The synthesis and chemistry of azolenines. Part 4. Preparation and rearrangement of some 3,5-diaryl-2H-pyrrole-2,2-dicarboxylic esters

Abstract

Oxidation of diethyl 3,5-diaryl-3,4-dihydro-2H-pyrrole-2,2-dicarboxylates (3) with chloranil in refluxing xylene gives not the 3,5-diaryl-2H-pyrrole-2,2-dicarboxylates (4) as reported by an earlier group, but instead the rearranged, isomeric, 1H-pyrrole-1,2-dicarboxylates (5). 2H-Pyrroles (4) may be obtained from the dihydropyrroles (3) using DDQ in benzene at room temperature; they rearrange to the isomers (5) in boiling xylene via an acyl [1,5]-sigmatropic shift from carbon to nitrogen, a novel process in the 2H-pyrrole series. Thermal analyses indicate that the rearrangement is concerted, with negligible charge separation in the transition state. Other novel 3,5-diaryl-1H-pyrrole-2-carboxylic acid derivatives are described.