tion of â-ketonitriles has remained until now an unsolved
problem. It has been scarcely reported that â-ketonitriles
could be enantioselectively reduced to â-hydroxy nitriles by
bakers’ yeast and the fungus CurVularia lunata.17,18 However,
one of the most characteristic features of the bioreduction
of â-ketonitriles by the reported whole-cell biocatalysts is
the existence of a competing R-ethylation reaction (Scheme
1), resulting in low chemical yields of the desired â-hydroxy
or produced during the reaction. It has been found that a
carbonyl reductase from Candida magnoliae (CMCR) cata-
lyzed the reduction of a series of ketones, R- and â-ke-
toesters, to anti-Prelog configurated alcohols in excellent
optical purity.22 Therefore, this carbonyl reductase was
applied to the reduction of a series of aromatic â-ketonitriles
to test our hypothesis.
In this context, 3-oxo-3-phenylpropanenitrile (1a) was
treated with a catalytic amount of CMCR and cofactor
NADPH, which was regenerated with D-glucose and D-
glucose dehydrogenase (GDH) systems (Scheme 2), in
Scheme 1. Whole-Cell Reduction of
3-Oxo-3-phenylpropanenitrile Concomitant with R-Ethylation21
Scheme 2. CMCR-Catalyzed Reduction of â-Keto Nitriles
with D-Glucose and D-Glucose Dehydrogenase (GDH) Cofactor
Recyling Systems
nitriles. Further studies suggested that the ethyl group may
come from the ethanol produced by the yeast or fungal
metabolism.19,20 Delhi et al. surprisingly found that â-keto-
nitriles was reduced to (S)-â-hydroxy nitriles by fungus
CurVularia lunata in 41-77% yield and 40-98% enan-
tiopurity when methanol was used as the cosolvent.20
Recently, an E. coli whole-cell system overexpressing yeast
carbonyl reductases was used to improve the yield of the
reduction product, but the ethylated product had not been
completely eliminated.21
potassium phosphate buffer. The reaction mixture was
extracted with methyl tert-butyl ether. Fortunately, HPLC
analysis of the extract showed that 3-oxo-3-phenylpropaneni-
trile (1a) was completely converted to (R)-3-hydroxy-3-
phenylpropanenitrile (2a), and no ethylated product 2-ethyl-
3-oxo-3-phenylpropanenitrile was detected, indicating the
competing ethylation reaction was completely eliminated.
The reductions of 3-oxo-3-phenylpropanenitrile (1a) and
other aromatic â-keton nitriles (1b-j) bearing various
substituents on the phenyl ring with this carbonyl reductase
were carried out at about 1 mmole scale. The reaction
mixture was worked up as described in the experimental
section. The product â-hydroxy nitriles were isolated and
characterized by NMR analysis. The ee values were mea-
sured by chiral HPLC or GC analysis.14 The results are
presented in Table 1.
We thus reasoned that use of isolated carbonyl reductase
should solve this problem because no ethanol is introduced
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Chiral â-hydroxy carboxylic acids are widely used as
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