Pyrrole carboxylic esters rearrangement

The apparent abnormal formation of ethyl pyrrole-2-carboxylate from the reaction of ethyl chloroformate and alkali metal salts of pyrrole could be accounted for by a thermal rearrangement during the isolation of the products. However, whilst this may be partly correct, an analysis of the reaction products prior to their isolation has established (77CJC4103) the formation of the thermodynamically more stable 2-carboxylic ester from the 1-isomer by a reaction sequence shown in Scheme 36. 

Elimination of water and fluoroboric acid from 1073 (Ri = H) afforded the H-pyrrole 1074, directly. However, for 1073 (Ri H), elimination of water and fluoroboric acid produced the 3//-pyrrole-3-carboxylic acid methyl ester 1075 in modest to good yield. In these cases, 1075 (R2 = H) was readily converted to the 17/-pyrrole 1076 in refluxing toluene via a [1,5] sigmatropic rearrangement of the carbomethoxy group. 

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