General procedure for the synthesis of 4-chlorothiophene-2-carboxylic acid from methyl 5-chlorothiophene-2-carboxylate: to a solution of methyl 5-chlorothiophene-2-carboxylate (10.0 g, 61.5 mmol) in anhydrous methanol (55 mL) was slowly added concentrated sulfuric acid (5-7 mL). The reaction mixture was heated to 50 °C and maintained for 18 h for the reaction, followed by concentration under reduced pressure to remove the solvent. The concentrated residue was dissolved in dichloromethane and washed several times with saturated sodium bicarbonate solution to neutralize the acid. The organic layer was dried over anhydrous sodium sulfate and concentrated under reduced pressure to afford 10.85 g of the crude product of 4-chlorothiophene-2-carboxylic acid (~100% yield), which could be used in subsequent reactions without further purification.1H NMR (400 MHz, DMSO-d6): δ 7.66 (d, 1H, J = 4.0 Hz), 7.23 (d, 1H, J = 4.4 Hz), 7.23 (d, 1H, J = 4.4 Hz) 3.81 (s, 3H).
