loading . . . Accessing Medium-Sized Bridged Heterocycles via EnT-Catalyzed Intermolecular Dearomative (5 + 4) Cycloaddition of Furans and Oxazoles Medium-sized bridged heterocycles are highly attractive structural motifs in bioactive natural products and medicinal chemistry. However, their broader exploration remains limited due to the lack of concise and modular synthetic strategies, as their synthesis is challenged by unfavorable enthalpic and entropic constraints. Herein, we report an energy transfer (EnT)-catalyzed intermolecular (5 + 4) dearomative cycloaddition of furans with vinyl cyclopropanes, providing direct access to (Z)-10-oxabicyclo[5.2.1]deca-3,8-diene scaffolds in a single step. Visible light triplet sensitization unlocks a distinct diene-type 1,4-biradical activation mode in furans. This triplet-state reactivity enables intermolecular higher-order (5 + 4) cycloadditions that remain inaccessible under conventional thermal activation. Mechanistic investigations support an energy transfer pathway and provide insights into the origin of the observed regioselectivity. The resulting partially unsaturated cycloadducts offer multiple synthetic handles for downstream functionalization, enabling rapid and modular incorporation of heterobicyclo[5.2.1]alkane motifs and highlighting their potential as versatile building blocks in synthetic chemistry. https://pubs.acs.org/doi/10.1021/jacs.6c00671
