General procedure for the synthesis of methyl 3-(chlorosulfonyl)benzoate from methanol and 3-chlorosulfonylbenzoyl chloride: 3-(chlorosulfonyl)benzoic acid chloride (2.4 g) was dissolved in dichloromethane (20 mL) and cooled to 0 °C. Pyridine (791 mg) and methanol (320 mg) were added sequentially under stirring. The reaction mixture was stirred at room temperature for 2 hours. After completion of the reaction, the solvent was removed by evaporation under reduced pressure. The residue was filtered and washed with a solvent mixture of ethyl acetate and isopropyl ether. The filtrate was concentrated under reduced pressure and the resulting residue was purified by silica gel column chromatography with hexane-ethyl acetate (9:1→4:1) as eluent to afford the target compound methyl 3-(chlorosulfonyl)benzoate (2.17 g, 92% yield) as colorless oil. The structure of the product was confirmed by 1H-NMR (CDCl3): δ 3.99 (3H, s), 7.74 (1H, t, J=8.1 Hz), 8.21-8.24 (1H, m), 8.39-8.43 (1H, m), 8.69-8.70 (1H, m).
