Chemoenzymatic deracemization of (±)-2,2-disubstituted oxiranes

The preparation of vicinal diets in up to 98% e.e. and 98% yield from the corresponding (+/-)-2,2-disubstituted epoxides was achieved via an enantioconvergent two-step hydrolysis: Thus, enantioselective enzymatic hydrolysis of the (S)-epoxide proceeded with retention of configuration furnishing the corresponding (S)-diol. In a subsequent step, the remaining (R)-oxirane was hydrolyzed during workup under acid catalysis with complete inversion of configuration yielding the (S)-diol. A detailed study of the latter reaction revealed that the experimental conditions have to be carefully chosen with respect to (i) nature of the acid, (ii) the solvent, and (iii) its water content to avoid racemization. (C) 1997 Elsevier Science Ltd. All rights reserved.