Pericyclic Reactions: Cycloadditions and Diels-Alder Reaction
Cycloadditions are among the most important reactions in organic synthesis. As the name applies, this class of reactions is suitable for the preparation of cyclic compounds of different ring sizes. Additionally, most of these reactions are regio- and stereoselective. In order to utilize the high synthetic potential of cycloadditions, these reactions are frequently used to solve difficult synthetic problems even if additional synthetic steps are required.
Hoffmann suggested the following classification system to arrange the multitude of diverse reactions according to the number of participating electrons of each component:
Some of these reactions either carry special names or are named after their discoverer (Diels-Alder reaction, 1,3-dipolar cycloaddition, Paterno-Büchi reaction, etc.). An alternative classification system, suggested by Huisgen, counts the atoms in both of those components which participate in the reaction and are incorporated in the newly formed ring.
This method directly provides the ring size of the cycloadduct.