[OH-].[Na+].[CH3:3][N:4]1[C:8]([CH3:9])=[C:7]([C:10]([O:12]CC)=[O:11])[C:6]([C:15]2[CH:20]=[CH:19][CH:18]=[CH:17][CH:16]=2)=[C:5]1[C:21]([O:23]CC)=[O:22]>O.C(O)C>[CH3:3][N:4]1[C:8]([CH3:9])=[C:7]([C:10]([OH:12])=[O:11])[C:6]([C:15]2[CH:16]=[CH:17][CH:18]=[CH:19][CH:20]=2)=[C:5]1[C:21]([OH:23])=[O:22]A solution of sodium hydroxide (101 g, 2.53 mol) in water (340 mL) was added to a solution of diethyl 1,5-dimethyl-3-phenyl-pyrrole-2,4-dicarboxylate (80 g, 0.25 mol) in ethanol (340 mL) and heated at reflux for 15 h. Ethanol was removed under reduced pressure and the residue diluted with water (200 mL) and chilled to 0° C. Concentrated hydrochloric acid (˜150 mL) was slowly added to adjust pH to ˜2, while maintaining temperature below 10° C. and the mixture was stirred for 30 min. The precipitated solid was filtered, washed with water (100 mL) and petroleum ether (200 mL) and dried under vacuum at 60° C. to afford 1,5-dimethyl-3-phenyl-pyrrole-2,4-dicarboxylic acid (60 g, 91%) as a white solid.
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