General procedure for the synthesis of 5-chlorosulfonyl-2-methylbenzoic acid from o-toluic acid: chlorosulfonic acid (50 mL, 752 mmol) was added to a 250 mL round-bottomed flask and cooled to 0 °C under nitrogen protection. O-toluic acid (10.0 g, 73.4 mmol) was added in small portions over 2 min and the reaction mixture turned brown. The reaction solution was slowly warmed to room temperature and then heated to 145°C for overnight reaction. After the reaction was completed, the mixture was cooled to room temperature and poured into ice water (~750 mL). The precipitate was collected by filtration, washed with water and dried in a vacuum oven at 80 °C for 7 h to give 15.53 g (90% yield) of an off-white solid product. The structure of the product was confirmed by 1H NMR (DMSO-d6): δ 2.53 (s, 3H), 7.26 (d, J = 7.9 Hz, 1H), 7.63 (d, J = 7.9 Hz, 1H), 8.07 (s, 1H), 13.60 (br s, 1H).
