regioselective cycloaddition to benzonitrile oxide. In the reaction with the less reactive mesitonitrile oxide the sulfur dioxide deriving from the dimerization of the dipolarophile causes a catalytic decomposition of the nitrileoxide, which competes with the cycloaddition. Benzothiophene-1,1-dioxide and the vinyl sulfone system of 2,3-dihydrothiophene-1,1-dioxide and nitrileoxides with lower regioselectivity