Preference for an Endo Transition State

There are a number of factors influencing cis vs. trans ring fusion. These can be grouped into three general categories preference for an endo transition state, influence of diene substitution, and influence of the bridge between the diene and the dienophile. 

The endo rule, formulated for the intermolecular Diels-Alder reaction, is not rigorously followed even in the intermolecular case (33). For intramolecular cyclizations, geometric factors (vide infra) tend to be more important. With highly activated dienophiles, and especially under acid catalysis, endo cyclization can predominate (34) [28, 29]. In the second example, the role of the acidic reaction medium in overcoming the geometric preference for an exo (chair-like) transition state is apparent. Other examples of improved endo selection on acid catalysis have been reported (7, 38). 

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The dimerization of cyclopentadiene occurs primarily through an endo transition state, as is typical for Diels-Alder reactions, (a) In the reactants, draw red and blue shaded lobes for the orbitals that have favorable secondary interactions in the diene and dienophile, causing the preference for an endo transition state, (b) Using bond-line formulas, draw curved arrows to show the flow of electrons that leads to product formation, and draw a three-dimensional formula for the product, (c) The reaction produces a racemic mixture. Show how the reactants align in three dimensions to form each enantiomer. 

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