Pyrolytic generation and rearrangement of N-substituted 1,2-didehydrocarbazoles

Article abstract of DOI:10.1002/jlac.199719970920

The flash vacuum pyrolysis (900-940°C) of a series of N-R-substituted carbazole-1,2-dicarboxylic anhydrides (R = methyl, phenyl, o-tolyl, benzyl, and ethyl) leads to 1,2-didehydro-carbazoles which undergo ring expansion, cyclisation, and other reactions but which appear not to undergo ring contraction to exocyclic carbenes. Thus the N-methyl compound 4 gives phenanthridine in 27% yield, the N-phenyl compound 5 undergoes C-2′-C-1 cyclisation to indolo[3,2,1-jk]carbazole 27, and the N-o-tolyl compound 6 gives both C-6′-C-1 cyclisation and 2′-CH₃-C-1 cyclisation. The N-benzyl compound 7 mainly suffers loss of the benzyl group, but a minor product, benzo[α]carbazole 33, is probably formed by an intramolecular cycloaddition. The N-ethyl compound 8 forms mainly carbazole and phenanthridine and in addition the minor C-2′-C-1 cyclisation products pyrrolo[3,2,1-jk]carbazole 44 and its 4,5-dihydro derivative 45. The formation of phenanthridine from the N-methyl anhydride 4 has been investigated by labelling of 4 with ¹³C at C-9a. VCH Verlagsgesellschaft mbH, 1997.