Competition between hetero-Diels-Alder and cheletropic 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