Fischer indolisation and related compounds. Part 21. Direction on the cyclisation in the Fischer indolisation of ethyl pyruvate 2-(p- or m-substituted phenyl)phenylhydrazones

Abstract

The Fischer indolisation of three kinds of ethyl pyruvate 2-(p- or m-substituted phenyl)phenylhydrazones (8a–c) was carried out with hydrogen chloride in ethanol or under sigmatropic conditions, in order to clarify chemically the mechanism of the first cyclisation step of Fischer indolisation. The reaction under acidic conditions took place predominantly on the electron-enriched benzene nucleus whereas the reaction under sigmatropic conditions took place with lower regioselectivity and in lower yield than the former. These results indicate to us that Fischer indolisation is catalysed by acid and proceeds by electrophilic attack of the enehydrazine entity on the aromatic nucleus.