Light-Initiated Four-Step Domino-Multicomponent Synthesis of Functionalized Alkylidenecyclobutanes

A four-step domino-multicomponent reaction (domino-MCR) is described for the synthesis of functionalized E-alkylidenecyclobutanes from 4-hydroxy-2-methylcyclopent-2-enone derivatives and three other simple reagents. The domino-MCR is accomplished in a single protocol, comprising a tandem photochemical [2 + 2]-cycloaddition/Norrish-I/γ-H transfer reaction followed by an acetal protection and an allylic substitution reaction. In parallel, a consecutive process has been established with distinct photochemical and nonradiative sequences. An intramolecular version of these reactions provides access to complex fused-bicyclic alkylidenecyclobutanes.