Pericyclic reactions have fascinated organic chemists for many decades (1). They are characterized by the concerted formation and cleavage of multiple σ and π bonds through a single transition state. The name "pericyclic reaction" originates from the cyclic character of their transition state geometries. The concerted nature of these reactions results in an important characteristic -their stereospecificity. This property makes them essential tools in modern synthetic organic chemistry. Up to the middle of the 20th century, many examples of this class of reactions had been discovered. However, their stereospecific nature could not be understood with the available mechanistic concepts in organic chemistry. Therefore, they were also known as the class of "no mechanism" reactions (2; 3).