[CH2:1]([S:7][C:8]1[S:20][C:11]2=[N:12][C:13]([C:17]#[C:18][CH3:19])=[CH:14][C:15](=[O:16])[N:10]2[N:9]=1)[CH2:2][CH2:3][CH2:4][CH2:5][CH3:6].[OH:21]OS([O-])=O.[K+].[OH2:27]>CO>[CH2:1]([S:7]([C:8]1[S:20][C:11]2=[N:12][C:13]([C:17]#[C:18][CH3:19])=[CH:14][C:15](=[O:16])[N:10]2[N:9]=1)(=[O:21])=[O:27])[CH2:2][CH2:3][CH2:4][CH2:5][CH3:6]In 225 ml of methanol, 6.6 g of 2-hexylthio-7-(1-propynyl)-5H-1,3,4-thiadiazolo[3,2-a]pyrimidin-5-one was dissolved. A dispersion of 51.2 g of OXONE® suspended in 180 ml of water was added to the methanol solution, and the resulting mixture was warmed to 60°~65° C. and stirred for 50 minutes. After cooling, the mixture was extracted with chloroform, and the organic layer was washed with a sodium thiosulfate aqueous solution and water respectively, and dried over anhydrous sodium sulfate. The solvent was distilled off and the residue was purified by silica gel column chromatography using an ethyl acetate/toluene mixture as an eluent, and was recrystallized from ethanol. Thus 2.6 g of 2-hexanesulfonyl-7-(1-propynyl)-5H-1,3,4-thiadiazolo[3,2-a]pyrimidin-5-one was obtained. m.p. 122°~125° C. Yield 41%.
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