In a 500 mL three-necked flask, 150 mL of deionized water and 12 g (0.3 mol) of sodium hydroxide were added sequentially and stirred until the solid was completely dissolved. Subsequently, 100 mL of ethanol was added and the reaction system was cooled down to 20 °C. Under stirring, methyl 6-chloro-4-methyl-3,4-dihydro-3-oxo-2H-1,4-benzoxazine-8-carboxylate (0.1 mol) was slowly added and the reaction was continued with stirring for 3 hours. After completion of the reaction, the reaction mixture was cooled to below 10 °C and the pH was adjusted with dilute hydrochloric acid to 2. The precipitated solid was collected by filtration and dried to afford 6-chloro-4-methyl-3-oxo-3,4-dihydro-2H-1,4-benzoxazine-8-carboxylic acid as an off-white solid with a yield of 21.9 g and 91% yield.
