Cycloadditive coupling between 3,6-diphenyl-s-tetrazine and bicyclo[6.1.0]nona-2,4,6-trienes; pericyclic synthesis of pyridazinocyclononene and pyridazinoazonine frameworks

Cycloadditive coupling between 3,6-diphenyl-s-tetrazine (1) and the bicyclo[6.1.0]nona-2,4,6-trienes (2a–c) produces the novel [7.4.0] heterobicycles (3a–c) which undergo ready oxidation to the corresponding annulated pyridazines (6); preliminary mechanistic observations suggest that the cycloadduct (2c) and, possibly, (2a)[and (2b)] are formed directly from the bicyclo[6.1.0]nona-2,4,6-triene frame and not from trapping of the corresponding cis,cis,trans,cis-mono-cyclic tetraene.