Generation of the new quirogane skeleton by a vinylogous retro-Michael type rearrangement of longipinene derivatives

A new tricyclic hydrocarbon skeleton, named quirogane, was prepared by a vinylogous retro-Michael type molecular rearrangement of (4R,5S,7R,8R,9S,10R,11R)-7,8-diacetyloxy-9-mesyloxy-1-oxolongipin-2-ene (5). A remarkable difference in chemical behavior as compared to the corresponding 2,3-dihydroderivative of 5 is explained in terms of the stability of anionic intermediates, which were evaluated by AM1 calculations. The structure of the quirogane skeleton was confirmed by single crystal X-ray diffraction analysis of quirogadiene 6. A [2+2] photochemical cyclization of 6 afforded the highly strained pentacyclic sesquiterpenoid (10).