Synthesis of bicyclo[4.1.0]hept-2-enes (trinorcarenes) by photochemical reaction of bicyclo[2.2.2]oct-5-en-2-ones

Photochemical reaction of bicyclo[2.2.2[oct-5-en-2-ones has been investigated as a prelude to focused application to the synthesis of sesquiterpenes such as sesquicarene and sirenin. Diels-Alder reaction of cyclohexa-2,4-dienes, having different substituents (methylthiomethyl and methoxy) at the C-6 position, with a dienophile proceeds regio- and stereo-selectively to give bicyclo[2.2.2]oct-5-en-2-ones; their photolysis in benzene upon high-pressure Hg lamp irradiation affords decarbonylation products, bicyclo[4.1.0]hept-2-enes (trinorcarenes), stereoselectively. Replacement of the methylthiomethyl group with a 2-ethoxycarbonylvinyl group improves the sequential reaction.