Intermolecular Crossed [2 + 2] Cycloaddition Promoted by Visible-Light Triplet Photosensitization: Expedient Access to Polysubstituted 2-Oxaspiro[3.3]heptanes.

This paper describes an intermolecular cross-selective [2 + 2] photocycloaddition reaction of exocyclic arylidene oxetanes, azetidines, and cyclobutanes with simple electron-deficient alkenes. The reaction takes place under mild conditions using a commercially available Ir(III) photosensitizer upon blue light irradiation. This transformation provides access to a range of polysubstituted 2-oxaspiro[3.3]heptane, 2-azaspiro[3.3]heptane, and spiro[3.3]heptane motifs, which are of prime interest in medicinal chemistry as gem-dimethyl and carbonyl bioisosteres. A variety of further transformations of the initial cycloadducts are demonstrated to highlight the versatility of the products and enable selective access to either of a syn- or an anti-diastereoisomer through kinetic or thermodynamic epimerization, respectively. Mechanistic experiments and DFT calculations suggest that this reaction proceeds through a sensitized energy transfer pathway.