Thermal decarboxylation, 386 2- pyrrole

The main feature of the reactivity of pyrrole-2-carboxylic acids is the ease with which the carboxyl group is removed. Thermal decarboxylation is a preparatively useful reaction. 

This cyclocondensation is thought to proceed via Michael addition of deprotonated isocyanoacetate to the nitroalkene, intramolecular trapping of the resulting nitronate 61 by the isocyanide functionality (61 62) followed by reprotonation, HNO2-elimination, and tautomerization of the resulting 2H-pyrrole to the IH-pyrrole 59. On saponification and thermal decarboxylation, the 3,4-disubstituted pyrrole 60 is obtained. 

Pyrrole-2-carboxylic acid easily loses the carboxylic group thermally. Pyrrole-3-carboxylic acid and furan-2- and -3-carboxylic acids also readily decarboxylate on heating to about 200°C. Thiophene-carboxylic acids require higher temperatures or a copper-quinoline catalyst. In furans, 2-carboxylic acid groups are lost more readily than 3-carboxylic acid groups (Scheme 64). 

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