Efficient Synthesis of (22R, 23R, 24S)-22, 23-Isopropylidenedioxy-5α-ergost-2-en-6-one, a Key Intermediate in the Preparation of Brassinolide

Abstract

(22R, 23R, 24S)-22, 23-Isopropylidenedioxy-5α-ergost-2-en-6-one 2b is an important intermediate of brassinolide. We found that the enone 2b can be prepared by transformation of (22R, 23R, 24S)-3α, 5-cyclo-22, 23-isopropylidenedioxy-5α-ergostan-6-one 5b with catalytic amount of both p-TsOH and NaBr in DMF under reflux. 5b was prepared from (22R, 23R, 24S)-3α, 5-cyclo-22, 23-dihydroxy-6β-methoxy-5α-ergostane 9b or a 6β-benzyloxy compound 9c, which was obtained in a manner similar to Mori's brassinolide synthesis. The enone 2b was eventually prepared via a benzyl ether 9c from stigmasterol 3a in a 15.5% yield in 11 steps.