Angewandte
Chemie
DOI: 10.1002/anie.201203219
Enzyme Catalysis
Enantioselective Oxidation of Aldehydes Catalyzed by Alcohol
Dehydrogenase**
Paul Kçnst, Hedda Merkens, Selin Kara, Svenja Kochius, Andreas Vogel, Ralf Zuhse,
Dirk Holtmann, Isabel W. C. E. Arends, and Frank Hollmann*
Alcohol dehydrogenases (ADH) are enjoying increasing
interest as versatile and selective biocatalysts in the context
intrinsic drawbacks of the kinetic resolution of racemic
[
8]
profens, enantioselective routes are gaining relevance.
We envisaged an oxidative dynamic kinetic resolution
(DKR) as shown in Scheme 1. Compared to the previously
reported reductive DKRs of profen aldehydes yielding
[1]
of both academic research and industrial implementation.
The stereospecific reduction of prochiral ketones is an area of
[2]
application most attractive to preparative organic chemists.
In contrast, the oxidation of alcohols is less popular because
[
3]
chirality is mostly destroyed rather than generated. Fur-
thermore, ADH-catalyzed oxidation is limited to the con-
version of primary or secondary alcohols into the correspond-
ing aldehydes and ketones. The few biocatalytic examples
reporting on the complete oxidation of alcohols to the
corresponding carboxylic acids focused on either whole
[4]
cells or the combination of an ADH with an aldehyde
[5]
dehydrogenase (AldDH). Simplifying the reaction scheme
to ideally only one biocatalyst would greatly increase the
preparative value of the biocatalytic synthesis of carboxylic
acids from alcohols. In principle, the ADH-mediated oxida-
tion of aldehydes is well-known as the dismutation activity of
some ADHs. Quite surprisingly, however, this reactivity has
been largely considered a curiosity and preparative applica-
tions are, to the best of our knowledge, yet unknown.
Hence we became interested in evaluating the scope of
the ADH-catalyzed oxidation of aldehydes yielding the
corresponding carboxylic acids. As a model reaction we
choose the oxidation of (racemic) profen aldehydes. Profens
Scheme 1. The oxidative dynamic kinetic resolution of profen alde-
hydes. ADH: alcohol dehydrogenase.
[6]
[8a–c]
enantiopure profen alcohols
this approach produces the
desired product directly without further chemical oxidation.
To evaluate whether aldehyde oxidation is a common
activity amongst ADHs or rather an exception, we screened
the proprietary c-LEcta ADH collection, comprising 70
ADHs from diverse origins recombinantly expressed in
Escherichia coli for the oxidation of racemic 2-phenyl
(
2-methyl-arylpropionates), efficient nonsteroidal antiinflam-
matory drugs, are generally marketed as racemates. With the
growing demand for enantiomerically pure, active pharma-
ceutical ingredients, routes to enantiomerically pure profens
[9]
propionaldehyde. Regeneration of catalytic amounts of
[7]
+
are being increasingly investigated. To circumvent the
NAD(P) was achieved by using an H O-forming NAD(P)H-
2
[
10]
oxidase (NOX) from Lactobacillus sanfanciscensis.
ensure efficient regeneration of NAD(P) and to compensate
for the comparably poor stability of this enzyme, it was
To
+
[*] Dr. P. Kçnst, Dr. S. Kara, Dr. A. Vogel, Prof. Dr. I. W. C. E. Arends,
[11]
applied at more than 100-fold excess (based on activity).
Dr. F. Hollmann
Department of Biotechnology, Delft University of Technology
Julianalaan 136, 2628BL Delft (The Netherlands)
E-mail: f.hollmann@tudelft.nl
Out of the 70 ADHs screened, 9 showed significant accumu-
lation of the desired acid (Table 1). Both R- and one S-specific
[9]
ADHs were identified. Notably, there was a significant
Dr. H. Merkens
c-LEcta GmbH
Deutscher Platz 5, 04103 Leipzig (Germany)
background activity of the E. coli host enzymes with modest
R selectivity. Possibly, endogeneous aldehyde dehydrogen-
ases or ADHs exhibiting “dismutase activity” accounted for
this.
In analogy to the generally accepted mechanism of ADH-
catalyzed alcohol oxidation, we assumed that aldehyde
oxidation proceeds via the aldehyde hydrate form of the
aldehyde. Hence, we suspected an influence of pH on the rate
and equilibrium of the aldehyde hydrate formation. Indeed,
raising the pH generally increased the ratio of acid over
alcohol formed (Table 1). Interestingly, however, the initial
rates of the reactions were hardly influenced by the pH value,
S. Kochius, Dr. D. Holtmann
Biochemical Engineering Group, DECHEMA Research Institute
Theodor-Heuss-Allee 25, 60486 Frankfurt am Main (Germany)
Dr. R. Zuhse
Chiracon GmbH
Biotechnologiepark, 14943 Luckenwalde (Germany)
[
**] Support by the Deutsche Bundesstiftung Umwelt, ChemBioTec (AZ
13253) is gratefully acknowledged.
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
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