An Approach to Some Spiro Oxindole Alkaloids Through Cycloaddition Reactions of 3-Methylideneindolin-2-one

Abstract

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.