[CH3:1][C:2]1[S:6][C:5]([S:7](=[O:10])(=[O:9])[NH2:8])=[C:4]([C:11]([OH:13])=O)[CH:3]=1.P(Cl)(Cl)(Cl)(Cl)Cl>C1(C)C=CC=CC=1>[CH3:1][C:2]1[S:6][C:5]2[S:7](=[O:10])(=[O:9])[NH:8][C:11](=[O:13])[C:4]=2[CH:3]=1A mixture of 0.5 gm (2.26 millimols) of 5-methyl-2-sulfamoyl-thiophene-3-carboxylic acid, 0.5 gm (2.2 millimols) of phosphorus pentachloride and 30 ml of anhydrous toluene was refluxed for 9 hours. The mixture was worked up in analogy to Example 1 and yielded 0.41 gm (89% of theory) of 5-methyl-thieno[3,2-d]isothiazole-3-(2H)-one-1,1-dioxide. M.p. 231° C. (from methanol).
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