Bifunctional reagents in organic synthesis. Total syntheses of the sesquiterpenoids (±)-pentalenene and (±)-9-epi-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%).