Abstract The reactivity of sulfur dioxide toward variously substituted butadienes was explored in an effort to define the factors affecting the competition between the hetero-Diels- Alder and cheletropic additions. At low temperature (<− 70), 1-alkyl-substituted 1, 3-dienes 1 that can adopt s-cis-conformations add to SO 2 in the hetero-Diels-Alder mode in the presence of CF 3 COOH as promoter. In the case of (E)-1-ethylidene-2- ...
