STEREOSELECTIVE EPOXIDATION OF ALLYLIC CARBAMATES WITH M-CHLOROPERBENZOIC ACID - THE ROLE OF COOPERATIVE COORDINATION

Stereoselective epoxidations of a series of allylic carbamate methyl e sters 1a-d, homoallylic alcohols 1e-h, and acetates 1i-l have been per formed using m-chloroperbenzoic acid (m-CPBA) as epoxidizing agent. Th roughout, the formation of the three epoxides was favored. This select ivity is probably due to a directing effect of the carbamate group whi ch forms a hydrogen bond to the peracid. However, the threo/erythro is omeric ratio is also dependent on steric interactions and on the possi bility of cooperative coordination of the peracid to other suitably po sitioned functionalities such as allylic methyl ester, homoallylic alc ohol, and acetate groups. The results of the present study indicate th at the methyl ester is a weaker directing group than the carbamate. Ho wever the directing effect of the methyl ester is stronger than that o f the homoallylic alcohol and acetate groups. A thermodynamic study of the epoxidation of two epimeric carbamate esters giving considerably different isomeric product ratios (9:1 compared to 2:1) indicates that the order in the transition state structure influences the isomeric r atios.