Bifunctional reagents in organic synthesis. Total syntheses of the sesquiterpenoids (±)-pentalenene and (±)-9-<i>epi</i>-pentalenene

Abstract

Transformation of the readily available keto acetal 14 into the enone 23 was accomplished via a seven-step sequence of reactions. Reaction of 23 with the bifunctional Grignard reagent 5 in the presence of copper(I) bromide – dimethyl sulfide, followed by intramolecular alkylation of the resultant chloro ketone 24, gave the tetracyclic ketone 25. Treatment of 25 with hydrogen in the presence of platinum afforded a mixture of the ketones 27 and 28 (42:58, respectively), which was transformed into a separable mixture of (±)-pentalenene (1) (32%) and (±)-9-epi-pentalenene (30) (33%). Keywords: ±-pentalenene synthesis, ±-9-epi-pentalenene synthesis, sesquiterpenoid synthesis, methylenecyclopentane annulation, bifunctional reagents, triquinane synthesis.