[Br:1][C:2]1[CH:3]=[C:4]2[C:9](=[N:10][CH:11]=1)[N:8]([CH2:12][CH3:13])[CH:7]=[C:6]([C:14]([O:16]CC)=[O:15])[C:5]2=[O:19].[OH-].[K+]>C(O)C>[Br:1][C:2]1[CH:3]=[C:4]2[C:9](=[N:10][CH:11]=1)[N:8]([CH2:12][CH3:13])[CH:7]=[C:6]([C:14]([OH:16])=[O:15])[C:5]2=[O:19]To a solution of ethyl 6-bromo-1-ethyl-4-oxo-1,4-dihydro-1,8-naphthyridine-3-carboxylate (Intermediate 15, 470 mg, 1.44 mmol) dissolved in ethanol (30 mL) was added 10% potassium hydroxide (2.9 mL, 2.89 mmol) and the mixture was heated to 175° C. for 1 h. The solvent was evaporated under reduced pressure to a residue. Water (20 mL) was added to the residue and the pH was adjusted to 3 with dilute hydrochloric acid (2N) to give solid precipitate which was filtered and dried to afford 340 mg (80%) of 6-bromo-1-ethyl-4-oxo-1,4-dihydro-1,8-naphthyridine-3-carboxylic acid.
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