[CH2:1]([S:8][C:9]1[S:20][C:12]2=[N:13][C:14]([Cl:19])=[C:15]([CH3:18])[C:16](=[O:17])[N:11]2[N:10]=1)[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.[OH:21]OS([O-])=O.[K+].[OH2:27]>CO>[CH2:1]([S:8]([C:9]1[S:20][C:12]2=[N:13][C:14]([Cl:19])=[C:15]([CH3:18])[C:16](=[O:17])[N:11]2[N:10]=1)(=[O:21])=[O:27])[C:2]1[CH:3]=[CH:4][CH:5]=[CH:6][CH:7]=1Four point nine (4.9) grams of 2-benzylthio-7-chloro-6-methyl-5H-1,3,4-thiadiazolo[3,2-a]pyrimidin-5-one was dissolved in methanol. To this solution, an aqueous suspension of 69.2 g of OXONE® in 240 ml of water was added, and the mixture was stirred at 60° C. for 1 hour and 45 minutes. After cooling, the reaction mixture was extracted with chloroform, the organic layer was washed with an aqueous sodium thiosulfate solution and water respectively and dried over anhydrous sodium sulfate. The solvent was distilled off, and the residue was recrystallized from a toluene/ethanol (1:1) mixture. Thus 3.3 g of 2-benzylsulfonyl-7-chloro-6-methyl-5H-1,3,4-thiadiazolo[3,2-a]pyrimidin-5-one was obtained. m.p. 211°-213° C. Yield 62%.
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