[N:1]1[C:10]2[NH:9][C:8]3[CH:11]=[C:12]([C:15]([O:17]C)=[O:16])[CH:13]=[CH:14][C:7]=3[S:6][C:5]=2[N:4]=[CH:3][CH:2]=1.[OH-].[Na+]>CO>[N:1]1[C:10]2[NH:9][C:8]3[CH:11]=[C:12]([C:15]([OH:17])=[O:16])[CH:13]=[CH:14][C:7]=3[S:6][C:5]=2[N:4]=[CH:3][CH:2]=1To 50 ml of a solution of 2 g of methyl 10H-pyrazino[2,3-b][1,4]benzothiazine-8-carboxylate in methanol was added 0.8 g of sodium hydroxide and the resulting mixture was heated under reflux for 1 hour. After distilling off the methanol under reduced pressure, a 1 N aqueous solution of hydrochloric acid was added thereto so as to adjust the pH value to 5 to 6. Then the mixture was extracted with ethyl acetate/dichloromethane and the organic layer was dried over anhydrous magnesium sulfate. After distilling off the solvent under reduced pressure, 1.9 g of the title compound was obtained as a yellow solid.
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