Cyclization processes ionic species

Very different types of cyclization processes of isolated ionic species have been discussed in detail by Cooks90) and Hesse91). In the following chapter only the interactions of vicinally substituted functions will be described. Such an elimination/ cyclization interaction may be recognized by two facts ... 

Dipolar species have been observed in the cycloaddition of polar intermediates. Thus cyclobutanes can be formed by non concerted processes involving zwitter ionic intermediates. The combination of an electron rich alkene (enamimes, enol ethers) and an alkene having electron withdrawing groups (nitro a cyano substituted alkenes) first gives a zwitter ion which can rotate about the newly formed bond before cyclization and gives both a cis and a trans adduct. 

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