Enantioselective transesterification of a tertiary alcohol by lipase A from Candida antarctica
For the first time, the tertiary alcohol 2-phenylbut-3-yn-2-ol could be resolved by transesterification using lipase A from Candida antarctica (CAL-A). Under optimized conditions, 94%ee for the produced acetate at a conversion of 35% and an enantioselectivity of E=65 were achieved.
Introduction
Tertiary alcohols and their esters are an important class of compounds, which are found in numerous natural products (e.g. α-terpineol and linalool are important flavor compounds) and also represent a useful group of building blocks (e.g. 2-phenylbut-3-yn-2-ol and 3-methylpent-1-yn-3-ol).1., 2. Hydrolases constitute a group of versatile biocatalysts finding tremendous attention in organic synthesis3., 4. and industrial applications.5., 6., 7. However the vast majority of chiral substrates resolved have been secondary alcohols, carboxylic acids and primary alcohols. Only a few hydrolases were shown to be active on tertiary alcohols.1., 8., 9., 10., 11., 12. This is basically due to the adverse steric interactions caused by the tertiary alcohols, making them challenging and tougher candidates for both chemical and biotransformations.
Recently, we discovered that a certain amino acid motif (GGGX-motif, in single-letter amino acid code where X denotes any amino acid) located in the oxyanion binding pocket of lipases and esterases determines activity towards tertiary alcohols.13 All enzymes bearing this motif (e.g. CAL-A, lipase from Candida rugosa, pig liver esterase, acetyl choline esterases and an esterase from Bacillus subtilis) were active towards several acetates of tertiary alcohols, while enzymes bearing the more common GX-motif did not hydrolyze the model compounds.
For 2-phenylbut-3-yn-2-ol, which is an important building block,1., 2. we observed strong autohydrolysis during enzymatic hydrolysis, which circumvented high enantioselectivity in accordance with a previous study.1
In this report, we demonstrate that transesterification of 2-phenylbut-3-yn-2-ol (Scheme 1) avoids this autohydrolysis and thus it is possible to obtain enantiomerically enriched ester using CAL-A as biocatalyst. This is the first example for a highly enantioselective enzyme-catalyzed resolution of a tertiary alcohol.
Section snippets
Results and discussion
Initially eight different lipases and esterases were investigated for transesterification activity of 2-phenylbut-3-yn-2-ol with vinyl acetate in hexane (Scheme 1). As expected, one esterase (BsteE) and three lipases (RML, CAL-B and TLL), which do not contain the GGGX-motif, did not show any transesterification activity. Of the GGGX-containing lipases and esterases, CAL-A from C. antarctica immobilized on carrier 1 (CAL-A-C1) and a thermophilic esterase (PestE) from Pyrobaculum calidifontis12
Conclusion
In conclusion, it was shown that a CAL-A-catalyzed resolution of a tertiary alcohol is feasible by performing a transesterification reaction in an organic solvent. Under optimized conditions, 35% conversion and an enantioselectivity of E=65 can be achieved.
Enzymes and chemicals
Vinyl acetate (>99%), isopropenyl acetate (>99%), (R,S)-2-phenylbut-3-yn-2-ol (>98%), 2,2,4-trimethylpentane (isooctane) (99%), tert-butyl-methylether (MTBE) (>99.8%) and n-hexane (>99%) were purchased from Fluka. The ionic liquid 1-butyl-3-methyl imidazolium bis[(triflouromethyl)sulfonyl]amide, [BMIM][BTA] (>97%) was a gift from Solvent Innovation (Cologne, Germany).
Polypropylene powder Accurel EP-100 (200–1000 μm; Akzo, Obernburg, Germany) was kindly donated by Professor Patrick Adlercreutz
Acknowledgements
S.H.K. is grateful to the Alexander von Humboldt Foundation (Bonn, Germany) for the award of a fellowship. M.P. acknowledges the financial support from the EU project ENDIRPRO (HPRN-CT-2002-00239) and from the BMBF (Bonn, Germany). The authors thank H. Atomi (Kyoto University, Kyoto, Japan) for providing the gene encoding PestE and E. Henke (Greifswald University) for useful discussions.
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