Spiroannulation approach to pentalenene, an angular triquinane sesquiterpene

Geminal acylation of the acetal of isophorone by 1, 2-bisubstituted cyclobutene in the presence of borontrifluoride etharate yields angular triquinane plus or minus pentalenene. The spiro (4.4) nonane system, required for the formation of the final central ring, was obtained through ring cleavage and reclosure. Reduction of the double bond and ketone and dehydration after cyclization to the enone completed the synthesis.