Abstract
Thermal cyclization of arylhydrazones in the absence of acid catalyst has been investigated. Indole was not obtained from acetaldehyde phenylhydrazone; aniline, N-ethylaniline, and an unidentified compound were isolated. Phenylhydrazones of cyclohexanone and cyclopentanone and their 2-substituted derivatives were cyclized, often in good yield, to the indole and indolenine; the nature of the substituent markedly influences the ratio of the two products. Gas–liquid chromatographic analysis of the reaction mixture from indolization of two arylhydrazones revealed no crossed products. Nitrophenylhydrazones do not cyclize in good yield, but thermal indolization of some pyridylhydrazones is a good method for the preparation of pyrrolopyridines.
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