Trifluoroacetic acid → 2-carboxymethyl-4-methyl-1H-pyrrole-3-carboxylic acid ethyl ester

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A solution of 5-carboxymethyl-3-methyl-1H-pyrrole-2,4-dicarboxylic acid 2-tert-butyl ester 4-ethyl ester (6.3 g, 20 mmol) in 600 ml of dichloromethane and trifluoroacetic acid (120 ml, 1.55 mol) was heated to reflux under a nitrogen atmosphere for 2.5 hours. The reaction mixture was stirred in the dry ice-ethanol bath and added with aqueous sodium hydroxide solution (1.55 mol sodium hydroxide in 100 ml of water) while maintaining the temperature at −30° C. Upon completion of the addition, the mixture was extracted with dichloromethane (100 ml×7). The combined organic extracts were washed with brine (100 ml), dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure to give 2-carboxymethyl-4-methyl-1H-pyrrole-3-carboxylic acid ethyl ester (3.6 g, 85.7%) as a white solid.

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