Fischer Indolization and Its Related Compounds. XXIII. Fischer Indolization of Ethyl Pyruvate 2-(2, 6-Dimethoxyphenyl)phenylhydrazone

In order to clarify the mechanism of Fischer indolization of 2-methoxyphenylhydrazones, Fischer indolization of ethyl pyruvate 2-(2-methoxyphenyl)phenylhydrazone (2) was carried out with hydrochloric acid in ethanol and zinc chloride in acetic acid. The reactions proceeded smoothly to give N-arylindoles (11-14) and some chlorinated diphenylamine derivatives (8-10) as by-products. Consideration of the indole products revealed that the Fischer indolization proceeded mainly at the unsubstitued phenyl nucleus rather than at the 2-methoxyphenyl nucleus. This result is inconsistent with the previous result that Fischer indolization of diarylhydrazones proceeded at the electron-richer nucleus. The structures of the diphenylamines were determined by chemical means and the mechanism of their formation is discussed.