[CH2:1]([O:3][C:4]1[CH:5]=[C:6]2[C:11](=[N:12][C:13]=1[CH3:14])[N:10]=[CH:9][C:8]([C:15]([O:17][CH2:18][CH3:19])=[O:16])=[C:7]2[OH:20])[CH3:2].O.[C:22]([OH:26])(=[O:25])[CH:23]=O>FC(F)(F)C(O)=O>[CH2:1]([O:3][C:4]1[C:13]([CH:14]=[CH:23][C:22]([OH:26])=[O:25])=[N:12][C:11]2[C:6]([CH:5]=1)=[C:7]([OH:20])[C:8]([C:15]([O:17][CH2:18][CH3:19])=[O:16])=[CH:9][N:10]=2)[CH3:2]A mixture of ethyl 6-ethoxy-4-hydroxy-7-methyl-1,8-naphthyridine-3-carboxylate (2.8 g), glyoxylic acid monohydrate (1.5 g) acetic acid (20 ml) and trifluoroacetic acid (10 ml) was heated at 90°-95° C. for 2 hours, with stirring. The mixture was evaporated down under reduced pressure. The residue was triturated with hot ethanol and filtered to give 3-(3-ethoxy-6-ethoxycarbonyl-5-hydroxy-1,8-naphthyridin-2-yl)acrylic acid, m.p.>280° C.
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