General procedure for the synthesis of methyl 2-methyl-3-thiophenecarboxylate from methanol and 2-methyl-3-thiophenecarboxylic acid: thionyl chloride (200 mL, 2.76 mol) was added slowly and dropwise to a solution of 2-methyl-3-thiophenecarboxylic acid (100 g, 703 mmol) in methanol (500 mL). After stirring the reaction mixture under reflux conditions for 3 h, it was concentrated under reduced pressure. The concentrated residue was diluted with dichloromethane, washed sequentially with water, saturated aqueous sodium bicarbonate solution and brine, and then dried over anhydrous sodium sulfate. After filtration, the solvent was removed by concentration under reduced pressure to give 105 g (96% yield) of methyl 2-methyl-3-thiophenecarboxylate as a brown oil. Its NMR hydrogen spectrum (400 MHz, CDCl3) data were as follows: δ 7.38 (d, J = 5.4 Hz, 1H), 6.98 (d, J = 5.4 Hz, 1H), 3.85 (s, 3H), 2.74 (s, 3H); Mass Spectrometry (ESI) m/z: 157 (M + H)+.
