[O:1]1[CH2:6][CH2:5][O:4][C:3]2[CH:7]=[C:8]([N:11]3[C:20]4[C:15](=[CH:16][CH:17]=[CH:18][CH:19]=4)[N:14]=[C:13]([C:21]([O:23]CC)=[O:22])[C:12]3=[O:26])[CH:9]=[CH:10][C:2]1=2.[OH-].[Na+].Cl>C(O)C>[O:1]1[CH2:6][CH2:5][O:4][C:3]2[CH:7]=[C:8]([N:11]3[C:20]4[C:15](=[CH:16][CH:17]=[CH:18][CH:19]=4)[N:14]=[C:13]([C:21]([OH:23])=[O:22])[C:12]3=[O:26])[CH:9]=[CH:10][C:2]1=24.5 g (13 mmol) of ethyl 1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-2-oxo-1,2-dihydroquinoxaline-3-carboxylate was dissolved in ethanol (100 ml), and 4.1 g (26 mmol) of a 25% aqueous solution of sodium hydroxide was added thereto. The solution was stirred for 2 hours at room temperature. The reaction mixture was concentrated under reduced pressure and water was added to the residue thus obtained. The mixture was acidified using 10% hydrochloric acid. A solid precipitated therefrom was separated by filtration, washed with water, and then dried to obtain 3.9 g (yield: 94%) of 1-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-2-oxo-1,2-dihydroquinoxaline-3-carboxylic acid as a pale yellow powder.
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