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
作者:Hideharu Seto、Mikio Hoshino、Shozo Fujioka、Toshio Suenaga、Takeshi Shimizu、Shigeo Yoshida
DOI:10.3987/com-00-s(i)39
日期:——
Irradiation of gibberellic acid derivatives in the presence of alpha -diketones in an oxygen saturated solution led to 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 -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 alpha -acylperoxy carbon radical determines the stereochemical course.