Spiroannulation approach to pentalenene, an angular triquinane sesquiterpene

Abstract

The route to the angular triquinane (±)-pentalenene (1a) followed from an efficient geminal acylation of the acetal of isophorone or its homolog 6b by 1,2-bis((trimethylsilyl)oxy)cyclobutene (3) in the presence of boron trifluoride etherate. Ring cleavage and reclosure (9→11) secured the required spiro[4.4] nonane system with a pendant ketone function in place for the closure of the final, central ring. The relative stereochemistry at C-9 was established during hydrogenation over palladium of the C-3 double bond of 11, but the facial selectivity was determined by the mode of reduction of the conjugated, orthogonal double bond at C-6 of 11. After cyclization to the enone 17b, the synthesis was completed by reductions of both the double bond and the ketone and simple dehydration. The 9-epi isomer (26) of (±) pentalenene was synthesized