Step A: 3,5-difluorophenol (2.6 g, 20 mmol) was dissolved in acetonitrile (50 mL) and triethylamine (14 mL, 100 mmol) was added. Magnesium chloride (3.8 g, 40 mmol) and paraformaldehyde (6.4 g, 200 mmol) were then added sequentially. The resulting inhomogeneous mixture was reacted at 60 °C with vigorous stirring for 16 hours. Upon completion of the reaction, the mixture was diluted with deionized water (200 mL) and the pH was adjusted to <2 with 1 M aqueous hydrochloric acid. the reaction mixture was extracted with ethyl acetate (200 mL). The organic layers were combined, washed with saturated brine, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure to give 2,4-difluoro-6-hydroxybenzaldehyde (2.6 g, 82% yield) as a red oily product. Mass spectrometry analysis showed m/z 157.1 [M-H]-.
