CO.C[O-].[Na+].C(O)(=O)C.[CH:10]([NH2:12])=[NH:11].S(=O)(=O)(O)O.O.[F:19][CH:20]([CH3:28])[C:21](=O)[CH2:22][C:23](OC)=[O:24]>CO>[F:19][CH:20]([C:21]1[N:12]=[CH:10][NH:11][C:23](=[O:24])[CH:22]=1)[CH3:28]To a solution in which 9.33 g of methyl 4-fluoro-3-oxopentanoate had been dissolved in 115 ml of methanol were successively added 36.5 g of a 28% sodium methoxide methanol solution and 9.84 g of formamidine acetate at room temperature, and the mixture was stirred at 40° C. for 12 hours. To the mixture was further added 0.66 g of formamidine acetate and after stirring the mixture at 50° C. for 2 hours, the mixture was cooled to 10° C. or lower. After cooling, a mixture of 9.51 g of conc. sulfuric acid and 8.5 g of water was added to the reaction mixture. After stirring the mixture at 50° C. for 30 minutes, insolubles were removed by filtration and the filtrate was quantitated by the liquid chromatography internal standard method. As a result, 7.99 g of 6-(1-fluoroethyl)-4-pyrimidone was found to be formed (yield: 89.2%). The filtrate was concentrated under reduced pressure and the concentrated solution was recrystallized from 40 ml of 2-propanol to obtain 5.82 g of 6-(1-fluoroethyl)-4-pyrimidone.
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