Pyrolytic Generation and Rearrangement of N ‐Substituted 1,2‐Didehydrocarbazoles

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‐didehydrocarbazoles 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′‐CH 3 C‐1 cyclisation. The N ‐benzyl compound 7 mainly suffers loss of the benzyl group, but a minor product, benzo[ a ]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 13 C at C‐9a.