Isothiazoles. Part 3. Cycloadditions of diazoalkanes to 3-dialkylaminoisothiazole 1,1-dioxides. Competitive ring cleavage in 3a,4-dihydro-6aH-pyrazolo[3,4-d]isothiazole 1,1-dioxides: Formation of 2-thia-3-azabicyclo[3.1.0]hex-3-ene 2,2-dioxides and/or pyrazoles

3-Dialkylaminoisothiazole 1,1-dioxides 1 readily undergo cycloadditions with diazoalkanes 2. The reaction is characterized by high site- and regio-selectivity. Cycloadducts 3 and 4 were found to undergo straightforward thermolysis reactions at elevated temperature through two different paths characterized respectively by loss of nitrogen or sulfur dioxide and diethylcyanamide. The different transformations affording pyrazoles 6 and derivatives of the new heterocycle 2-thia-3-azabicyclo[3.1.0]hex-3-ene 2,2-dioxide 5 are discussed.