The purple pigments produced by acetylation of 2-(2-oxoindolin-3-yl)glyoxylates in the presence of pyridine. Some new evidence
Abstract
As a result of the analysis of 13C n.m.r. spectra, a new structure (7) is preferred for the purple pigments obtained by the reactions of 2-(2-oxoindolin-3-yl)glyoxylates with acetic anhydride in the presence of pyridine. A mechanistic rationalisation of the formation of the pigments and their unusual rearrangement on oxidation with chromic acid are discussed.