General procedure for the synthesis of 3,5-dimethylisoxazole-4-carboxylic acid from ethyl 3,5-dimethylisoxazole-4-carboxylate: to a mixture of ethyl 3,5-dimethyl-4-isoxazolecarboxylate (2.4 g, 14 mmol) in a solution of tetrahydrofuran (THF, 8 mL) and methanol (MeOH, 8 mL), an aqueous solution of 5 N sodium hydroxide (NaOH) (8.5 mL ). The reaction mixture was stirred at room temperature for 8 hours. Upon completion of the reaction, the solvent was removed by distillation under reduced pressure, followed by acidification with 6 N aqueous hydrochloric acid (HCl) to pH = 2. The precipitated white solid product was filtered, washed with water, and dried to afford 3,5-dimethylisoxazole-4-carboxylic acid (2.1 g, 94.0% yield). The structure of the product was confirmed by 1H NMR (300 MHz, CDCl3): δ 2.72 (s, 3H), 2.49 (s, 3H).
