Symmetry-Based Selection Rules for Cyclization Reactions

8 SYMMETRY-BASED SELECTION RULES FOR CYCLIZATION REACTIONS 

As a practical matter, the second condition need not be fulfilled rigorously. If the reacting skeleton of atoms by itself has certain symmetry elements, rules based on these symmetry elements may often be expected to hold even if substituents on some of the skeletal atoms formally destroy the skeletal symmetry. This will be true when the substituents do not differ much from each other in an electronic sense. For instance, the diene 7.8-1 has C2l. symmetry. If one methyl group is replaced by an ethyl group, to give 7.8-II, the symmetry is reduced to no more than Cs. However, the electronic similarity 

Among the many types of reactions which may be treated by Woodward-Hoffmann rules, cyclizations in which open-chain olefins are converted, either thermally or photochemically, to cyclic species are especially important and can serve well to illustrate the principles involved in this type of analysis. We 

This is the simplest practical example of an olefin cyclization. The reaction is represented schematically as follows  

If the reactant molecules approach each other with their molecular planes parallel and then pass into the product, cyclobutane, with a planar C4 skeleton, there are a number of persisting symmetry elements. Until the product molecule is actually reached, the symmetry is for the final product with a planar ring the symmetry is D4h. Since Dy, is a subgroup of D4/l, the symmetry elements giving rise to DVt are the persisting ones. 

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