How Torsional Effects Cause Attack at Sterically Crowded Concave Faces of Bicyclic Alkenes

Cycloadditions of 1,3-dipoles and related species to a cis- oxabicyclo(3.3.0)octenone occur on the more sterically crowded concave face. These cycloadditions were studied experimentally by Gais and co-workers in 1998 (Eur. J. Org. Chem. 1998, 257�273) and have now been studied computationally with density functional theory (DFT). Transition states have been computed for various types of (3 + 2) cycloadditions, including diazomethane 1,3-dipolar cycloadditions, a thermally promoted methylenecyclopropane acetal cycloaddition, and a Pd-catalyzed cycloaddition of methylenecyclopropane to an oxabicyclo(3.3.0)octenone. The concave stereoselectivities arise from alkene predistortion that leads to torsional steering in the transition states.