A Versatile Chemo-Enzymatic Route to Enantiomerically Pure β-Branched α-Amino Acids

Article abstract of DOI:10.1021/ja049499d

A series of diastereoisomers of β-methyl-β-phenylalanine analogues 1a?f have been prepared in enantiomerically pure form using a combination of chemo- and biocatalysis. Starting from l-threonine methyl ester 2, a range of β,β-disubstituted didehydroamino

Full text of DOI:10.1021/ja049499d

Published on Web 03/16/2004
A Versatile Chemo-Enzymatic Route to Enantiomerically Pure â-Branched
r-Amino Acids
Geoffrey J. Roff,† Richard C. Lloyd,‡ and Nicholas J. Turner*,†
School of Chemistry, UniVersity of Edinburgh, King’s Buildings, West Mains Road,
Edinburgh, EH9 3JJ, U.K., Dowpharma, Chirotech Technology Ltd., Unit 321 Cambridge
Science Park, Milton Road, Cambridge, CB4 0WG, U.K.
Received January 28, 2004; E-mail: n.j.turner@ed.ac.uk
Scheme 1. Stereoinversion of â-Methylphenylalanine 1a (Ar )
Enantiomerically pure â-branched R-amino acids constitute
valuable building blocks for the synthesis of modified peptides
possessing activity as enzyme inhibitors.1 The incorporation of
bulky â-substituents into an amino acid leads to conformational
restriction in peptides and allows for the development of high
affinity ligands for receptors. Existing methods for the synthesis
of the individual stereoisomers of â-methyl R-amino acids generally
rely upon either stereoselective approaches employing chiral
auxiliaries2 or alternatively nonselective synthesis followed by
separation of the isomers by fractional recrystallization of diaster-
eomeric salts.3 Although high enantiomeric excesses can be
achieved via the latter process, the limiting factor is the maximum
yield of 50%. An attractive, recently developed alternative is the
catalytic asymmetric hydrogenation of â,â-disubstituted didehy-
droamino acids using chiral bis-phosphine ligands.4 However, it
has been found that although the (Z)-aryl-didehydroamino acids
undergo fast and highly selective hydrogenation, the corresponding
(E)-isomers react more slowly and with much lower selectivities,
resulting in access to only two of the four possible stereoisomers.5
Herein, we report a combined chemobiocatalytic method for the
synthesis of all four stereoisomers of a series of enantiomerically
pure â-methyl-â-arylalanine analogues.
Ph)
Scheme 2. Preparation of Z-5 from L-Threonine Methyl Ester 2a
a
Reagents and conditions: (i) Ac2O, NaOAc. (ii) Et3N, MeOH. (iii)
NXS, CH2Cl2. (iv) Et3N.
Scheme 3. Suzuki Couplings with Vinyl Iodide Z-5a
a
Reagents and conditions: (i) ArB(OH)2, Pd(OAc)2. (ii) Na2CO3, EtOH.
We have recently developed a method for the deracemization
of R-amino acids6 and amines7 via a cyclic oxidation-reduction
sequence and have shown that it can be extended to the stereo-
inversion of â- and γ-substituted R-amino acids.8 The catalytic cycle
involves the combined action of an enantioselective amino acid
oxidase (AAO), which oxidizes the R-amino acid to the corre-
sponding imine, together with a nonselective reducing agent
(e.g., ammonia-borane), which effects reduction back to the
starting material. For example, the (2R,3R)- and (2S,3R)-isomers
of â-methylphenylalanine 1a were successfully interconverted
by this method in high yield and enantio/diastereoselectivity
(Scheme 1).8 Since the (2S,3R) and (2R,3S) pair of diastereomers
are easily accessed via catalytic asymmetric hydrogenation of
the (Z)-didehydroamino acids as described above, we envisaged
using the AAO stereoinversion procedure to convert them, re-
spectively, to the less accessible (2R,3R)- and (2S,3S)-diastereo-
isomers.
Methyl (Z)-2-(N-acetylamino)but-2-enoate 3 was prepared by
dehydration of L-threonine methyl ester 2 according to the method
of Nugent et al.9 Subsequent bromination using N-bromosuccin-
imide followed by triethylamine gave the vinyl bromide 4 (X )
Br) in 48% yield as a 1:1 mixture of (E/Z)-isomers10 which could
be separated (Scheme 2). However, the subsequent Suzuki couplings
were generally found to proceed in higher yields with the (E)- rather
than (Z)-isomers, and hence, we decided to examine the more
reactive vinyl iodide, which to our knowledge has not previously
been reported.11
Treatment of (Z)-3 with N-iodosuccinimide followed by Et3N
yielded 5 as a 1:1 mix of (E/Z)-isomers. However, addition of 2%
12
TFA to the solvent CH2Cl2 gave 5 in 48% yield predominantly
as the (Z)-isomer (Z/E ) 5:1). The (Z)-isomer was separated by
chromatography and then subjected to a range of Suzuki couplings
(Scheme 3), which proceeded in good yields (Table 1) to give a
range of (Z)-didehydroamino acids (6a-f).
Asymmetric hydrogenation of the (Z)-didehydroamino acids
6a-f was performed at 100 psi hydrogen for 18 h in methanol
using either the [Rh(R,R)-Et-DuPhos(COD)]BF4 or [Rh(S,S)-
Et-DuPhos(COD)]BF4 catalyst to yield the (2R,3S)-7a-f or (2S,3R)-
7a-f diastereoisomers respectively (Scheme 4). Purification through
a short column of silica resulted in quantitative yields of the
products 7a-f with ee’s >98%, with the exception of (2R,3S)-7f
(ee ) 96%) and (2S,3R)-7f (ee ) 93%).13 Deprotection by refluxing
in 4 M HCl gave the enantiomerically pure amino acids 1a-f as
the hydrochloride salts in high yields and >99% ee after trituration
with acetone (Table 2).
We initially screened the (2R,3S)-isomers of 1a-f against D-AAO
from pig kidney, which we had previously shown to be effective
for the stereoinversion of (2R,3S)-1a to (2S,3S)-1a.8
However, this particular enzyme was unable to transform all of
the substrates, and thus, we turned to the D-AAO from Trigonopsis
† University of Edinburgh.
‡ Dowpharma.
9
4098
J. AM. CHEM. SOC. 2004, 126, 4098-4099
10.1021/ja049499d CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 3. Stereoinversions of 1a-f Using D- and L-AAO
Table 1. Yields of 6a-f from Suzuki Couplings
product
Ar
yield %
6a
6b
6c
6d
6e
6f
C6H5
81
77
84
72
82
73
p-F-C6H4
p-CF3-C6H4
2-naphthyl
2,5-di-Me-C6H3
p-MeO-C6H4
Scheme 4. Asymmetric Hydrogenation of Z-6a-f
substrate
Ar
enzyme
product
yield %
(2R,3S)-1a
(2R,3S)-1b
(2R,3S)-1c
(2R,3S)-1d
(2R,3S)-1e
(2R,3S)-1f
(2S,3R)-1a
(2S,3R)-1b
(2S,3R)-1c
(2S,3R)-1d
(2S,3R)-1e
(2S,3R)-1f
C6H5
C6H4-p-F
D-AAO
D-AAO
D-AAO
D-AAO
D-AAO
D-AAO
L-AAO
L-AAO
L-AAO
L-AAO
L-AAO
L-AAO
(2S,3S)-1a
(2S,3S)-1b
(2S,3S)-1c
(2S,3S)-1d
(2S,3S)-1e
(2S,3S)-1f
(2R,3R)-1a
(2R,3R)-1b
(2R,3R)-1c
(2R,3R)-1d
(2R,3R)-1e
(2R,3R)-1f
69
68
81
80
72
81
83
85
92
80
71
80
C6H4-p-CF3
2-naphthyl
C6H3-(2,5-DiMe)
C6H4-p-OMe
C6H5
C6H4-p-F
C6H4-p-CF3
2-naphthyl
Table 2. Yields and Enantiomeric Excesses of (2R,3S)-1a-f and
(2S,3R)-1a-f
C6H3-(2,5-DiMe)
C6H4-p-OMe
substrate
Ar
product
yield %
ee %
(2R,3S)-7a
(2S,3R)-7a
(2R,3S)-7b
(2S,3R)-7b
(2R,3S)-7c
(2S,3R)-7c
(2R,3S)-7d
(2S,3R)-7d
(2R,3S)-7e
(2S,3R)-7e
(2R,3S)-7f
(2S,3R)-7f
C6H5
C6H5
4-F-C6H4
4-F-C6H4
4-CF3-C6H4
4-CF3-C6H4
2-naphthyl
(2R,3S)-1a
(2S,3R)-1a
(2R,3S)-1b
(2S,3R)-1b
(2R,3S)-1c
(2S,3R)-1c
(2R,3S)-1d
(2S,3R)-1d
(2R,3S)-1e
(2S,3R)-1e
(2R,3S)-1f
(2S,3R)-1f
88
89
78
76
71
84
68
87
89
73
66
67
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
ening this approach to other classes of chiral compounds are
ongoing and will be reported in due course.
Acknowledgment. This work was supported by funds provided
by the Biotechnology and Biological Sciences Research Council
(CASE award to G.J.R.) and the Wellcome Trust.
2-naphthyl
Supporting Information Available: Experimental procedures for
the preparation of vinyl iodide Z-5, asymmetric hydrogenation reactions,
and the stereoinversion procedures (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
2,5-di-Me-C6H3
2,5-di-Me-C6H3
4-MeO-C6H4
4-MeO-C6H4
References
Variabilis, which in combination with ammonia-borane complex
gave good yields (68-81%) of the desired (2S,3S)-diastereomers
of 1a-f with excellent selectivity (>99% de) (Table 3).13 The
corresponding L-â-arylphenylalanine analogues (2S,3R)-1a-f were
converted to the remaining set of (2R,3R)-diastereomers using snake
venom L-AAO and ammonia-borane complex with good to
excellent yields (80-92%) and again very high selectivity (de >
99%) (Table 3). One reaction, namely that involving (2S,3R)-1e
with L-AAO, did not go fully to completion, resulting in only a
71% conversion and 49% de. In this case, an alternative L-amino
acid oxidase would need to be identified with the appropriate
substrate specificity.
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scale, using D-AAO from T. Variabilis and ammonia-borane
complex as the reducing agent, yielding the isolated product in 55%
yield. Workup of these reactions is very straightforward, involving
simply filtration to remove the enzyme catalyst, followed by
evaporation and recrystallization from ethanol/ethyl acetate.
In summary, we have developed a highly versatile and efficient
route to all but one of a set of 24 â-methylarylalanine analogues
1a-f in high yields and excellent enantiomeric/diasteroemeric
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to access all 23 products via asymmetric hydrogenation of the (Z)-
â,â-disubstituted didehydroamino acids 6a-f followed by stereo-
inversion using D- and L-AAO’s. Further studies aimed at broad-
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for full details).
JA049499D
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