Conjugated polyenes electrocyclic ring closure

In an electrocyclic reaction, a cyclic system (ring closure) is formed through the formation of a a-bond from an open-chain conjugated polyene system at the cost of a multiple bond and vice versa (ring opening). These reactions are unimolecular in nature as the rate of reactions depends upon the... 

The Diels-Alder reaction couples the ends of two separate tt systems. Can rings be formed by the linkage of the termini of a single conjugated di-, tri-, or polyene Yes, and this section will describe the conditions under which such ring closures (and their reverse), called electrocyclic reactions, take place. Cycloadditions and electrocyclic reactions belong to a class of transformations called pericyclic (peri, Greek, around), because they exhibit transition states with a cyclic array of nuclei and electrons. 

In electrocyclic reactions as already mentioned there is ring-opening or ring-closure of polyenes. In these reactions an open chain conjugated olefinic system with nn-electrons undergoes ring-closure to a system with (n -2) n-electrons + one o-bond and converse of this process is also feasible under changed reaction conditions. Some examples of electrocyclic reactions are cited below ... 

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