Microbial desymmetrization of 3-arylglutaronitriles, an unusual enhancement of enantioselectivity in the presence of additives

Article abstract of DOI:10.1016/S0040-4039(00)01518-5

In the presence of an organic additive such as acetone or β-cyclodextrin or in a biphasic system of hexane and aqueous phosphate buffer, microbial desymmetrization of 3-arylglutaronitriles catalyzed by Rhodococcus sp. AJ270 cells proceeded regiospecifically and enantioselectively to produce S-(+)-3-aryl-4-cyanobutyric acids in high enantiomeric excess. Convenient chemoenzymatic syntheses of optically active R-(-)-4-amino-3-phenylbutyric acid and 4R-(-)-4-phenyltetrahydropyran-2-one are described. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01518-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8549–8552
Microbial desymmetrization of 3-arylglutaronitriles, an
unusual enhancement of enantioselectivity in the presence of
additives
a,
a
a
b
Mei-Xiang Wang, * Chu-Sheng Liu, Ji-Sheng Li and Otto Meth-Cohn
a
Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
b
Chemistry Department, University of Sunderland, Sunderland SR1 3SD, UK
Received 7 July 2000; revised 22 August 2000; accepted 7 September 2000
Abstract
In the presence of an organic additive such as acetone or b-cyclodextrin or in a biphasic system of
hexane and aqueous phosphate buffer, microbial desymmetrization of 3-arylglutaronitriles catalyzed by
Rhodococcus sp. AJ270 cells proceeded regiospecifically and enantioselectively to produce S-(+)-3-aryl-4-
cyanobutyric acids in high enantiomeric excess. Convenient chemoenzymatic syntheses of optically active
R-(−)-4-amino-3-phenylbutyric acid and 4R-(−)-4-phenyltetrahydropyran-2-one are described. © 2000
Elsevier Science Ltd. All rights reserved.
Desymmetrization of prochiral compounds is one of the most important methods in asymmet-
1
ric synthesis and much progress has been made during the past decade. Enzymatic desym-
metrization of prochiral diesters and diols has been extensively studied and used in the
2
preparation of homochiral compounds in enantiopure form. Desymmetrization of prochiral
3
4
dinitriles, however, was not known until recently. Ohto and Turner and their co-workers
independently investigated hydrolysis of O-substituted 3-hydroxyglutaronitriles using different
Rhodococcus whole-cell catalysts and obtained the corresponding monocyanocarboxylic acids in
good enantiomeric excess. When 3-benzylglutaronitrile was used as substrate, the stereoselectiv-
3
ity of the reaction was, however, extremely low. It is also interesting to note that biotransfor-
mation of a disubstituted malononitrile catalyzed by Rhodococcus rhodochrous IFO15564 yielded
5
an amido-acid with excellent enantioselectivity.
Our previous studies have demonstrated that Rhodococcus sp. AJ270 is a powerful and robust
nitrile hydratase/amidase-containing microorganism. It can efficiently hydrolyze a variety of
6
7
nitriles and dinitriles with excellent chemo- and regioselectivity. It also showed high enantiosel-
8
ectivity against racemic nitriles such as a-substituted phenylacetonitriles and trans-2-arylcyclo-
*
Corresponding author. Fax: 86-10-62569564; e-mail: mxwang@infoc3.icas.ac
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01518-5

8550
9
propanecarbonitriles. To further explore its potential in asymmetric synthesis we undertook the
current study of desymmetrization of 3-arylglutaronitriles, envisaging a simple and convenient
synthesis of optically active 3-aryl-4-cyanobutyric acids which are important intermediates for
10
the synthesis of chiral b-aryl-g-aminobutyric acids (b-aryl-GABA) such as Baclofen. We wish
to report herein the Rhodococcus sp. AJ270-catalyzed desymmetrization of 3-arylglutaronitriles
and an unusual enhancement of enantioselectivity caused by additives.
Interaction of 3-arylglutaronitriles 1 with Rhodococcus sp. AJ270 whole cells under various
11
conditions yielded S-(+)-3-aryl-4-cyanobutyric acids 2 as the sole isolated product (Scheme 1).
In contrast to this observed regiospecificity, however, both the chemical conversion and
enantioselectivity of the reaction were strongly dependent upon the nature of the substituent X
at the para position of the aromatic ring and upon the reaction conditions employed (Table 1).
The presence of either a bulkier electron-donating or electron-withdrawing group resulted in the
decrease of the conversion rate, indicating a steric limitation of the nitrile hydratase although the
substituent X is remote from the cyano function. Halogen substituents usually gave low
enantioselectivity. Prolonged incubation times led to a slight increase in chemical conversion,
but to a dramatic decrease of the enantiomeric excess of 2. Surprisingly, addition of acetone (3
ml) as a co-solvent in the reaction gave rise to a significant improvement of enantioselectivity in
all cases. Similar enhancement of enantioselectivity was also observed when b-cyclodextrin
(
b-CD) was used as an additive or when the incubation was performed in a biphasic system of
hexane and aqueous phosphate buffer. The precise reason for the improvement of enantioselec-
tivity in the presence of an organic additive is not clear at the moment, although these
Scheme 1.
Table 1
a
Microbial desymmetrization of 3-arylglutaronitriles 1
b
c
b
Entry
X
Time (h)
Additive
2 (% )
Ee (% )
1 (% )
1
2
3
4
5
6
7
8
9
H
H
H
H
12
24
24
12
8
24
24
72
72
24
24
–
–
2a (56)
2a (88)
2a (67)
2a (61)
2a (48)
2b (42)
2b (25)
2d (37)
2d (25)
2e (81)
2e (16)
61
39
88
80
84
64
95
26
63
25
76
32
2
Acetone
b-CD
Hexane
–
Acetone
–
Acetone
–
Acetone
21
21
39
44
61
49
63
6
H
CH3
CH3
Cl
Cl
F
1
1
0
1
F
70
a
b
c
Reaction was carried out in an aqueous phosphate buffer solution.
Isolated yield.
Determined by chiral HPLC analysis.
12

8551
compounds may possibly act as inhibitors to deactivate a racemase. It should be pointed out
that in some cases addition of acetone or hexane may also inhibit the nitrile hydratase, leading
to lower catalytic efficiency.
S-(+)-3-Aryl-4-cyanobutyric acids 2 are useful chiral intermediates in organic synthesis. A
straightforward Curtius rearrangement of 2a, followed by acidic hydrolysis therefore afforded
10a
R-(−)-4-amino-3-phenylbutyric acid 4, a clinically used mood elevator and tranquillizer, whilst
the reduction of carboxylic acid ester of 2a, followed by intramolecular hydrolytic cyclization
yielded 4R-(−)-4-phenyltetrahydropyran-2-one 6, a very useful intermediate in natural products
1
3
synthesis (Scheme 2).
Scheme 2.
In summary, desymmetrization of 3-arylglutaronitriles catalyzed by Rhodococcus sp. AJ270
gave optically active S-(+)-3-aryl-4-cyanobutyric acids and the enantiomeric selectivity was
improved dramatically by adding additives such as acetone and b-cyclodextrin or by using a
biphasic system of hexane and an aqueous phosphate buffer. We have also provided novel and
convenient chemoenzymatic methods for the preparation of optically active R-(−)-4-amino-3-
phenylbutyric acid and 4R-(−)-4-phenyltetrahydropyran-2-one.
Acknowledgements
We thank the Major Basic Research Development Program (No. G2000077506), the National
Natural Science Foundation of China and the Chinese Academy of Sciences for their financial
support. M.-X.W. and O.M.-C. also thank The Royal Society, London, for supporting a Joint
Research Project between Chinese and UK groups.
References
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2
. For a useful review, see: Schoffers, E.; Golebiowski, A.; Johnson, C. R. Tetrahedron 1996, 52, 3769–3826.
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8552
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1. General procedure for the biotransformation: Dinitrile (2 mmol) was incubated with Rhodococcus sp. AJ270 (2
g wet weight) in an aqueous phosphate buffer (pH 7.0) at 30°C for a period of time. After removal of biomass
through filtration, the resulting solution was acidified using HCl (2 M) to pH 2 and extracted with diethyl ether.
Acid was obtained after removal of the solvent. All products were characterized by their spectral data and
microanalysis.
1
1
2. Chiral HPLC analysis of monocyano-acid products was performed using a Chiracel OD column with hexane:2-
propanol [180:1 or 90:10] as the solvent at a flow rate of 0.2 or 0.8 ml/min.
3. (+)-(S)-4-Phenyltetrahydropyran-2-one, 42% ee, [h]D 1.73° (c 6); 98% ee, [h]D 3.80°. See: (a) Lam, L. K. P.; Hui,
R. A. H. F.; Jones, J. B. J. Org. Chem. 1986, 51, 2047–2050. (b) Meyers, A. I.; Smith, R. K.; Whitten, C. E. J.
Org. Chem. 1979, 44, 2250.
25
25
1
.