The Conformations of Caryophyllene

Caryophyllene, a constituent of many essential oils, include clove oil, has a trans alkene contained in a 9-membered ring. One interesting property is that it has 4 diastereoisomers possible, originating from a total of three disymmetric centres present in the molecule. Two of these are conventional chiral centres, one is present in the form of a disymmetric trans double bond. To understand why such a bond can result in two configurations, one must appreciate that (concurrent) rotation about the two C-C single bonds adjacent to the alkene is in fact restricted. This phenomenon, also known as atropisomerism, is a feature of a number of natural products, and arises from a combination of angle strain in a medium sized ring caused by the rotation, together with trans-annular steric effects. The transition state for the process in caryophyllene ketone is shown below.
Caryophyllene Ketone isoCaryophyllene Ketone
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Transition state for atropisomerisation: Barrier = 22 kcal/mol
See also
  1. C. S. Wannere and P v R. Schleyer, "How Do Ring Currents Affect 1H NMR Chemical Shifts?", Organic Letters, 2003, 5(5), 605-608.
  2. M. Clericuzio, G. Alagona, G. Giuliano; T. Caterina. "Ab Initio and Density Functional Evaluations of the Molecular Conformations of b-Caryophyllene and 6-Hydroxycaryophyllene", J. Org. Chem. 2000, 65(21), 6910-6916.

(C) H. S. Rzepa, 2003.