The cycloaddition-cycloelimination pathway to homotropilidenes - Synthesisand properties of homotropilidenes

The cycloaddition-cycloelimination reaction sequence between 3,6-disubstitu ted 1,2,4,5-tetrazines 9 and various cyclopropenes 10 provides 3,4-diazanor caradienes 11, which undergo [4+2] cycloadditions with additional cycloprop enes 10 to form tetracyclic azo compounds 12. Photolysis of these compounds , with accompanying loss of nitrogen, affords homotropilidenes (bicyclo[5.1 .0]octa-2,5-dienes) 13-26. Substituents at various positions of the homotro pilidene skeleton have been observed to exert significant influences on the Cope rearrangement rate and, in cases of unsymmetrically substituted homot ropilidenes, on the equilibrium positions.