Desymmetrization of approach to Diels-Alder reaction

The initial orientations for the two WH-allowed pathways for the reaction between butadiene and ethylene are shown at the right side of Fig. 7.1. In the chosen axis convention, the appropriate geometry for embarkation on the [As pathway is and that for [Aa +A] is C. These two groups are 

A glance at the illustration of planar cyclohexene in Fig. 7.2 triggers an immediate objection The molecule is too strained to remain in plane, but would relax spontaneously from to either the cis (CJ ) or the trans (C0 conformation. As in the case of Fig. 5.6, where neither the reactant nor the product is in its stable geometry, planar cyclohexene must be understood to be a formal construct representing both accessible conformations of the molecule. The procedure was formally justified in the paper outlining the theoretical ba- 

OCAMS makes two tacit assumptions that are fundamental to all discussions of mechanism in terms of orbital symmetry  

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Figure 7.1. Desymmetrization of approach to Diels-Alder reaction ...

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