Pericyclic reaction - the third type of organic reaction mechanism along with ionic and radical reactions - include some of the most powerful synthetically useful reactions, like the Diels - Alder reaction, 1,3- dipolar cycloadditions, the Alder ene reaction, Claisen rearrangements, the 2,3-Wittig rearrangement, diimide reduction, sulfoxide elimination and many others. These reactions are characterised by having cyclic transition structures, and also...