[CH2:1]([O:3][C:4]1[C:8]([Cl:9])=[C:7](Cl)[S:6][N:5]=1)[CH3:2].[CH3:11][S:12][O-:13].[Na+].CN(C=[O:19])C>C(OCC)C>[CH2:1]([O:3][C:4]1[C:8]([Cl:9])=[C:7]([S:12]([CH3:11])(=[O:19])=[O:13])[S:6][N:5]=1)[CH3:2]To a solution of 3-ethoxy-4,5-dichloroisothiazole (19.8 g, 0.10 mol) in DMF (50 ml) under a nitrogen atmosphere was added sodium methanesulfenate (20.4 g, 0.20 mol). The mixture was warmed to 100° C. and stirred for two days. The reaction mixture was then cooled, diluted with diethyl ether, washed with water, dried over anhydrous sodium sulfate, filtered, and concentrated in vacuo. The residue was chromatographed on a silica gel column, eluting with chloroform, to give 7.1 g of the title compound, mp 85°-87° , crystallized from cyclohexane. A by-product, 3-ethoxy-4,5-bis(methylsulfonyl)isothiazole (3.9 g) was also obtained, mp 158°-160° (crystallized from cyclohexane).
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