The flash vacuum pyrolysis (900-940°C) of a series of N-Rsubstituted carbazole-l,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-l cycli-sation and 2'-CH3-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 cyclisationproducts pyrrolo[3,2,1-jk]carbazole 44 andits 4 5 dihydro derivative 45. The formation of phenanthridine from the N-methyl anhydride 4 has been investigated by labelling of 4 with ' :' C at C-9a.

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3-Methylideneindolin-2-one (3-methylideneoxindole) (1) was prepared in 60–89% yield by flash vacuum pyrolysis of the acetate or methyl carbonate of 3-hydroxy-3-methylindolin-2-one, and was fully characterized, but application of the same procedure to 3-hydroxy-5-methoxyindolin-2-one did not give useful yields. Cycloaddition reactions of CH2= +NR–CH2 – and of the related ylide (25) (from 1-(trimethylsilylmethyl)piperidine-2-carbonitrile and AgF) to 3-methylideneindolin-2-one (1) gave 4–20% yields of spiro oxindoles, but yields were markedly below those achieved in cycloadditions to the more electrophilic N-phenylmaleimide (70–79%). Attempted alkylation of weakly basic secondary amines, e.g. piperidine-2-carbonitrile, with Me3SiCH2Cl in dimethyl sulfoxide/K2CO3 gave only carbamates Me3SiCH2OOCNR2.

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