Cl[C:2]1[N:3]=[C:4]2[S:12][C:11]([S:13][CH2:14][CH2:15][CH2:16][CH2:17][CH2:18][CH2:19][CH3:20])=[N:10][N:5]2[C:6](=[O:9])[C:7]=1[CH3:8].[F-:21].[K+].S1(CCCC1)(=O)=O>O>[F:21][C:2]1[N:3]=[C:4]2[S:12][C:11]([S:13][CH2:14][CH2:15][CH2:16][CH2:17][CH2:18][CH2:19][CH3:20])=[N:10][N:5]2[C:6](=[O:9])[C:7]=1[CH3:8]A solution of 8.6 g of the thus obtained 7-chloro-2-heptylthio-6-methyl-5H-1,3,4-thiadiazolo[3,2-a]pyrimidin-5-one, 3.0 g of spray dry potassium fluoride and 32 ml of sulfolane was heated at 205°~225° C. in a stream of nitrogen for 1 and half hours. After cooling, water was added to the mixture, and the mixture was extracted with toluene. The toluene layer was collected and washed with water. The toluene phase was dried over anhydrous sodium sulfate, the solvent was distilled off. The residue was purified by column chromatography on a silica gel with an ethyl acetate/n-hexane (1:3) mixture as an eluent. After the solvent was distilled off, 5.2 g of 7-fluoro-2-heptylthio-6-methyl-5H-1,3,4-thiadiazolo[3,2-a]pyrimidin-5-one was obtained.
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