Substituent Effect in Pericyclic Reactions

In analyzing the individual pericyclic reactions we were so far restricted to only the simplest cases of parent nonsubstituted systems. Such a simplification is certainly valuable for the clear demonstration of the principles of the selection rules. Since, however, the majority of really observed processes proceeds on substituted systems, it is necessary to analyze the role of the substitution in the pericyclic reactions in details. In this connection there is a munber of various questions. The most important of these questions concerns the elucidation of the well known 

In cormection with such examples of the violation of the Woodward-Hoffinann rules it is necessary to say, however, that the products are not usually formed in these reactions by the symmetry-forbidden concerted mechanism but by the nonconcerted stepwise process involving the participation of appropriate biradical or zwitterionic intermediates [39-42]. 

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