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Stereoisomers of Cyclic Compounds


A cycloalkene may be chiral even if it contains no asymmetric carbon. Such a cycloalkene enantiomer's chirality is the result of another chirality element, the chirality helix. (E)-cyclooctene is chiral due to its internal helical twist, which is caused by the strong ring strain of the planar form. The enantiomers of (E)-cyclooctene (enantiomer I and II in the table below) differ in their rotational sense of the helical shape. During the interconversion of these enantiomers by a conformational change, a strong ring strain is built in the planar form. As interconversion then takes place, the conformational change incurs a very high energy barrier (151 kJmol-1 ). This would explain some characteristic qualities of these enantiomers, such as their optical activity and their ability to be isolated at room temperature.


Enantiomers of (E)-cyclooctene
Enantiomer IEnantiomer II

In addition, (E,Z) isomerism of the ring's double bond and cis,trans isomerism of the substituents are common in cycloalkenes.

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