Isomeric ellipticines. Part 1. Synthesis of two linear isomers of the antitumour alkaloid ellipticine

Two linear isomers of the parent 5,11-dimethyl-6H-pyrido[4,3-b]carbazole ellipticine 1a, belonging to the 6H- and 10H-pyridocarbazoles series and their methoxylated derivatives, have been synthesized by three different methods using the Skraup reaction and the Fischer–Borsche cyclization. One of these (method A) afforded an angular monomethyl-7H-pyridocarbazole 9 instead of the expected linear 6H-isomer 8a on cyclisation of 5,8-dimethyl-6-hydrazinoquinoline 21 with cyclo-hexanone via the Fischer–Borsche indole synthesis. The two other methods (B and C) afforded the desired linear isomers 7 and 8 when the Skraup reaction was performed with aminocarbazoles 26 or with tetrahydroaminocarbazoles 17 and 27 respectively, followed by aromatization of the A ring. Oxidized species were isolated after the aromatization step in method C and were tentatively assigned structures 30–33.