[C:1]([C:3]1[CH:21]=[CH:20][C:6]2[O:7][CH:8]([C:16]([O:18]C)=[O:17])[CH2:9][N:10]([C:11]([O:13][CH2:14][CH3:15])=[O:12])[C:5]=2[CH:4]=1)#[N:2].[OH-].[Na+].Cl>C(O)C>[C:1]([C:3]1[CH:21]=[CH:20][C:6]2[O:7][CH:8]([C:16]([OH:18])=[O:17])[CH2:9][N:10]([C:11]([O:13][CH2:14][CH3:15])=[O:12])[C:5]=2[CH:4]=1)#[N:2]To a solution of 4-ethyl 2-methyl 6-cyano-2H-benzo[b][1,4]oxazine-2,4(3H)-dicarboxylate (665 mg, 2.29 mmol) in ethanol (12.5 mL) was added NaOH (2.3 mL of 1 M, 2.30 mmol). The mixture was stirred at room temperature for 10 min. A solution of 1 N HCl was slowly added until the pH was acidic. After removing the ethanol, the aqueous layer was extracted with dichloromethane (3×20 mL), dried over Na2SO4 and concentrated in vacuo to provide 6-cyano-4-ethoxycarbonyl-2,3-dihydro-1,4-benzoxazine-2-carboxylic acid (590 mg, 2.14 mmol). 1H NMR (400.0 MHz; DMSO-d6) δ 13.57 (s, 1H), 8.12 (s, 1H), 7.50 (dd, J=2.0, 8.5 Hz, 1H), 7.15 (d, J=8.5 Hz, 1H), 5.20 (s, 1H), 4.44-4.40 (m, 1H), 4.20 (dd, J=7.2, 8.0 Hz, 2H), 3.72 (dd, J=3.3, 14.0 Hz, 1H), 1.25 (t, J=7.1 Hz, 3H).
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