Biocatalytic Racemization of a-Hydroxy Ketones (Acyloins)
yeast Trichosporon cuntaneumMY 1506) acting on sub-
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strate 5 was initially postulated, but later disproven.
[
[
[
In summary, biocatalytic racemization of a range of
structurally diverse linear and cyclic acyloins was ac-
complished for the first time using whole (resting) cells
of Lactobacillus paracasei DSM 20207. Due to the
mild (physiological) reaction conditions, isomerizations
proved to be “clean” and side products were detected
only in trace amounts, if any. The fact that only marginal
racemization took place using heat-deactivated cells
proved the enzymatic character of this isomerization.
The enzyme(s) involved in this biotransformation and
their substrate tolerance are currently being investigat-
ed in detail. These results will allow us to extend our re-
cently developed one-pot two-enzyme deracemization
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strategy towards acyloins.
Experimental Section
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General Procedure for the Biocatalytic Racemization
of Acyloins
2
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Lyophilized cells (50 mg) were rehydrated in BIS-TRIS buffer
À2
(
0.5 mL, 50 mM, 10 M MgCl , pH 6) for 1 h at 428C with
2
shaking at 150 rpm. Substrate 1–6 (5 mg) was added followed
by shaking of the reaction mixture at150 rpmand 428C. After a
given time, the cells were removed by centrifugation. The su-
pernatant was extracted with ethyl acetate and the organic
phase was dried over sodium sulfate. The determination of con-
version and the enantiomeric excess was carried out by HPLC
on a chiral stationary phase (see Supporting Information, Ta-
ble S1). For HPLC analysis, the organic phase was evaporated
under reduced pressure and the residue was dissolved in HPLC
eluent.
[
[
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Supporting Information
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12] W. Hummel, M.-R. Kula, Eur. J. Biochem. 1989, 184, 1–
For the synthesis of substrates, determination of absolute con-
figuration by optical rotation, preparation of biocatalyst, and
analytical procedures see the supporting information.
1
3; for a detailed mechanistic investigation on the bioca-
talytic racemization of a-hydroxy carboxylic acids, see:
B. M. Nestl, S. M. Glueck, B. Hauer, R. Stuermer, W.
Kroutil, K. Faber, manuscript in preparation.
[
[
13] For a mechanistically related interconversion of benzoin
enantiomers by microbial stereo-inversion mediated by
Acknowledgements
[8d]
whole fungal cells see ref.
This study was performed within the Research Centre Applied
Biocatalysis in cooperation with BASF AG (Ludwigshafen)
and financial support by the FFG, the City of Graz and the Prov-
ince of Styria is gratefully acknowledged. B. Hauer and R.
Stürmer (BASF AG) are cordially thanked for their valuable
contributions.
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[15] Elimination is extremely facile and was even promoted
by exposure to neutral silica gel during column chroma-
tography.
References and Notes
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