Phosphoryl chloride (12 g, 77 mmol) was slowly added dropwise to anhydrous N,N-dimethylformamide (8.4 g, 0.12 mol) at 0 °C and under nitrogen protection while maintaining stirring. After the dropwise addition, the reaction mixture was continued to be stirred at this temperature for 30 min. Subsequently, a solution of N,N-dimethylformamide (50 mL) of 2,4-difluorophenol (5.0 g, 39 mmol) was slowly added dropwise to the above mixture. After completion of the dropwise addition, the reaction mixture was transferred to room temperature conditions with continuous stirring for 5 hours. Upon completion of the reaction, the reaction was carefully quenched with water and extracted with ethyl acetate (3 x 150 mL). The organic phases were combined, washed sequentially with water and saturated brine, dried over anhydrous sodium sulfate, and finally concentrated under reduced pressure to afford the target product 3,5-difluoro-2-hydroxybenzaldehyde (5.5 g, 90% yield) as a yellow oil.
