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.