Can Diels-Alder Reactions Lead to trans-Fused Products? A Computational Study of the Competitive [4+2] and [2+4] Cycloaddition of Dienes to -Aryl-Substituted Cyclohexenones

The Diels-Alder reaction is one of the most interesting processes known in organic chemistry. Its one-step nature, which allows exceptional control of the stereoselectivity of the reaction, is one of its most important characteristics. However, it is also a limitation of the process, as it prevents the formation of trans products. In spite of this, it is believed that trans products can be formed in two-step Diels-Alder reactions, and there is a classic example that illustrates this possibility. In this paper we study this system by theoretical methods and have concluded that not only the reported Diels-Alder reaction follows a one-step mechanism, but also that the trans product observed by the authors can only originate by a [3,3] sigmatropic rearrangement of a previously formed Diels-Alder intermediate. The study of a model system also suggests that trans fusion in two-step Diels-Alder reactions will not happen under conventional reaction conditions as the transition states involved in such processes would have prohibitive activation energies. The large calculated values result from very poor stabilization of the zwitterionic intermediate when it adopts the conformation that allows the trans approach of the diene moiety to the activated dienophile. The analysis of the potential energy surfaces obtained by two-dimensional scans indicates that the one-step DA reaction that would lead to trans-fused products also will not occur under conventional reaction conditions.