Synthesis of (22R,23R,24S)-24-Methyl-5α-cholestane-3β,6α,22,23-tetraol, a Biosynthetic Precursor of Brassinolide

A biosynthetic precursor of brassinolide, (22R,23R,24S)-24-methyl-3α-cholestane-3β,6α,22,23-tetraol was synthesized from Δ23-22-keto steroid which was obtained from the corresponding 20-carbonitrile oxide. The side chain was built up by a series of successive transformations: hydride reduction of the initial enone, epoxidation of the allyl-like alcohol, and copper(I) cyanide-catalyzed opening of the 23,24-epoxy ring. The cyclic fragment was completed by opening of the cyclopropane ring with subsequent hydroboration and oxidation of the Δ5-bond.