H-atom near to the SO2 group give easily an intramolecular ring-closure rection by action of potassiumt-butoxide producing the 3-amino-4,5-dihydro-4-hydroxy-isothiazole S,S-dioxides 4. Compounds 4 are transformed by thionyl chloride into the corresponding chloro-derivatives 5 which in turn are dehydrochlorinated by potassium carbonate to substituted 3-amino-isothiazole S,S-dioxides 6.