56083-42-2 4a,4b,5,8,8a,8b-Hexahydro-1,4-di(2-pyridinyl)-5,8-ethenobenzo[3,4]cyclobuta[1,2-d]pyridazine-6,7-dicarboxylic acid dimethyl ester 56083-43-3 4a,4b,5,8,8a,8b-Hexahydro-1,4-diphenyl-5,8-ethenobenzo[3,4]cyclobuta[1,2-d]pyridazine-6,7-dicarboxylic acid dimethyl ester