General procedure for the synthesis of methyl 2-chloro-4-hydroxybenzoate from methanol and 2-chloro-4-hydroxybenzoic acid: sulfoxide chloride (SOCl2, 1.5 mL) was slowly added to a solution of methanol (10 mL) containing 2-chloro-4-hydroxybenzoic acid (5 g, 0.03 mol). The reaction mixture was gradually heated from room temperature to reflux temperature and reacted under reflux conditions for 12 hours. Upon completion of the reaction, the solvent was removed by concentration under reduced pressure to afford the target product methyl 2-chloro-4-hydroxybenzoate (Compound 132A, red solid, 5.4 g, 100% yield). The structure of the product was confirmed by 1H NMR (400 MHz, DMSO-d6): δ 10.71 (s, 1H), 7.84-7.71 (m, 1H), 6.91 (d, J = 2.4 Hz, 1H), 6.82 (dd, J = 2.4, 8.7 Hz, 1H), 3.79 (s, 3H).
