An Asymmetric, Chemo‐Enzymatic Synthesis of O‐Acetylcyanohydrins

Abstract

AbstractA one‐pot chemo‐enzymatic synthesis of highly enantiomerically enriched O‐acetylcyanohydrins has been developed. The bimetallic (salen)titanium complex 1 is used to convert aldehydes into nonracemic (R)‐O‐acetylcyanohydrins with 61 to 93 % enantiomeric excess. A lipase enzyme is then used to hydrolyse the unwanted (S) enantiomer of the product, leaving (R)‐O‐acetylcyanohydrins with 80 to >99 % enantiomeric excess and in 75 to 96 % overall yield. Of ten lipase enzymes investigated, Candida antarctica lipase‐B (CAL‐B) has been shown to be the most suitable and the conditions for its use have been optimised. Although no single solvent has been found in which both catalyst 1 and CAL‐B gave high reaction rates and enantioselectivities, two procedures to allow their sequential use without purification of the O‐acetylcyanohydrin produced by catalyst 1 have been developed. In the first of these, the reaction with catalyst 1 is carried out in dichloromethane which is then removed and replaced with methyl tert‐butyl ether prior to addition of the enzyme. In the second procedure, the first reaction is carried out in concentrated dichloromethane solution, and this is then just diluted with methyl tert‐butyl ether prior to addition of the lipase. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)