Using 3-chloro-5-methoxybenzaldehyde (22.8 g, 134 mmol) as starting material, it was dissolved in dichloromethane (250 mL) and the solution was cooled to 0°C. Boron tribromide (15.8 mL, 167 mmol) was slowly added dropwise over a period of 15 min. After the dropwise addition was completed, the reaction mixture continued to be stirred at 0°C for 2 hr. Subsequently, water (50 mL) was added slowly to quench the reaction. The reaction mixture was extracted with ether (2 x 100 mL), the organic layers were combined, dried over anhydrous sodium sulfate, filtered and concentrated in vacuum. Purification by silica gel column chromatography using hexane: ethyl acetate (4:1) as eluent gave the target product 3-chloro-5-hydroxybenzaldehyde (5.2 g, 25% yield). The product was confirmed by 1H NMR (300 MHz, CDCl3): δ 9.85 (s, 1H), 7.35 (s, 1H), 7.20 (s, 1H), 7.10 (s, 1H), 3.68 (s, 1H).
