Pericyclic Reactions: Cycloadditions and Diels-Alder Reaction
Cycloadditions: Stereochemistry in Supra-supra and Supra-antara Cycloadditions
The principle of the aromaticity of transition states defines which of the two modes of attack is being preferred.
The number of electrons participating in the cycloaddition determines wether the reaction follows either a supra-supra (s+s) or a supra-antara (s+a) mechanism. A purely formal way of determining the participating electrons consists of simply counting the electron flow arrows that are used to illustrate the mechanism and multiplying by two. Each arrow moves two electrons. Examples:
The stereochemical outcome of a reaction indicates whether a (s+s) or a (s+a) approach is involved. (s+s) Cycloadditions preserve the stereochemistry of the starting materials. For example, cis-substituted alkenes produce syn products in [2+4] cycloadditions:
At first it cannot be determined how the substituents of alkene and diene relate to each other in the product since the two possibilities A and B mentioned above exist. Depending on the substitution one product can be the larger product. In this case the description endo- or exo-selectivity is being used.