Photosensitized [2 + 2]-Cycloaddition of Complex Acceptor-Donor Combinations: A Regio/Diastereoselectivity Study

The photosensitized [2 + 2]-cycloaddition of chalcones, conjugated cyclopentenones, and cyclohexenones with electron-rich alkenes such as cyclic enolethers and polymethylated alkenes was investigated. While cyclic enones showed high regio- and stereoselectivity, acyclic enones resulted in a more complex product mixture containing dimers as well as four dominant regio- and diastereoisomers. This complex product mixture can be controlled by adjusting the reaction conditions such as sensitizer, solvents, or additives.

Intermolecular [2π+2σ]-photocycloaddition enabled by triplet energy transfer

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 enormous progress within the last century, research has been focused on such [2π+2π]-systems, in which two π-bonds are converted into two new σ-bonds^1,2. Here we report an intermolecular [2+2]-photocycloaddition that uses bicyclo[1.1.0]butanes as 2σ-electron reactants^3-7. This strain-release-driven [2π+2σ]-photocycloaddition reaction was realized by visible-light-mediated triplet energy transfer catalysis^8,9. A simple, modular and diastereoselective synthesis of bicyclo[2.1.1]hexanes from heterocyclic olefin coupling partners, namely coumarins, flavones and indoles, is disclosed. Given the increasing importance of bicyclo[2.1.1]hexanes as bioisosteres-groups that convey similar biological properties to those they replace-in pharmaceutical research and considering their limited access^10,11, there remains a need for new synthetic methodologies. Applying this strategy enabled us to extend the intermolecular [2+2]-photocycloadditions to σ-bonds and provides previously inaccessible structural motifs.