Methylene-indolines, indolenines and indoleniniums, XXII. (1) The Fischer indolization of some substituted cyclopentanones

Article abstract of DOI:10.1016/S0040-4020(01)80957-1

When submitted to the acidic conditions of the Fischer indolization reaction, the phenylhydrazone of 2,5-dimethylcyclopentanone 1 did not give the expected indolenine, 1c but carbinolamines 1d. Depending on the substitution pattern of the aromatic ring, or that of the cyclopentanone, dimers, 3j, 4j ketone 3e, or compounds 5m and 5n resulting from the cleavage of the cyclopentane ring were obtained. NaBH₃CN reduction of the reaction mixture mainly gave indolines such as 1f.