Enantioselective Synthesis of Chiral Medium-Sized Cyclic Compounds via Tandem Cycloaddition/Cope Rearrangement Strategy

The
nine-membered ring-bearing bicyclo[5.2.2]­tetrahydrooxonines
frameworks have enantioselectively been constructed via a tandem [3
+ 2] cycloaddition/Cope rearrangement reaction of vinylethylene carbonates
(VECs) with coumalates or pyrones. Under mild conditions, palladium-catalyzed
asymmetric tandem reaction of various substituted VECs and coumalates
or pyrones proceeds smoothly to produce the corresponding medium-sized
heterocyclic compounds in high yields with very high enantioselectivities.
Moreover, the reaction on the gram scale and further diverse transformations
of the products were workable. The reaction mechanism was investigated
through control experiments and DFT calculations, which show the reaction
proceeds via a tandem [3 + 2] cycloaddition/Cope rearrangement pathway
rather than via a [5 + 4] cycloaddition pathway.