CHLOROPEROXIDASE-CATALYZED ASYMMETRIC OXIDATIONS - SUBSTRATE-SPECIFICITY AND MECHANISTIC STUDY

The substrate specificity of chloroperoxidase from Caldaromyces fumago in a number of halide-independent reactions was investigated and the ability of this enzyme to perform benzylic hydroxylations with high en antioselectivity is revealed. The substrate repertoire of chloroperoxi dase is expanded and the enantioselectivity data for synthetically use ful epoxidations are reported. The enzyme epoxidizes straight chain al iphatic and cyclic cis-olefins in a highly stereoselective manner favo ring small unsubstituted substrates in which the double bond is not mo re than two carbon atoms from the terminal. The epoxidation of short-c hain prochiral terminal dienes proceeds with high diastereoselectivity and moderate enantioselectivity, yielding monoepoxides exclusively. U nsubstituted straight-chain terminal olefins seven carbons or longer a re epoxidized poorly. Aliphatic and aromatic alcohols are efficiently oxidized to aldehydes and acids. The utilization of radical probe subs trate trans-2-phenyl-1-methylcyclopropane revealed that the mechanism of chloroperoxidase-catalyzed hydroxylation is incompatible with the e xistence of a discrete radical intermediate and most likely proceeds v ia transfer of the oxygen atom from the high valent iron oxo intermedi ate directly to the substrate.