Efficent and Stereoselective #beta#-Epoxidation of the 16(17)-Double Bond of Gibberelic Acid Derivatives with An Acylperoxy Radical Generated by Irradiation of #alpha#-Diketones and Oxygen

摘要

Irradiation of gibberellic acid derivatives in the presence of #alpha#-diketones in an oxygen saturated solution led ot efficient epoxidation of the 16(17)-double bond to give the 16#beta#, 17-epoxides predominantly. The observed stereoselectivity of #alpha#-to #beta#-epoxidation ranging from 0:100 on 3,13-di-O-acetylgibberellic acid methyl ester (7) through 11:89 on its 3,13-di-O-methoxymethyl congener, i.e., #beta#-selectivity, was in contrast to the #alpha#-selectiviyt of 74:26 resulted from a peracid epoxidation on 7. This clearly illustrates the radical reaction mechanism involving an acylperoxy radical as a one-oxygen transfer species, in which the stability of the diastereomeric transition states of the intermediary #alpha#-acylperoxy carbon radical determines the stereochemical course.