Stereoselective Epoxidation of Germacradienes by Enzymes and Selective Transannular Cyclization of Germacradienes and Their Epoxides

Abstract

Microbial transformation of germacrone (5) using Cunninghamella blakesleeana has resulted in the stereoselective attack of oxidation enzymes to give three optically active epoxides (6, 7, and 8) whose stereostructures have been elucidated by chemical and physicochemical evidence. Transannular cyclization of germacrone 1, 10-epoxide (6) and germacrone 4, 5-epoxide (7) by acids has been performed and it has been found that the former gives the eudesmanes (10 and 11) while the latter affords the guaianes (9 and 12-15). The mechanism of acid-induced transannular rearrangement of germacra-1 (10), 4-dienes and their epoxides is discussed.