Cover Image, Volume 35, Issue 5



The endo selectivity of the archetypal Diels–Alder cycloaddition between cyclopentadiene and maleic anhydride is caused by an unfavorable steric arrangement in the transition-state region of the exo pathway, as shown on the back cover. The methylene moiety of cyclopentadiene runs into the oxygen lone-pair electrons of maleic anhydride, which translates into a more destabilizing activation strain. Neither the donor–acceptor orbital interactions nor the total interaction between the deformed reactants contributes to the endo selectivity, as detailed by Israel Fernández and F. Matthias Bickelhaupt on page 371.