General procedure for the synthesis of methyl 4-(chlorosulfonyl)benzoate from methanol and 4-(chlorosulfonyl)benzoyl chloride: 4-chlorosulfonylbenzoic acid (5 g, 23 mmol) was dissolved in dichloroethane (10 mL), thionyl chloride (20 mL) was added, and the reaction mixture was heated to 80 °C, and the reaction was maintained at this temperature for 2 hours. Upon completion of the reaction, the reaction mixture was concentrated by rotary evaporator to give a brown solid. The resulting solid was cooled in an ice bath for 5 min, followed by the slow addition of pre-cooled methanol (40 mL) at 0 °C with stirring, a process that lasted for 5 min. The reaction mixture was gradually warmed up to room temperature and stirring was continued for 10 minutes. Finally, ice-cold water (40 mL) was added, at which time a white solid precipitated, and the solid product was collected by filtration and dried under vacuum to afford pure methyl 4-(chlorosulfonyl)benzoate (4.5 g, 84% yield). The product was characterized by NMR (400 MHz, DMSO-d6): δ, ppm: 3.84 (s, 3H), 7.70 (d, 2H, J = 8.4Hz), 7.93 (d, 2H, J = 8.4Hz).
