Oxidopyrylium-Alkene [5 + 2] Cycloaddition Conjugate Addition Cascade (C3) Sequences: Scope, Limitation, and Computational Investigations

Oxidopyrylium-alkene [5 + 2] cycloaddition conjugate addition cascade (C3) sequences are described. Intramolecular cycloadditions involving terminal alkenes, enals, and enones were investigated. Substrates with tethers of varying lengths delivered five- and six-membered carbocycles and heterocycles thus demonstrating the scope and limitation of the cycloaddition–conjugate addition cascade. Several experiments and theoretical calculations provide evidence for the proposed mechanistic pathway.