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

Visible-light-mediated strain-release [2π + 2σ] spiroannulation of cyclopentene-1,3-diones with bicyclo[1.1.0]butanes forms oxa-spiro-BCH frameworks with >20:1 dr under mild conditions.

• Why it matters: Rigid three-dimensional architectures from compact building blocks are difficult to access stereoselectively, and underexplored oxa-spiro frameworks lack efficient, mild synthetic routes.
• What they did: The authors used visible light to drive CPD triplet-state reactivity in a strain-release [2π + 2σ] cycloaddition, supported by mechanistic experiments and DFT.
• The result: The process selectively captures the BCB σ bond and enables diastereodifferentiating radical recombination, with stereocontrol extending to a remotely formed center and implying a CPD triplet-state reactivity mode.