For more than one century, photochemical [2+2]-cycloadditions have been used by synthetic chemists to make cyclobutanes, four-membered carbon-based rings. In this reaction, typically two olefin subunits (two π-electrons per olefin) cyclize to form two new C–C σ-bonds. Although the development of photochemical [2+2]-cycloadditions has made significant progress within the last century, research has been focused on such [2π+2π]-systems, where two π-bonds are converted into two new σ-bonds1,2. Here, we report an intermolecular [2+2]-photocycloaddition which uses bicyclo[1.1.0]butanes (BCBs) as 2σ-electron reactants3–7. This strain-release-driven [2π+2σ]-photocycloaddition reaction was realized by visible light-mediated triplet energy transfer catalysis8,9. A simple, modular, and diastereoselective synthesis of bicyclo[2.1.1]hexanes (BCHs) from heterocyclic olefin coupling partners, namely coumarins, flavones and indoles, is disclosed. Given the increasing importance of BCHs as bioisosteres – groups which convey similar biological properties to those they replace – in pharmaceutical research and considering their limited access10,11, there remains a need for new synthetic methodologies. Applying this strategy allowed us to extend the intermolecular [2+2]-photocycloadditions to σ-bonds and provides previously inaccessible structural motifs.

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