Photochemically Produced Aminocyclobutanes as Masked Dienes in Thermal Electrocyclic Cascade Reactions

Cyclobutane products of a triplet sensitized enamide-alkene intramolecular [2 + 2] photocycloaddition have been shown to undergo fragmentation under acidic conditions. This lability has been exploited by inducing a complexity-generating thermal electrocyclic cascade sequence involving the in situ formation of a cyclobutene, followed by electrocyclic ring opening, Diels–Alder cycloaddition, and subsequent lactamization. This combination of excited state photochemistry and thermal electrocyclic cascade reactions allows simple planar molecules to be rapidly transformed into sp3-rich scaffolds