Pericyclic Reactions: Further Examples
Pericyclic Reactions: Ene- and Retro-Ene Reactions
The ene reaction is a frequent and mostly interfering side reaction during cycloadditions. Dienophiles (electron-poor alkenes) can add to double bonds with abstraction and migration of an allylic hydrogen.
Using the reaction between optically active 3-phenyl-but-1-ene and maleic acid anhydride as an example it could be shown that the ene reaction takes place in a suprafacial manner. However, many ene reactions proceed nonstereospecifically via diradical intermediates. Sometimes retro-ene reactions, the reverse of ene reactions, are of synthetic value in stereoselective eliminations:
Eliminations proceed stereoselectively and yield syn compounds.