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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.

Fig.1
Ene reaction

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:

Fig.2
Ester pyrolysis
Fig.3
Chugaev elimination
Fig.4
Cleavage of β-ketocarboxylic acids
Fig.5
Pyrolysis of β-hydroxyketones
Fig.6
Cope elimination

3D Animation of the Chugaev elimination

Eliminations proceed stereoselectively and yield syn compounds.

Fig.7
syn-Elimination
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