acids, the D-counterparts were obtained with a lipase from Aspergillus niger. Furthermore, the racemization rate of the two configurationally unstable substrate
antipodes under weakly basic conditions at pH 7.6 is sufficiently rapid to provide a
dynamic resolution with a theoretical yield of 100% (k rac
Sub
! k R or k S , respectively),
whereas the products are configurationally stable (k rac
Prod
% 0, compare Fig. 2.9).
Candida sp. Lipases
Several crude lipase preparations are available from the yeasts Candida lipolytica,
C. antarctica (CAL), and C. rugosa (CRL, syn. C. cylindracea). The latter enzyme,
the three-dimentional structure of which has been resolved by X-ray analysis [357],
has been frequently used for the resolution of esters of secondary alcohols [429–434]
and, to a lesser extent, for the resolution of α-substituted carboxylates [435, 436]. The
CRL preparations from several commercial sources which contain up to 16% of
protein [437] differ to some extent in their activity but their selectivity is very similar
[438]. As CRL is able to accommodate relatively bulky esters in its active site, it is
the lipase of choice for the selective hydrolysis of esters of cyclic secondary alcohols.
To illustrate this point, some representative examples are given below.
The racemic cyclohexyl enol ester shown in Scheme 2.54 was enzymatically
resolved by CRL to give a ketoester with an (S)-stereocenter on the α-position (77%
e.e.) coupled with a diastereoselective protonation of the liberated enol, which led
to an (R)-configuration on the newly generated center on the γ-carbon atom, only a
trace of the (S,S)-diastereomer was formed. The remaining (R)-enol ester was
obtained in optically pure form [439]. In accordance with the substrate model
Type IV (Scheme 2.45), no significant hydrolysis on the fully substituted ethyl
carboxylate was observed.
COOH
O
Ph
HN
R
COOH
O
Ph
HN
R
Ph
R
O
O
N
base-catalyzed in-situ racemization
4
Aspergillus
sp. lipase
PPL
buffer pH 7.6
L
buffer pH 7.6
L
D
D
R
Lipase
Configuration e.e. [%]
PhPPL
L
76
CH 3 -S-CH 2 -CH 2 -
PPL
L
80
Ph-CH 2 -
PPL
L
>99
PhAspergillus sp.
D
80
CH 3 -S-CH 2 -CH 2 -
Aspergillus sp.
D
83
Ph-CH 2 -
Aspergillus sp.
D
>99
Scheme 2.53 Lipase-catalyzed dynamic resolution of oxazolin-5-ones
CO 2 Et
OAc
CO 2 Et
O
OH
CO 2 Et
CO 2 Et
OAc
spontaneous
e.e. > 99%
e.e. = 77%
γ
α
rac
buffer
CRL
+
tautomerisation
+ trace of (S,S)-diastereomer
R
S
R
S
Scheme 2.54 Enzymatic resolution of a cyclic enol ester by Candida rugosa lipase
92
2 Biocatalytic Applications
antipodes under weakly basic conditions at pH 7.6 is sufficiently rapid to provide a
dynamic resolution with a theoretical yield of 100% (k rac
Sub
! k R or k S , respectively),
whereas the products are configurationally stable (k rac
Prod
% 0, compare Fig. 2.9).
Candida sp. Lipases
Several crude lipase preparations are available from the yeasts Candida lipolytica,
C. antarctica (CAL), and C. rugosa (CRL, syn. C. cylindracea). The latter enzyme,
the three-dimentional structure of which has been resolved by X-ray analysis [357],
has been frequently used for the resolution of esters of secondary alcohols [429–434]
and, to a lesser extent, for the resolution of α-substituted carboxylates [435, 436]. The
CRL preparations from several commercial sources which contain up to 16% of
protein [437] differ to some extent in their activity but their selectivity is very similar
[438]. As CRL is able to accommodate relatively bulky esters in its active site, it is
the lipase of choice for the selective hydrolysis of esters of cyclic secondary alcohols.
To illustrate this point, some representative examples are given below.
The racemic cyclohexyl enol ester shown in Scheme 2.54 was enzymatically
resolved by CRL to give a ketoester with an (S)-stereocenter on the α-position (77%
e.e.) coupled with a diastereoselective protonation of the liberated enol, which led
to an (R)-configuration on the newly generated center on the γ-carbon atom, only a
trace of the (S,S)-diastereomer was formed. The remaining (R)-enol ester was
obtained in optically pure form [439]. In accordance with the substrate model
Type IV (Scheme 2.45), no significant hydrolysis on the fully substituted ethyl
carboxylate was observed.
COOH
O
Ph
HN
R
COOH
O
Ph
HN
R
Ph
R
O
O
N
base-catalyzed in-situ racemization
4
Aspergillus
sp. lipase
PPL
buffer pH 7.6
L
buffer pH 7.6
L
D
D
R
Lipase
Configuration e.e. [%]
PhPPL
L
76
CH 3 -S-CH 2 -CH 2 -
PPL
L
80
Ph-CH 2 -
PPL
L
>99
PhAspergillus sp.
D
80
CH 3 -S-CH 2 -CH 2 -
Aspergillus sp.
D
83
Ph-CH 2 -
Aspergillus sp.
D
>99
Scheme 2.53 Lipase-catalyzed dynamic resolution of oxazolin-5-ones
CO 2 Et
OAc
CO 2 Et
O
OH
CO 2 Et
CO 2 Et
OAc
spontaneous
e.e. > 99%
e.e. = 77%
γ
α
rac
buffer
CRL
+
tautomerisation
+ trace of (S,S)-diastereomer
R
S
R
S
Scheme 2.54 Enzymatic resolution of a cyclic enol ester by Candida rugosa lipase
92
2 Biocatalytic Applications