Enantioselective N-Heterocyclic Carbene Catalyzed Diene Regenerative (4 + 2) Annulation.

An enantioselective N-heterocyclic carbene (NHC)-catalyzed diene regenerative (4 + 2) annulation has been achieved through the use of highly nucleophilic morpholinone-derived catalysts. The reaction proceeds with good to excellent yields, high enantioselectivity (most >92% ee), and good diastereoselectivity (most >7:1). The generality of the reaction is high, with 19 examples reported. The utility of the products has been examined with subsequent derivatization in Diels-Alder reactions using electron-poor dienophiles. Furthermore, interception of the proposed β-lactone intermediate has been achieved, allowing the synthesis of compounds bearing four contiguous stereocenters with high levels of enantio- and diastereoselectivity.