Efficient intermolecular 3+2 trapping of the Nazarov intermediate with vinyl sulfides

Simple cross-conjugated dienones undergo Nazarov cyclization and in situ trapping in the presence of vinyl sulfides, yielding functionalized bicyclo[2.2.1]heptanone products. The formal 3+2 cycloaddition is presumed to be a stepwise process, but furnishes the adducts with good diastereoselectivity.