Chemistry Reference
In-Depth Information
development of immobilisates, which allow easy separation from the reaction mixture
and which are more stable regarding abrasion. Such a technology has been developed
jointly by Vorlop and Gröger et al. using oxynitrilase entrapped in highly fl exible
polyvinyl alcohol lenses [127]. A prerequisite is the extension of the molecular weight
by cross-linking the oxynitrilase by means of glutaraldehyde and chitosane. The result-
ing immobilized oxynitrilase turned out to be suitable for recycling within at least 20
recycling cycles. The preparation of cross-linked enzyme aggregates represents a
further effi cient immobilization methodology for oxynitrilases as well as sol-gel immo-
bilization [128,129]. This has been demonstrated by Hanefeld and Sheldon et al. for
various oxynitrilases. For example, in the presence of immobilized oxynitrilase from
Linum usitatissimum
as cross-linked enzyme aggregate, the desired product (
R
) - 2 -
butanone cyanohydrin was formed enantioselectively, and subsequent hydrolysis
gave (
R
) - 2 - hydroxy - 2 - methylbutyric acid in 85% yield (from 2 - butanone) and with an
enantiomeric excess of 87% ee.
An alternative process technology for the suppression of the unwanted formation of
racemic cyanohydrins was reported by the Kula group [130]. To carry out the oxyni-
trilase-catalyzed reaction in pure aqueous media, however, the pH has to be kept below
pH 3.25. A broad range of substrates was converted enantioselectively into the desired
cyanohydrins under these reaction conditions.
A further milestone in this fi eld was achieved by the Griengl group developing an
asymmetric hydrocyanation in an aqueous-organic two-phase solvent system in the
presence of a recombinant oxynitrilase from
Hevea brasiliensis
[131 - 133] . Excellent
experimental data were obtained for the expression of this enzyme, which ensures avail-
ability for large-scale applications. The process concept of the Griengl group is currently
applied at DSM for the production of (
S
) - 3 - phenoxybenzaldehyde cyanohydrin ((
S
) -
94
),
which is a valuable intermediate in industrial pyrethroid manufacture. Impressive space-
time yield of 2.1 mol/(L·h) has been reported for the synthesis of (
S
) -
94
as well as both
excellent yields of 98% and enantioselectivity of 99% ee (Scheme 6.37) [134]. In addi-
tion, numerous aldehydes turned out to be suitable substrates [135-137]. Since recom-
binant (
R
)-oxynitrilases are also available, the Griengl process has been extended to the
synthesis of (
R
)-cyanohydrins products as well [138-140]. In summary, this technology
is already regarded as a well-established and mature technology for large-scale
applications.
O
OH
(
S
)-oxynitrilase from
Hevea brasiliensis
O
O
H
+
N
CN
Aqueous buffer/
methyl
tert
-buty lether
93
(
S
)-
94
98% yield
99% ee
Scheme 6.37.
Besides the use of isolated oxynitrilases, in particular in recombinant form, much
work has been done with defatted almond meal powder as biocatalyst. This type of
biocatalyst is easily accessible and can be used directly in the corresponding hydrocyana-
