Pyrrole studies. Part 35. Aerial oxidation of 4-substituted 3-amino-2,5-dimethylpyrroles. An X-ray crystallographic and spectroscopic study of the oxidation products

Abstract

The attempted preparation of ethyl 3-amino-2,5-dimethylpyrrole-4-carboxylate and 4-acetyl-3-amino2,5-dimethylpyrrole by standard ring synthesis procedures resulted in the exclusive isolation of 2-hydroxy-2H-pyrroles, as indicated by ¹H and ¹³C n.m.r. spectral data. The structure of ethyl 3-amino-2-hydroxy-2,5-dimethyl-2H-pyrrole-4-carboxylate (5a) has been confirmed by single crystal X-ray crystallographic analysis and circumstantial evidence that the 2-hydroxy-2H-pyrroles (5a,b) arise from the initially formed 1H-pyrroles (2; R = R′= Me, Z = CO₂Et or COMe) is provided by the observation that ethyl 3-amino-5-methyl-2-phenylpyrrole-4-carboxylate (7) undergoes aerial oxidation to give ethyl 3-amino-2-hydroxy-5-methyl-2-phenyl-2H-pyrrole-4-carboxylate (8).