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To a 100 mL round bottom flask was added 2-chloro-3-thiophenecarboxylic acid (3 g, 18.45 mmol) and methanol (50 mL). With stirring at 0 °C, thionyl chloride (4.4 g, 37.29 mmol) was added slowly and dropwise. The reaction mixture was continued to be stirred at room temperature for 3 hours. Upon completion of the reaction, the reaction solution was concentrated under reduced pressure by rotary evaporator. The concentrated residue was purified by silica gel column chromatography using ethyl acetate/petroleum ether (1:10 to 1:8) as eluent. Finally, 3.2 g of methyl 2-chlorothiophene-3-carboxylate was obtained in 98% yield as a yellow oil.

[1] Patent: WO2017/184624, 2017, A1. Location in patent: Page/Page column 180; 181
[2] Patent: WO2018/109643, 2018, A1. Location in patent: Page/Page column 104; 106