A chemical model for a possible oxidative rearrangement in the biosynthesis of triterpenes : the rearrangement of 7α,8α- and 8,9-epoxytirucallanes

Abstract

7α,8α-Epoxytirucallane derivatives, including methyl 3α-acetoxy-7α,8α-epoxytirucall-24-en-21-oate (XXIV) have been prepared by a method involving oxidation of the 7,8-double bond of the corresponding olefin with ozone. These derivatives have been shown to rearrange when treated with boron trifluoride in benzene to give the corresponding 7α-hydroxyapotirucall-14-enes such as methyl 3α-acetoxy-7α-hydroxyapotirucall-14,24-dien-21-oate (XXXIV). This rearrangement provides a chemical model for a possible stage in the biosynthesis of the limonoid bitter principles. Treatment of the 8α,9α- and 8β,9β-epoxytirucallane derivatives (XLVIII) and (XLIX) with boron trifluoride–ether complex in benzene affords the 7,9(11)-diene (LI) from the α-epoxide, but the β-epoxide affords a product in which methyl migration has occurred. Structure (LIV) is proposed for the latter.