Thermally induced [4+2] cycloaddition reactions of 5-ethanesulfonyl2-methylpyridazin-3(2H)-one (2) with pyrano[3,4-b]indol-3(9H)-ones (1, 4) affords isomeric pyridazino[4,5-b]carbazolones (3a,b, 5a,b), the product ratio depending on the substitution pattern of the diene. Two carbazolocarbazoles (6, 7) were obtained as side products. The antineoplastic activity of ellipticine (5,11-dimethyl-6H-p...