An approach to some spiro oxindole alkaloids through cycloaddition reactions of 3-methylideneindolin-2-one

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.