Pericyclic Reactions: Cycloadditions and Diels-Alder Reaction
Cycloadditions: Introduction to Facial Selectivity
Woodward-Hoffmann rules predict facial selectivity with the following possibilities:
If both newly formed bonds are generated by attack from the same face of the π system the formation is called suprafacial. However, if attack occurs from opposite sides of the π system, the orientation of the newly formed bonds is called antarafacial.
The π systems of both components are nearly parallel during the supra-supra facial attack, thus allowing for optimal overlap of the orbitals. In the case of an antara-supra facial attack, the π system of the antarafacial component is initially located vertical to the suprafacial reacting partner. For steric reasons, such an arrangement is difficult to realize in most cases and the overlap is less effective. Therefore, supra-antara cycloadditions are relatively rare, even when allowed. An antara-antara overlapping configuration of reaction partners is even more difficult to achieve. Such a reaction is not known experimentally, it is a purely hypothetical example.