[2+2] Photocycloaddition of Symmetrically Disubstituted Alkenes to 2(5H)-Furanones: Diastereoselective Entry to 1,2,3,4-Tetrasubstituted Cyclobutanes

A study on the [2+2] photochemical cycloaddition of 1,4‐difunctionalized 2‐butenes to 2(5H)‐furanones is presented. These reactions deliver 1,2,3,4‐tetrasubstituted cyclobutanes with suitable functionalization in the four side chains for further synthetic elaboration. The effects of the substituents in both the lactone and the 2‐butene on the stereoselectivity of the photochemical reaction have been evaluated. Starting from (Z)‐2‐butenes, under photosensitized conditions, a competitive cis/trans isomerization of the olefin inhibits the cycloaddition process. It was found that the presence of a cis double bond confined within a medium‐size ring does not prevent bond rotation in the intermediate 1,4‐biradicals. The photochemical [2+2] cycloaddition of 2(5H)‐furanones to symmetrical 1,4‐difunctionalized 2‐butenes has been studied. The use of the chiral lactone 11 as the starting substrate enables stereoselective access to densely functionalized 1,2,3,4‐tetrasubstituted cyclobutanes.