A MODEL FOR TRITERPENE SIDE-CHAIN SYNTHESIS. 2

Article abstract of DOI:10.1016/S0040-4020(01)85880-4

A bicyclic model for 17-keto derivatives of triterpenes, trans-1,6-dimethyl-2-methoxybicyclo<4.3.0>nonan-7-one, 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.