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The reductive condensation of 2,5-disubstituted pyrroles White, James David

Abstract

The problem initially presented was the structural elucidation of a compound obtained when 2,5-dimethylpyrrole was subjected to conditions of acidic reduction. Previous workers had assigned a molecular formula C₁₂H₁₇N to this product and a partial structure had been put forward based on the indolenine system. In the course of this work it was found that the compound obtained by these earlier workers was the result of a reductive self-condensation of 2,5-dimethylpyrrole, and Its structure was conclusively established as 1,3,4,7-tetramethylisoindoline. The methods used in the structural elucidation of this product included elemental analysis of its derivatives, measurement of its basicity and equivalent weight, infrared and ultraviolet spectroscopic evidence, oxidative degradation, and its proton magnetic resonance spectrum. Two related isoindolines were prepared by different routes. 2,4,7-trimethylisoindoline was synthesised by methods analogous to those already known, and the ultraviolet spectrum of its methiodide, when compared with that of the methiodide from 1,3,4,7-tetramethylisoindoline, reinforced the structural assignment of the latter. 1,3-diphenyl-4,7-dimethylisoindoline was obtained by the reductive condensation of acetonylacetone with 2,5-diphenylpyrrole (which did not undergo self-condensation). The favourable result of this reaction suggested that a similar condensation may have occurred to give the 1,3,4,7-tetramethylisoindoline and also admitted the possibility of a general synthesis of substituted isoindolines by this route. An attempt was made to resolve the mechanism of the 2,5-dimethylpyrrole condensation, for which either a Diels-Alder reaction or a ring-opening process may be postulated. The failure of the dimethylpyrrole to show dienic character, even in the presence of very strong dienophiles, together with positive evidence for ring-opening and ketone-pyrrole condensation argued forcibly for the latter mechanism.

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