Photocatalytic Reconstruction of Bicyclo[1.1.0]butanes to Oxygenated Bicycles.

Saturated benzene bioisosteres alleviate the constraints of aromatic planarity; however, oxygen-containing variants remain underdeveloped with limited positional diversity. Here we report a visible-light-driven oxygenation of bicyclo[1.1.0]butanes that encodes ring-open-close programmability to construct unique oxygenated bicycles, 5-oxa-2-oxobicyclo[2.1.1]hexanes (5-oxa-2-oxo-BCHex), bearing new exit vectors. Pyridine N-oxides act as oxene surrogates, effecting selective cleavage of the bridge C-C bond to generate β,γ-unsaturated 1,2-diketones that rapidly undergo a photoinduced intramolecular Paternò-Büchi [2 + 2] cycloaddition to reclose the bicycle. Although either carbonyl can engage, a reversible retro-[2 + 2] equilibrium self-corrects off-pathway adducts, funneling reactivity to 5-oxa-2-oxo-BCHexs. Notably, further pathway control enables divergence: Lewis-acid activation furnishes hydroxy-cyclopentenones, whereas sequential phototransformation affords cyclopropane-fused γ-lactones. Uncovering a new BCB reactivity, this programmed ring-open-close logic expands the oxygenated bicyclic chemical space available from a single BCB precursor and enriches the repertoire of site-diversified benzene bioisosteres.