The first generation and trapping of a five-membered ring allene: 2-dehydro-3a,4,5,6,6a-pentahydropentalene

3-Bromo-3-fluorotricyclo[3.3.0.02,4]octane (8) was prepared by addition of bromofluorocarbene to bicyclo[3.2.0]hept-6-ene (5). Treatment of a solution of 8 in ether with MeLi in the presence of furan afforded the trapping product 11. The formation of the trapping product is consistent with the first generation of a five-membered ring allene, namely, 2-dehydro-3a,4,5,6,6a-pentahydropentalene (4), a reactive intermediate.