[C:1]([O:9]CC)(=O)[CH2:2][C:3]([O:5][CH2:6][CH3:7])=[O:4].[H-].[Na+].[CH2:14]([N:21]1[C:26]2[N:27]=[CH:28][CH:29]=[CH:30][C:25]=2[C:24](=O)[O:23]C1=O)[C:15]1[CH:20]=[CH:19][CH:18]=[CH:17][CH:16]=1.Cl>CC(N(C)C)=O>[CH2:6]([O:5][C:3]([C:2]1[C:1](=[O:9])[N:21]([CH2:14][C:15]2[CH:20]=[CH:19][CH:18]=[CH:17][CH:16]=2)[C:26]2[C:25]([C:24]=1[OH:23])=[CH:30][CH:29]=[CH:28][N:27]=2)=[O:4])[CH3:7]Diethyl malonate (0.6 mL, 4 mmol) was added slowly to a suspension of NaH (60% in mineral oil, 164 mg, 4.1 mmol) in dimethylacetamide (20 mL) and stirred at room temperature for 0.5 h under inert atmosphere. 1-Benzyl-1H-pyrido[2,3-d][1,3]oxazine-2,4-dione (2) (1 g, 4 mmol) was added to the solution and heated at 110° C. for 4 h (TLC control). The solution was cooled and poured into ice water. The pH of the solution was adjusted to 3 by cold 10% HCl. The solids formed were filtered, washed by excess water, and dried in a vacuum oven to yield 940 mg (72%) of 1-benzyl-4-hydroxy-2-oxo-1,2-dihydro-[1,8]-naphthyridine-3-carboxylic acid ethyl ester (3) as white solids. MP: 143° C.; 1H-NMR (DMSO-d6): δ 1.29 (t, J=6.9 Hz, 3H), 4.31 (q, J=6.9 Hz, 2H), 5.55 (s, 2H), 7.23 (m, 5H), 7.36 (m, 1H), 8.45 (dd, J=1.5, 7.5 Hz, 1H), 8.70 (dd, J=1.5, 7.5 Hz, 1H); EIMS: 325 (M+1), 347 (M+23).
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