[OH-].[Na+].C([O:5][C:6]([C:8]1[CH:9]=[N:10][C:11]2[C:16]([C:17]=1[Cl:18])=[N:15][C:14]([O:19][CH3:20])=[CH:13][CH:12]=2)=[O:7])C>O1CCCC1.O>[Cl:18][C:17]1[C:16]2[C:11](=[CH:12][CH:13]=[C:14]([O:19][CH3:20])[N:15]=2)[N:10]=[CH:9][C:8]=1[C:6]([OH:7])=[O:5]Sodium hydroxide (5.86 g, 140.6 mmol, 2.5 eq) is added protionwise at room temperature to a stirred solution of 4-chloro-6-methoxy-[1,5]naphthyridine-3-carboxylic acid ethyl ester (15.0 g, 56.25 mmol, 1.0 eq) in tetrahydrofuran (150 mL) and water (80 mL). After 15 hours stirring at room temperature, tetrahydrofuran is removed, the aqueous layer is cooled down to 0° C. and the pH is adjusted to 3 by the addition of a 2N hydrochloric acid aqueous solution. The resulting precipitate is collected by filtration, washed with water and dried under high vacuum to afford 4-chloro-6-methoxy-[1,5]naphthyridine-3-carboxylic acid as a light red solid (12.9 g, 96% yield).
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