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However, it is also a limitation of the process, as it prevents the formation of <jats:italic>trans<\/jats:italic> products. In spite of this, it is believed that <jats:italic>trans<\/jats:italic> products can be formed in two\u2010step Diels\u2013Alder 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\u2013Alder reaction follows a one\u2010step mechanism, but also that the <jats:italic>trans<\/jats:italic> product observed by the authors can only originate by a [3,3] sigmatropic rearrangement of a previously formed Diels\u2013Alder intermediate. The study of a model system also suggests that <jats:italic>trans<\/jats:italic> fusion in two\u2010step Diels\u2013Alder 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 <jats:italic>trans<\/jats:italic> approach of the diene moiety to the activated dienophile. The analysis of the potential energy surfaces obtained by two\u2010dimensional scans indicates that the one\u2010step DA reaction that would lead to <jats:italic>trans<\/jats:italic>\u2010fused products also will not occur under conventional reaction conditions.<\/jats:p>","DOI":"10.1002\/ejoc.201201754","type":"journal-article","created":{"date-parts":[[2013,5,31]],"date-time":"2013-05-31T09:41:54Z","timestamp":1369993314000},"page":"5171-5179","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":2,"title":["Can Diels\u2013Alder Reactions Lead to <i>trans<\/i>\u2010Fused Products? A Computational Study of the Competitive [4+2] and [2+4] Cycloaddition of Dienes to \u03b1\u2010Aryl\u2010Substituted Cyclohexenones"],"prefix":"10.1002","volume":"2013","author":[{"given":"Snezhana M.","family":"Bakalova","sequence":"first","affiliation":[]},{"given":"A. 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