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

Cross-conjugated 1,4-pentadien-3-ones undergo electrocyclization to 2-oxidocyclopentenyl cations upon treatment with BF₃·OEt₂. When these reactions are carried out in the presence of electron-rich vinyl sulfides, nucleophilic trapping followed by ring-closure furnishes bridged bicyclic products that can be viewed as formal 3+2 cycloadducts. This process results in three new carbon-carbon bonds and establishes up to five new stereocenters.