Simple 1, 3-dienes add to SO2 in the presence of acid catalysts to generate the corresponding 3, 6-dihydro-1, 2-oxathiin-2-oxides (sultines) 1 at low temperature. This observation led us to invent a new carbon-carbon bond forming reaction in which electronrich dienes are condensed with electron-rich alkenes giving sulfinic acids that undergo stereoselective retro-ene elimination of SO2. Polypropionate fragments ...