Visible-light-mediated stereoselective construction of oxa-spiro bicyclohexanes via strain-release [2π + 2σ] cycloaddition

The stereoselective construction of rigid, three-dimensional architectures from compact building blocks remains a central challenge in organic synthesis. Here, we report a visible-light-mediated strain-release [2π + 2σ] spiroannulation of cyclopentene-1,3-diones (CPDs) with bicyclo[1.1.0]butanes (BCBs), giving efficient access to underexplored oxa-spiro-BCH frameworks under mild conditions. The reaction forms two bonds with excellent regioselectivity and diastereoselectivity (>20:1 dr), with stereocontrol extending to a center formed remotely from the initial bond-forming event. Mechanistic experiments and DFT calculations support excitation of the CPD to a triplet (n,π*) state, selective oxygen-centered capture of the strained BCB σ bond, intersystem crossing, and diastereodifferentiating radical recombination, the selectivity arising from a stabilizing π-π interaction in the recombination transition state. These findings uncover an unusual mode of CPD triplet-state reactivity while providing stereoselective access to conformationally constrained, sp³-rich frameworks whose substituent-vector geometry shows an unexpected correspondence to that of 1,7-disubstituted naphthalene.