[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. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.