Chemistry Reference
In-Depth Information
effi cient recombinant whole-cell biocatalysts, consisting of an ADH and GDH, are
Shimizu and coworkers [221]. As a GDH, the GDH from
Bacillus megaterium
, which
accepts both NADH and NADPH as a cofactor, was used. Already in the 1990s, Shimizu
et al. developed an effective
E. coli
catalyst, as well as a highly effi cient reaction system
for the reduction of 4 - chloro - 3 - oxobutanoate [222 - 226] . The use of these effi cient
recombinant whole - cell catalysts in the asymmetric reduction of 4 - chloro - 3 - oxobutano-
ate (
122
) forming the corresponding pharmaceutically important alcohol (
R
) -
123
has
been intensively investigated and optimized by the Shimizu group. As a reaction media,
an
n
-butyl acetate/water two-phase solvent system turned out to be suitable [227]. When
using the
E. coli
host organism overexpressing an NADP
+
- dependent ADH from
Spo-
robolomyces salmonicolor
, and an isolated GDH enzyme or GDH-expressing cells as
biocatalysts, the desired optically active (
R
) - alcohol (
R
) -
123
was formed with up to
255 g/L in the organic phase under optimized conditions [228,229]. The conversion
reached 91% and an enantioselectivity of 91% ee was found. Besides glucose as a cosub-
strate, a low amount of NADP+ is required. A further improvement has been achieved
when using
E. coli
, co-expressing both the ADH from
S. salmonicolor
and the GDH
from
B. megaterium
, resulting in the formation of the desired optically active (
R
) - alcohol
with 94.1% conversion and an enantioselectivity of 91.7% ee when operating at a sub-
strate concentration of 300 g/L and adding a catalytic amount of the NADP
+
- cofactor
[230]. Scheme 6.58 illustrates the concept of this application of a tailor-made whole-cell
biocatalyst in a two-phase reaction media, as well as experimental results.
It is noteworthy that the Shimizu group also designed a whole-cell catalyst for the
synthesis of the analogue (
S
) - enantiomeric form of ethyl 4 - chloro - 3 - hydroxybutanoate
[231]. In addition, Kaneka researchers jointly with the Shimizu group reported the exten-
sion of this reduction technology for the reduction of other type of functionalized
-
keto ester substrates, for example, 4-bromo-3-oxobutanoate [232], and a range of other
substrates [231]. This impressive biocatalytic reduction technology developed by the
Shimizu group has already been commercialized. Since 2000, Kaneka Corporation
applies this methodology for the manufacture of ethyl (
S
) - 4 - chloro - 3 - hydroxybutanoate
on industrial scale [231].
A recombinant whole-cell catalyst, containing an ADH and GDH, has also been
developed by Patel et al., and successfully applied for the reduction of an acetophenone
substituted with a keto ester-containing moiety [233]. The reaction proceeded with a
reaction yield of 95% and gave an excellent enantioselectivity of 99.9% ee. Notably, this
biotransformation has been scaled up to a 500-L scale. The construction of an
E. coli
whole-cell catalyst, harboring the widely used (
R
) - selective ADH from
L. kefi r
and a
GDH from
Bacillus subtilis
, has also been successfully accomplished by the Hummel
group [234] .
In addition, Degussa researchers jointly with the Hummel group reported the appli-
cation of recombinant whole-cell biocatalysts in asymmetric reductions of a range of
ketones at high substrate input, exceeding 150 g/L, in pure aqueous media, and in general
without the need of addition of external amount of cofactor [235]. Both types of enan-
tiomers are available due to the use of (
S
) - and (
R
) - selective whole - cell biocatalysts.
This methodology, which is both economical and simple to be carried out, has been used
for the preparation of a wide range of optically active alcohols (
S
) - and (
R
) -
141
. Typi-
cally, the substrate concentrations are in the range of 1 M, thus exceeding 100 g/L. The
reduction proceeds with high conversions of up to
β
>
95%, and with high enantioselectivi-
ties of up to
99.4% ee. An overview about selected examples is given in Scheme 6.59.
The synthesis of a fl uorinated 4-phenylethan-1-ol as well as aliphatic halohydrins also
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