THERMAL INDOLIZATION OF ARYLHYDRAZONES

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.