To a solution of 2-chloro-5-hydroxybenzoic acid (4 g, 23.6 mmol) in methanol (50 mL) cooled to 0 °C was added thionyl chloride (2.5 mL, 35 mmol) dropwise. The reaction mixture was stirred at room temperature for 3 hours. Upon completion of the reaction, the solvent was removed by evaporation under reduced pressure and the residue was dissolved in ethyl acetate (100 mL). The organic phase was washed sequentially with saturated sodium bicarbonate solution (3 x 50 mL), water (50 mL) and brine (50 mL), dried over anhydrous sodium sulfate and concentrated under reduced pressure. The product was white crystals in 99% yield, melting point 96-97 °C. 1H NMR (400 MHz, CDCl3) δ: 3.91 (s, 3H, OCH3), 6.91 (dd, J=8.7, 3.1 Hz, 1H, Ar-H), 7.27 (d, J=8.7 Hz, 1H, Ar-H), 7.31 (d, J=3.1 Hz, 1H Ar-H).13C NMR (100MHz, CDCl3) δ: 52.79, 118.15, 120.33, 124.67, 130.25, 132.14, 154.51, 166.77.ESI HRMS Calculated value [M+H]+: 187.0162, Measured value: 187.0155.HPLC retention Time tR = 10.618 min (100% detection at both 220 nm and 254 nm).
