[3,3]-Claisen rearrangements in 24alpha-methyl steroid synthesis. Application to campesterol, crinosterol, and Delta25-crinosterol side chain construction.

This paper elaborates an improved synthesis of crinosterol and campesterol starting from stigmasterol. The proposed approach is based on Claisen rearrangement of Delta23-22-allylic alcohols with various configurations of the 22-hydroxy group and geometry of the Delta23-double bond. It allows complete use of the starting steroid for preparing 24alpha-methyl derivatives. It was possible to partially control the stereochemistry at C-25. Hydrogenation of the Delta22-double bond was shown to proceed with partial isomerization of the C-24 alkyl substituent. The Ireland ester enolate variant of the Claisen rearrangement was demonstrated to be useful for preparing 24alpha-methyl steroids containing the Delta22,25-system.