[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:17].[CH2:18](O)[CH2:19][OH:20].C1(P(C2C=CC=CC=2)C2C=CC=CC=2)C=CC=CC=1.N(C(OCC)=O)=NC(OCC)=O>O1CCCC1>[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][CH2:18][CH2:19][OH:20])=[O:15])[C:5]2=[O:17]6-Bromo-1-ethyl-4-oxo-1,4-dihydro-1,8-naphthyridine-3-carboxylic acid (Intermediate 22, 1.7 g, 5.72 mmol), ethane-1,2-diol (1.91 g, 34.34 mmol) and triphenylphosphine (9.0 g, 34.34 mmol) were dissolved in tetrahydrofuran (100 mL). Diethyl azodicarboxylate (DEAD) (4.97 g, 28.61 mol) was added drop wise and the mixture was stirred for 16 h at room temperature. The mixture was concentrated under reduced pressure, and water (100 mL) was added. The aqueous mixture was extracted with ethyl acetate (2×100 mL) and the combined organic layers were dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure to give a crude compound. The crude compound was purified by flash column chromatography over silica gel (100-200 mesh) using eluent gradient of ethyl acetate/petroleum ether: 1:1 to 100:0 to afford 1.6 g (82%) of 2-hydroxyethyl 6-bromo-1-ethyl-4-oxo-1,4-dihydro-1,8-naphthyridine-3-carboxylate.
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