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Pericyclic Reactions: Introduction

Pericyclic Reactions: Classification System and Stereochemistry

According to Woodward and Hoffmann pericyclic reactions are classified as follows:

Alternatively, reactions can be classified based on a formal definition which states that bonds arranged in a cycle are broken and formed alternately during the reaction. Given the ringsize and the number of changed bonds one can define a finite number of reactions that meet the requirements. From this set of reactions a "periodic system" of possible reactions can be generated.

Classification system of pericyclic reactions

The nomenclature of the reactions is based on the number of σ- and π-electrons participating in the reaction. Therefore, the Diels-Alder reaction is a [4π + 2π] addition and the retro-ene reaction represents a [2π + 2σ + 2π] reaction.

Diels-Alder and ene reaction
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