Stereocontrolled synthesis of the brassinolide side-chain: formal synthesis of brassinolide

Abstract

The stereoselective introduction of the brassinolide side-chain, having a (20S,22R,23R,24S)-22,23-diol functionality, was examined. The catalytic reduction of (20S,22R,24Z)-6β-methoxy-23-oxo-3α,5-cyclo-5α-stigmast-24(28)-eno-29,22-lactone (11), derived from the 20-carbaldehyde (7) and 3-isopropyl-2-lithiofuran in three steps, afforded (20S,22R,23R,24S)-22-hydroxy-6β-methoxy-3α,5-cyclo-5α-stigmastano-29,23-lactone (14), an important intermediate for the stereoselective synthesis of brassinolide.