Preparation and rearrangement of 6a-methyl-6aH-benzo[a]carbazole and 11b-methyl-11bH-benzo[c]carbazole

Abstract

The benzo derivatives (3) and (4) of 4a-methyl-4aH-carbazole are isolable compounds. The 6a-methyl-6aH-benzo[a]carbazoles (3a and b) are formed by photolysis of the benzotriazoles (7a and b)(Scheme 3), but are transformed on further irradiation into the linear indenoquinolines (10a and b) by an aza-di-π-methane rearrangement. Compound (3a) was also prepared by bromination-dehydrobromination of its dihydro derivative (13)(Scheme 4); 11b-methyl-l 1bH-benzo[c]carbazole (4) was prepared by oxidation of its dihydro derivative (18) with benzeneseleninic anhydride (Scheme 5). Whilst compound (3a) is readily isomerised to the indenoquinoline (10a) on irradiation, compound (4) is unchanged; this is in keeping with the diradical structures proposed as intermediates in the aza-di-π-methane rearrangement. Flash vacuum pyrolysis of the benzo[a]carbazole (3a) gives benzo[a]carbazoles (21a) and (21b), together with the angular indenoquinoline (22). Flash vacuum pyrolysis of the benzo[c]carbazole (4) similarly gives benzo[c]carbazoles (23a) and (23b), and the same indenoquinoline (22)(Scheme 7). The Spiro compound (25) is proposed as a common intermediate in the conversion of both compounds (3a) and (4) into the quinoline (22).