[Cl:1][C:2]1[CH:3]=[C:4]([C:13]([O:15]C)=[O:14])[C:5]2[O:10][CH2:9][CH2:8][N:7]([CH3:11])[C:6]=2[CH:12]=1.[OH-].[Na+].CO.Cl>O>[Cl:1][C:2]1[CH:3]=[C:4]([C:13]([OH:15])=[O:14])[C:5]2[O:10][CH2:9][CH2:8][N:7]([CH3:11])[C:6]=2[CH:12]=17.1 Grams of methyl 6-chloro-4-methyl-3,4-dihydro-2H-1,4-benzoxazine-8-carboxylate and 3.8 g of sodium hydroxide were dissolved in a mixed solvent obtained by mixing 50 ml of methanol and 100 ml of water, and the resulting solution was heated at 70° C. for 2 hours with stirring. The liquid reaction mixture was added to 300 ml of water, and 6 ml of concentrated hydrochloric acid was added thereto. The resulting insoluble substance was filtered off, washed with water, and dried, thereby obtaining 5.8 g of 6-chloro-4-methyl- 3,4-dihydro-2H-1,4-benzoxazine-8-carboxylic acid. Melting point: 202°-204° C.
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