Tetracyclic triterpenes. Part 2. A synthetic approach to cucurbitacins

Authors:

• Zdzisław Paryzek

Abstract

Efficient 19(10 → 9β) methyl migration leading to compounds with the cucurbitane skeleton occurs when 3β-acetoxy-9β,11β- epoxylanostan-7-one (1) is treated with boron trifluoride-diethyl ether in acetic anhydride. The major product (2) has the double bond in the 5,6-position which is common for natural cucurbitacins. The factors influencing the direction of the rearrangement are discussed. Several transformations including the removal of the 7-carbonyl group and epimerization of C-10 are described.