It has been established that the interaction of diazomethane with 4,5-dialkyl-substituted 3-oxo-(2H)-isothiazole 1,1-dioxides proceeds in two stages. Initially alkylation of the sulfonimide nitrogen atom and the carbonyl group oxygen atom occurs (in a ratio of similar to 3:2), then there is a regioselective cycloaddition of diazomethane at the C=C double bond with the formation of the corresponding N-methyloxoisothiazolopyrazolines and 3-methoxyisothiazolopyrazolines.