Ethanol → 4-(2-carboxyethyl)-3-methyl-5-(2-oxo-1,2-dihydroindol-3-ylidenemethyl)-1H-pyrrole-2-carboxylic acid

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4-(2-Methoxycarbonylethyl)-3-methyl-5-(2-oxo-1,2-dihydro-indol-3-ylidenemethyl)-1H-pyrrole-2-carboxylic acid ethyl ester (81.7 g) ), 56.5 g of potassium hydroxide, 200 mL of ethanol and 200 mL of water were charged to a 1 L, 3 neck round bottom flask equipped with mechanical stirring and a thermometer. The mixture was stirred at 90° C. for 90 minutes, cooled to room temperature, and acidified with acetic acid until a precipitate formed. The precipitate was collected by vacuum filtration, washed with 50 mL of water and vacuum dried to give 69.1 g (85 %) of 4-(2-carboxyethyl)-3-methyl-5-(2-oxo-1,2-dihydroindol-3-ylidenemethyl)-1H-pyrrole-2-carboxylic acid as a red solid.

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