3,5-difluoro-4-formylbenzoic acid.

3,5-difluoro-4-formylbenzoic acid.

To a solution of 3,5-difluorobenzoic acid (291 g, 1.84 mol) in 2-methyltetrahydrofuran (4.35 L) was added TMEDA (604 mL, 4.03 mol) at room temperature. The resulting solution was cooled to -78°C. Afterward, /?-BuLi (2.5 M in hexane) (1.77 L, 4.43 mol) was added drop-wise, during which the temperature of the mixture remained at less than -65 °C. The mixture was then stirred at -78 °C for 1.5 hr. Anhydrous MeOCHO (239 mL, 3.88 mol) was added dropwise at a rate that allowed the temperature to be maintained at less than -65 °C. The resulting solution was allowed to warm at room temperature, and then maintained a room temperature while being stirred for 18 hr. The mixture was then cooled to 0-5°C, and excess base was quenched with 6M aqueous HC1 (2.2 L, 13.2 mol). The phases were then separated, and the aqueous layer was extracted 3 times with 2- methyltetrahydrofuran (3 x 500 mL). The combined organic phases were washed with saturated brine, dried over MgS0₄, filtered, and concentrated under vacuum. The residue was dissolved in ethyl acetate (350 mL) at reflux, and cooled to room temperature.

Hexanes (480 mL) were then added, and the resulting mixture was further cooled to -15°C. The solid was collected by filtration, rinsed with hexanes, and dried under mechanical vacuum to form the title compound (122 g, 35%) as a solid. 

H NMR (300 MHz, DMSO- d₆) δ ppm 7.63-7.70 (m, 2 H), 10.23 (s, 1 H); 

MS m/z (ESI) 187.17 [M+H]⁺.