[CH3:1][C:2]1[NH:3][C:4]2[C:9]([C:10](=O)[C:11]=1[C:12]([O:14][CH2:15][CH3:16])=[O:13])=[CH:8][CH:7]=[CH:6][N:5]=2.[OH-].[Na+].O=P(Cl)(Cl)[Cl:22]>>[Cl:22][C:10]1[C:9]2[C:4](=[N:5][CH:6]=[CH:7][CH:8]=2)[N:3]=[C:2]([CH3:1])[C:11]=1[C:12]([O:14][CH2:15][CH3:16])=[O:13]Ethyl 2-methyl-4-oxo-1,4-dihydro-1,8-naphthyridine-3-carboxylate (5.18 mg, 2.23 mmol) was dissolved in POCl3. The mixture was heated to 100° C. for 1.0 h. The mixture was allowed to cool and then it was carefully poured into ice-water (250 ml). The mixture was neutralized with solid NaOH. The product was extracted with EtOAc. The combined organic extracts were dried (MgSO4), filtered and concentrated. Purification was done by flash chromatography (CH2Cl2/MeOH 97:3→96:4) giving 531 mg (95%) of the title compound.
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