9<i>H</i>‐Pyrimido[4,5‐<i>b</i>]indole‐2,4‐diones. The acid‐catalyzed cyclization of 6‐(phenylhydrazino)uracils

Abstract

Abstract 6‐(Phenylhydrazino)uracils undergo facile Fischer‐type cyclization in both N hydrochloric acid and in formic acid at reflux to give 9H‐pyrimido[4,5‐b]indole‐2,4‐diones. Yields appear to be related to substituent effects and side reactions, and these are discussed in light of the mechanism of this reaction. In the reaction of 6‐(phenylhydrazino)uracil itself with hydrochloric acid, the major competing reaction is hydrolysis to barbituric acid and phenylhydrazine, whereas in formic acid a novel cyclization occurs to give I‐phenyl‐3‐carboxamidomethyl‐1,2,4‐triazole as the major product. Nmr spectra of pyrimido[4,5‐b]indole‐2,4‐diones provide interesting examples of peri effects on proton chemical shifts.