Sodium Hydroxide → 5-(Difluoromethyl)furan-2-carboxylic acid

5-Formyl-2-furancarboxylic acid (2.9 g) was dissolved in N,N-dimethylformamide (30 ml), and potassium carbonate (2.9 g) and iodoethane (3.6 g) were added. The mixture was stirred at room temperature for 12 hours, poured into ice water and extracted with diethyl ether. The extract was washed with water and saturated aqueous sodium chloride, dried over anhydrous magnesium sulfate and concentrated under reduced pressure. The residue was subjected to silica gel column chromatography, and the desired fractions were concentrated under reduced pressure to give 5-formyl- 2-furancarboxylic acid ethyl ester (1.3 g). 5-Formyl-2-furancarboxylic acid ethyl ester (1.3 g) in methylene chloride (5 ml) was slowly added dropwise to diethylaminosulfur trifluoride (DAST) (1.3 g) in methylene chloride (5 ml) at room temperature. The mixture was stirred at room temprature for 30 minutes, and water was added to the mixture. The resulting mixture was extracted with diethyl ether, and the extract was washed with water and saturated aqueous sodium chloride, dried over anhydrous magnesium sulfate and concentrated under reduced pressure. The residue was subjected to silica gel column chromatography, and the desired fractions were concentrated under reduced pressure to give 5-difluoromethyl-2-furancarboxylic acid ethyl ester (0.7 g). 5-Difluoromethyl-2-furancarboxylic acid ethyl ester (2.3 g) synthesized in this manner was dissolved in ethanol (20 ml), and 1N aqueous sodium hydroxide (15 ml) was added. The mixture was stirred at room temperature and concentrated under reduced paressure. The residue was dissolved in water and washed with diethyl ether. The aqueous layer was acidified with 1N hydrochloric acid and extracted with ethyl acetate. The extract was washed with water and saturated aqueous sodium chloride, dried over anhydrous magnesium sulfate and concentrated under reduced pressure. Precipitated crystals were collected by filtration and washed with hexane-isopropyl ether to give 5-difluoromethyl-2-furancarboxylic acid (1.2 g) as crystals.