A Convenient Synthetic Route to Spiro[indole-3,4'-piperidin]-2-ones

Starling from 1-[(tert-butoxy)carbonyl]piperidine-4-carboxylic acid and 2-bromoaniline, the spiro[indole-3,4'-piperidin]-2-one system was obtained in three high-yielding steps: anilide formation, N(1)-protection, and intramolecular cyclization under Pd catalysis as the key reaction. The preparation of the corresponding 2-bromoanilide was studied. In extension, the same sequence was developed with 4-methyl- and 4-nitro-2-bromoaniline. In the key step, the NO, group led to a rather diminished yield. The transformation of the protected spiro[indole3,4'-piperidin] -2-one to the corresponding unprotected dihydroindoles is discussed.