The general procedure for the synthesis of methyl 5-carboxylic acid-2-thiophenecarboxylate from dimethyl thiophene-2,5-dicarboxylate is as follows: to a mildly refluxed solution of dimethyl thiophene-2,5-dicarboxylate (0.430 g, 2.15 mmol) in dioxane/methanol (1:2, 1.5 ml) was added an aqueous sodium hydroxide solution (0.086 g, 2.15 mmol, dissolved in 0.5 ml of water). The reaction mixture was kept under stirring for 5 hours and subsequently cooled to room temperature. After adjusting the pH of the reaction solution to 2 with 1N hydrochloric acid, it was extracted with ethyl acetate (3 x 30 ml). The organic phases were combined, washed with brine (1 x 5 ml), dried over anhydrous magnesium sulfate and concentrated under reduced pressure to afford monomethyl thiophene-2,5-dicarboxylate as a white solid (0.342 g, 86% yield). The product was characterized by 1H NMR (CD3OD): δH 7.72-7.73 (d, 1H), 7.69-7.70 (d, 1H), 3.87 (s, 3H).
