Competition between hetero-diels-alder and chelotropic addition of sulfur dioxide. Theoretical and experimental substituent effects on the relative stability of 3,6-dihydro-1,2-oxathiin-2-oxides (Sultines) and 2,5-dihydrothiophene-1,1-dioxides (Sulfolenes). Anomeric effects in sultine and 6-substituted derivatives.