Chemistry Reference
In-Depth Information
O
O
Cl
O
Benzaldehyde
lyase
MeO
MeO
H
H
+
OH
Cl
ThDP, Mg
2+
,
buffer, DMSO,
30°C
OMe
OMe
97
98
(
R
)-
99
>99% conversion
95% selectivity
>99% ee
Scheme 6.39.
attractive catalysts for the benzoin condensation [147]. Notably, due to the use of whole
cells with cofactor therein, the addition of external amount of cofactor is not required.
In addition, high substrate concentrations were reported, which represents a further
prerequisite for a technical process with favorable economical data.
Recent achievements have been made in the condensation of aliphatic aldehydes.
This reaction has also been known for a long time for acetoin condensation. However,
extension to other types of aldehydes has been a challenge for a long time. Enzymatic
carboligation of linear aldehydes has been reported recently by Trauthwein and Müller
et al. resulting in high conversions and enantioselectivities of up to 80% ee [148]. When
using branched aliphatic aldehydes in the presence of a benzaldehyde lyase, high con-
version and an enantioselectivity of up to 89% ee were obtained. Very recently, the
Müller group also reported the suitability of unsaturated aldehydes for such types
of umpolung reactions [149]. Depending on the type of enzyme, regioselectivity can
be varied, and the desired α-hydroxy ketones, for example, (
R
) -
102
, are formed regi-
oselectively and with enantiomeric excess of up to >99%. An example is shown in
Scheme 6.40 .
Benzaldehyde
lyase
O
O
O
Me
Ph
H
+
H
Me
Ph
Me
ThDP, Mg
2+
,
buffer
Me
OH
101
100
(
R
)-
102
75% yield
>96% ee
Scheme 6.40.
6.4.4. Aldol Reactions
Aldol reactions are one of the most important transformations in organic chemistry, and
play a key role in biochemical processes for the construction of sugars. A key feature
of biocatalytic aldol reactions [150,151] is the high specifi city with respect to the donor
component, whereas a broad substrate range is usually observed for the acceptor mol-
ecules. Accordingly, the aldolases are typically divided according to the type of required
donor. Some examples of donors are dihydroxyacetone phosphate (DHAP), pyruvate,
acetaldehyde, and glycine. Aldolases have found many synthetic applications, which
