General procedure for the synthesis of 3-chloro-4,5-dihydroxybenzoic acid from 3-chloro-4-hydroxy-5-methoxybenzoic acid:
Step (i): tribromoborane (7.86 mL, 82 mmol) was added dropwise to a stirred solution of 3-chloro-4-hydroxy-5-methoxybenzoic acid (6.61 g, 32.6 mmol) in dichloromethane (50 mL) under nitrogen protection. The reaction mixture was continued to be stirred at 0 °C for 2 h and subsequently poured in batches into ice/brine (250 mL). The aqueous phase was extracted with ethyl acetate (2 x 150 mL) and the combined organic phases were dried over magnesium sulfate and filtered. The solvent was removed by concentration under reduced pressure to give 3-chloro-4,5-dihydroxybenzoic acid (5.11 g, 79% yield). Mass spectrum (electrospray ionization, negative mode) m/z 187 [M-H]? ; 1H NMR (400 MHz, DMSO-d?) δ: 12.69 (1H, broad single peak), 10.14 (2H, broad single peak), 7.35 (1H, double peak), 7.32 (1H, double peak).
