Pericyclic reaction

In organic chemistry, a pericyclic reaction is a type of chemical reaction between organic compounds. In the case of pericyclic reactions, the transition state of the molecule is a ring (has a cyclic geometry), and the reaction goes forward in a concerted way. Pericyclic reactions are usually rearrangement reactions. The most important groups of pericyclic reactions are:
In general, pericyclic reactions are equilibrium processes. However, it is possible to push the reaction in one direction if the product is at a significantly lower energy level. This is applying Le Chatelier's principle to a reaction involving a single molecule.
Many pericyclic reactions have similar stepwise radical processes connected with them. Chemists disagree whether some reactions are pericyclic reactions. For example, it is not definitively known whether the [2+2] cycloaddition mechanism is concerted (or may depend on the reactive system). Many pericyclic reactions have similar reactions that are metal-catalyzed. But these metal-catalyzed reactions are also not really pericyclic. The metal catalysts stabilize the reaction intermediates. So the reaction is not concerted, but rather metal-stabilized.
A large photoinduced hydrogen sigmatropic shift was utilized in a corrin synthesis performed by Albert Eschenmoser containing a 16π system. 
Due to the principle of microscopic reversibility, there is a parallel set of "retro" pericyclic reactions, which perform the reverse reaction.
Pericyclic reactions in biochemistry.
Pericyclic reactions also happen in several biological processes:
References.
Organic Chemistry