[reaction: see text] Introduction of a homochiral p-tolylsulfinyl group at the trans terminal position of an alkene induces a total diastereodifferentiation in the intramolecular cycloaddition of the latter to 3-oxidopyrylium ylide precursors. The cycloadducts can be readily desulfinylated to afford enantiomerically pure oxa-bridged bicyclo[5.3.0]decane ring systems. Theoretical calculations confirm that diastereoselectivity stems from the conformational preferences of the alkenylsulfoxide unit in the transition state of the reaction.