[CH2:1]([O:3][C:4](=[O:27])[C:5]([C:12](=[O:26])[C:13]1[CH:18]=[C:17]([F:19])[C:16]([F:20])=[C:15]([O:21][C:22](=O)C)[C:14]=1F)=[CH:6][NH:7][CH:8]([CH3:11])CO)[CH3:2].[F-].[K+]>CN(C)C=O>[CH2:1]([O:3][C:4]([C:5]1[C:12](=[O:26])[C:13]2[CH:18]=[C:17]([F:19])[C:16]([F:20])=[C:15]3[O:21][CH2:22][CH:8]([CH3:11])[N:7]([CH:6]=1)[C:14]=23)=[O:27])[CH3:2]In 30 ml of dimethylformamide was dissolved 4.30 g of the ethyl-2-(3-acetoxy-2,4,5-trifluorobenzoyl)-3-(2-hydroxy-1-methylethyl)aminoacrylate (X) thus obtained and 1.92 g (0.033 mole) of potassium fluoride was added to the mixture and the mixture was stirred at 140° to 150° C. for 2 hours. After completion of the reaction, the solvent was distilled under reduced pressure. To the residue was added water and the mixture was extracted with dichloromethane, and the organic layer was washed with water, dried and then condensed under reduced pressure. Then, the residue was washed with ethanol, and the residue was recrystallized from acetone to give 1.40 g of ethyl-9,10-difluoro-3-methyl-7-oxo-2,3-dihydro-7H-pyrido[1,2,3-de][1,4]benzoxazine-6-carboxylate (IV) as pale brown fine needle crystals.
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