Electrocyclic and Sigmatropic Reactions as Cycloadditions

Electrocyclic and Sigmatropic reactions can be considered in terms of cycloadditions. It has been shown that it is possible if we consider sigma bond as a component of cycloaddition. Selection rules for cycloadditions can be applied to electrocyclic as well as sigmatropic reactions. Involvement of obond in these reactions is possible in following manner  

Classification of o-bond as a component of cycloaddition came out of consideration of suprafacial and antarafacial addition to double bond which can be demonstrated by ball and stick-model. During retention there is interaction of bonding lobe (large one) of a-bond with the other component of cycloaddition. During inversion interaction of smaller lobe takes place with other component of cycloaddition. 

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