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■ Efficient and Stereoselective β-Epoxidation of the 16(17)-Double Bond of Gibberellic Acid Derivatives with an Acylperoxy Radical Generated by Irradiation of α-Diketones and Oxygen

Hideharu Seto,* Mikio Hoshino, Shozo Fujioka, Toshiro Suenaga, Takeshi Shimizu, and Shigeo Yoshida

*The Institute of Physical and Chemical Research, Wako-shi, Saitama 351-0198, Japan

Abstract

Irradiation of gibberellic acid derivatives in the presence of α-diketones in an oxygen saturated solution led to efficient epoxidation of the 16(17)-double bond to give the 16β,17-epoxides predominantly. The observed stereoselectivity of α- to β-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., β-selectivity, was in contrast to the α-selectivity 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 α-acylperoxy carbon radical determines the stereochemical course.