A model for triterpene side-chain synthesis. 2

Abstract

A bicyclic model for 17-keto derivatives of triterpenes, trans -1,6-dimethyl-2-methoxybicyclo[4.3.0] nonan-7-one (1), was used to explore side-chain construction concurrent with oxidation at C-8 (triterpene C-16). Oxidation of methylene derivative 2 with selenium dioxide gave β alcohol 14, and borohydride reduction of the corresponding ketone ( 15) led to the α epimer 16. Each allylic alcohol ( 14 and 16) reacted stereospecifically with phenylsulfenyl chloride to give, respectively, diastereoisomeric sulfoxides 20 and 19. The latter yielded a conjugate base that was methylated selectively to give sulfoxide 21. Desulfuration of 21 with trimethylphosphite generated the ethylidene α alcohol 22, and following PDC oxidation, E enone 25.This enone was a minor product from an alternative synthesis beginning with ethylidene derivative 3. Treatment of 3 with selenium dioxide followed by PDC gave the isomeric Z enone 24 as the major product. Reaction of 24 with lithium bis-4-methyl-3-pentenylcuprate proceeded by a facial-selective conjugate addition to the euphane CD model 26. The E isomer 25, under similar conditions, gave only an unresolved mixture. These results are compared with similar steroid reactions reported by Trost and Schmuff 11.