enone enolates and chiralN-phosphonyl imines.

The reaction was found to tolerate a wide scope of imine substrates

with good chemical yields and high diastereoselectivities (up to

98:2 dr). This methodology provides an easy and efficient strategy

for the synthesis of chiral b0-amino-a,b-enones. The chiral auxil-iary, phosphonyl moiety can be easily cleaved under simple and

mild condition.

Acknowledgments

We gratefully acknowledge the financial support from the

National Natural Science Foundation of China (No. 21102071)

and the Fundamental Research Funds for the Central Universities

(Nos. 1107020522 and 1082020502). The Jiangsu 333 program

(for Pan) and Changzhou Jin-Feng-Huang program (for Han) are

also acknowledged.

Supplementary data

Supplementary data associated with this article can be found, in

the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.03.

005.

References and notes

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Jr.J. Med. Chem.1996,39, 47564761; (d) Li, W.Z.; Ma, B.J. Org. Chem.2005, 70,32773280.

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NP

N

Ph Ph

O NH

Ph

O

Ph

NH

Ph

O

Ph

O

O

a, b

3a

4a

[] = +4.9 (c = 0.66, CHCl3)

[]lit = +6.6 (c = 0.94, CHCl3)

81% yield

Scheme 2. Cleavage of chiral auxiliary: (a) HCl/MeOH, rt; (b) (EtO) 2CO, Et3N, DCM.

Li

O

NH

H

PhNP

N

Ph Ph

O N

Ph+

OLi

N

PN

Ph

Ph

O

Ph

Scheme 3. Proposed transition state.

2478 Y. Xiong et al./ Tetrahedron Letters 55 (2014) 24762479

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