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
C. S. Wannere and P v R. Schleyer, "How Do Ring Currents
Affect 1H NMR Chemical Shifts?", Organic
Letters, 2003, 5(5), 605-608.
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.