[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].[C:18]1(P(C2C=CC=CC=2)C2C=CC=CC=2)[CH:23]=CC=C[CH:19]=1.N(C(OCC)=O)=NC(OCC)=[O:40]>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][CH2:19][CH2:18][CH2:23][OH:40])=[O:15])[C:5]2=[O:17]6-Bromo-1-ethyl-4-oxo-1,4-dihydro-1,8-naphthyridine-3-carboxylic acid (2.0 g, 6.73 mM), propane-1,3-diol (Intermediate 22, 2.047 g, 26.93 mmol) and triphenylphosphine (7.058 g, 26.93 mmol) were dissolved in tetrahydrofuran (80 mL). Diethyl azodicarboxylate (DEAD) (3.51 g, 20.02 mmol) was added and the mixture was stirred for 16 h at room temperature. The solvent was evaporated under reduced pressure and water (100 mL) was added to the residue. 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) and the product was eluted with a gradient of 50% ethyl acetate/petroleum ether to 100% ethyl acetate to afford 1.8 g (80%) of 3-hydroxypropyl 6-bromo-1-ethyl-4-oxo-1,4-dihydro-1,8-naphthyridine-3-carboxylate.
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