Site- and stereo-selectivity in the reaction of bicyclo[4,2,0]octa-2,4-dienes and related 2,4,7-trienes with 3-chloroperbenzoic acid: the formation ofacid-sensitive ene-epoxides and their conversion into 1,4-epoxides

Abstract

Controlled site selective peroxidation of the bicyclo[4,2,0]octa-2,4,7-trienes (1a), (1b) or the related 2,4-diene (9) by means of 3-chloroperbenzoic acid in a basic two-phase medium allows the isolation of ene epoxides; these are especially sensitive to acid and rearrange to 1,4-oxides upon treatment with commercial (D)chloroform. Regular 3-chloroperbenzoic acid peroxidation conditions yield the 1,4-oxides directly. A carbonium mechanism is proposed to account for the interconversion of the initially formed 1,2-oxides into the more stable 1,4-oxides; the benzocyclobutene and methylene alcohols isolated in some cases are accommodated within this mechanism. Structure determinations draw heavily on ¹³C n.m.r. spectroscopy, especially to distinguish between epoxides and 1,4-oxides. The X-ray structure of a key 1,4-Coxide (11) is presented. The concept of the steric bulk of a π-cloud is discussed in terms of its role in controlling the stereoselectivity of the epoxidation reaction.