Mechanistic Insights into the Factors Determining Exo-Endo Selectivity in the Lewis Acid-Catalyzed Diels-Alder Reaction of 1,3-Dienes with 2-Cycloalkenones.

We adduce evidence that the asynchronous, Lewis acid-catalyzed Diels-Alder reaction of 2-cycloalkenones with nonsimple alpha,beta-enones can proceed via transition states in which the 1,3-diene subunit is skewed, i.e., nonplanar, with profound effects on the ratio of exo and endo addition products.