Investigation of oxidopyrylium–alkene [5+2] cycloaddition conjugate addition cascade (C3) sequences
Novel oxidopyrylium–alkene [5+2] cycloaddition conjugate addition cascade (C3) sequences are described. Various acetoxypyranone–alkenes with pendant nucleophiles undergo [5+2] cycloaddition followed by conjugate addition from the concave face of the intermediate pyranone toward bridged, tetracyclic ethers. In several cases, 3 new rings, 4 new bonds, and 6 new contiguous stereocenters are constructed with excellent diastereoselectivity. Finally, an asynchronous concerted reaction pathway is proposed to explain the high diastereoselectivity of the oxidopyrylium–alkene [5+2] C3.