building blocks for the synthesis of cholesterol-lowering drugs from the statin
family. An impressive example for the development of stereoselective enzymes
derived from the metagenome is the discovery of >130 novel nitrilases from
biotope-specific environmental DNA libraries [764, 765]. Among these enzymes,
22 nitrilases showed (S)-selectivity for the desymmetrization of the unprotcted 3hydroxyglutarodinitrile (Scheme 2.103, R ¼ H), while one produced the mirrorimage (R)-enantiomer in 95–98% e.e. [766].
Kinetic and Dynamic Resolution of rac-Nitriles α-Hydroxy and α-amino acids
can be obtained from the corresponding α-hydroxynitriles (cyanohydrins) and
α-aminonitriles [767], which are easily synthesized in racemic form from the
corresponding aldehyde precursors by addition of hydrogen cyanide or a Strecker
synthesis, respectively (Schemes 2.104 and 2.105). In aqueous systems, cyanohydrins are stereochemically labile and undergo spontaneous racemization via HCN
elimination, which furnishes a dynamic resolution process. From aliphatic raccyanohydrins, whole cells of Torulopsis candida yielded the corresponding
(S)-α-hydroxy acids [768], while (R)-mandelic acid is produced from racmandelonitrile on an industrial scale by employing resting cells of Alcaligenes
faecalis [769] in >90 % yield [770, 771].
OH
O
N
OR
C
NH 2
O
N
OR
C
N
N
OR
C
C
H 2 N
OH
O
O
C
C
H 2 N
N H 2
O
O
C
C
N
N
C
C
Rhodococcus butanica ATCC 21197
99
90
88
Rhodococcus butanica ATCC 21197
Brevibacterium R312
E.e. [%]
Microorganism
-CH 2 -Ph
-CO-Ph
-CH 2 -Ph
selective
non-selective
hydratase
nitrile
Microbial cells
Rhodococcus rhodochrous
96% e.e., 92% yield
selective
amidase
non-selective
hydratase
nitrile
H 2 O
R
R
2 H 2 O
H 2 O
NH 3
amidase
H 2 O
NH 3
R
Scheme 2.103 Asymmetric microbial hydrolysis of a prochiral dinitrile
CN
OH
R
OH
CO 2 H
R
OH
CO 2 H
R
Alcaligenes
faecalis
Torulopsis
candida
R
S
in-situ racemization
R
S
2 H 2 O
NH 3
2 H 2 O
NH 3
R
Microorganism
e.e. α-Hydroxyacid [%]
(CH 3 ) 2 CHTorulopsis candida
>90
(CH 3 ) 2 CH-CH 2 -
Torulopsis candida
>95
PhAlcaligenes faecalis
~100
Scheme 2.104 Stereocomplementary enantioselective hydrolysis of α-hydroxynitriles
2.1 Hydrolytic Reactions
131
family. An impressive example for the development of stereoselective enzymes
derived from the metagenome is the discovery of >130 novel nitrilases from
biotope-specific environmental DNA libraries [764, 765]. Among these enzymes,
22 nitrilases showed (S)-selectivity for the desymmetrization of the unprotcted 3hydroxyglutarodinitrile (Scheme 2.103, R ¼ H), while one produced the mirrorimage (R)-enantiomer in 95–98% e.e. [766].
Kinetic and Dynamic Resolution of rac-Nitriles α-Hydroxy and α-amino acids
can be obtained from the corresponding α-hydroxynitriles (cyanohydrins) and
α-aminonitriles [767], which are easily synthesized in racemic form from the
corresponding aldehyde precursors by addition of hydrogen cyanide or a Strecker
synthesis, respectively (Schemes 2.104 and 2.105). In aqueous systems, cyanohydrins are stereochemically labile and undergo spontaneous racemization via HCN
elimination, which furnishes a dynamic resolution process. From aliphatic raccyanohydrins, whole cells of Torulopsis candida yielded the corresponding
(S)-α-hydroxy acids [768], while (R)-mandelic acid is produced from racmandelonitrile on an industrial scale by employing resting cells of Alcaligenes
faecalis [769] in >90 % yield [770, 771].
OH
O
N
OR
C
NH 2
O
N
OR
C
N
N
OR
C
C
H 2 N
OH
O
O
C
C
H 2 N
N H 2
O
O
C
C
N
N
C
C
Rhodococcus butanica ATCC 21197
99
90
88
Rhodococcus butanica ATCC 21197
Brevibacterium R312
E.e. [%]
Microorganism
-CH 2 -Ph
-CO-Ph
-CH 2 -Ph
selective
non-selective
hydratase
nitrile
Microbial cells
Rhodococcus rhodochrous
96% e.e., 92% yield
selective
amidase
non-selective
hydratase
nitrile
H 2 O
R
R
2 H 2 O
H 2 O
NH 3
amidase
H 2 O
NH 3
R
Scheme 2.103 Asymmetric microbial hydrolysis of a prochiral dinitrile
CN
OH
R
OH
CO 2 H
R
OH
CO 2 H
R
Alcaligenes
faecalis
Torulopsis
candida
R
S
in-situ racemization
R
S
2 H 2 O
NH 3
2 H 2 O
NH 3
R
Microorganism
e.e. α-Hydroxyacid [%]
(CH 3 ) 2 CHTorulopsis candida
>90
(CH 3 ) 2 CH-CH 2 -
Torulopsis candida
>95
PhAlcaligenes faecalis
~100
Scheme 2.104 Stereocomplementary enantioselective hydrolysis of α-hydroxynitriles
2.1 Hydrolytic Reactions
131