Energy-Transfer-Driven [2 + 2] Cycloaddition: Synthesis of Cyclobutane-Fused Tetracycles from Indenes and Maleimides

Molecular skeletons containing fused cyclobutane rings are of significant value in drug discovery as they can increase the fraction of sp3 carbons (Fsp3) and impart conformational rigidity. Herein, we report a visible-light photocatalytic strategy for the intermolecular [2 + 2] cycloaddition of indenes with maleimides, which enables the efficient construction of cyclobutane-fused tetracyclic skeletons. Mechanistic investigations confirm that the reaction proceeds via an energy-transfer (EnT) pathway, wherein the excited-state photosensitizer activates the maleimide, followed by cyclization with indene through a triplet biradical intermediate. This method not only establishes an efficient synthetic route toward cyclobutane-fused polycyclic compounds but also further expands the utility of EnT catalysis.