CC1(C)[O:7][C:6](=[O:8])[C:5](=[C:9]([OH:15])[CH:10]=[C:11]([NH:13][CH3:14])[CH3:12])[C:4](=O)[O:3]1.[F:18][B-](F)(F)F.F[B-](F)(F)F.F[N+]1C=CC=CC=1C1C=CC=C[N+]=1F>C(#N)C>[F:18][C:10]1[C:9]([OH:15])=[C:5]([C:6]([OH:7])=[O:8])[C:4](=[O:3])[N:13]([CH3:14])[C:11]=1[CH3:12]1.0 g of 2,2-dimethyl-5-(1-hydroxy-3-methylamino-2-butenylidene)-1,3-dioxane-4,6-dione was suspended in 20 ml of acetonitrile, 806 mg of N,N′-difluoro-2,2′-bipyridinium bis(tetrafluoroborate) was added thereto, and this was stirred at room temperature for 4 hours and at 50° C. for 30 minutes. The reaction mixture was added to 100 ml of ice water, and this was extracted with 100 ml of chloroform twice. The organic layer was dried with anhydrous magnesium sulfate and concentrated, and the residue was subjected to silica gel column chromatography (eluent: ethyl acetate) to obtain 480 mg of 5-fluoro-1,6-dimethyl-4-hydroxy-2-oxo-1,2-dihydropyridine-3-carboxylic acid represented by the formula:
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