General procedure for the synthesis of 5-(2-furanyl)isoxazole-3-carboxylic acid from ethyl 5-(furan-2-yl)isoxazole-3-carboxylate: 1N aqueous lithium hydroxide solution (80 mL) was slowly added to a mixed solution of THF (130 mL) and methanol (25 mL) containing ethyl 5-(furan-2-yl)isoxazole-3-carboxylate (4.14 g). The reaction mixture was stirred at room temperature for 15 hours. Upon completion of the reaction, the solvent was removed by distillation under reduced pressure. The residue was acidified by adding 1N hydrochloric acid to the residue until a solid precipitated. The solid product was collected by filtration and washed with distilled water. After drying, 3.22 g of 5-(2-furyl)isoxazole-3-carboxylic acid (yield: 90%) was obtained as a white solid. [178] 1H-NMR (acetone-d6, 200 MHz) δ: 7.90-7.86 (m, 1H), 7.19 (d, 1H), 7.00 (s, 1H), 6.77-6.73 (m, 1H).
