General procedure for the synthesis of 5-chloro-2,4-dihydroxybenzaldehyde from 2,4-dihydroxybenzaldehyde: Sulfonyl chloride (14 mL, 171.2 mmol) was added slowly and dropwise to a solution of 2,4-dihydroxybenzaldehyde (20 g, 144.92 mmol) in ether (500 mL) at 0 °C and under argon protection. After the dropwise addition was completed, the reaction mixture was warmed to room temperature and stirring was continued for 30 minutes. Subsequently, the reaction mixture was poured into ice water to separate the organic layer. The organic layer was washed sequentially with water and saturated saline, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure to give the crude product. The crude product was purified by silica gel column chromatography (eluent: petroleum ether/ethyl acetate=90:10) to afford 5-chloro-2,4-dihydroxybenzaldehyde as an off-white solid (6.7 g, 26% yield, 97% purity by LC/MS). eSI-MS, m/z 170.9 [M-H] ; elemental analysis consistent with C7H5ClO3; 1H NMR (DMSO-d6, 400 MHz): δ 11.39 (1H, s), 10.87 (1H, s), 9.98 (1H, s), 7.0 (1H, s), 6.58 (1H, s).
