Farnesene

The term farnesene refers to a set of closely related chemical compounds which are sesquiterpenes. α-Farnesene and β-farnesene are isomers, differing by the location of one double bond. α-Farnesene is 3,7,11-trimethyl-1,3,6,10-dodecatetraene and β-farnesene is 7,11-dimethyl-3-methylene-1,6,10-dodecatriene. The alpha form can exist as four stereoisomers that differ about the geometry of two of its three internal double bonds (the E and Z stereoisomers of the third internal double bond are identical). The beta form can exist as two stereoisomers about the geometry of its central double bond.

α-(E,E)-Farnesene is the most common isomer. It is found the coating of apples, and other fruits, and its oxidation by air produces compounds that are damaging to the fruit. It produces a characteristic green apple odour. The oxidation products result in injury to cell membranes which eventually result in cell death in the outermost cell layers of the fruit, resulting in a storage disorder known as scald. α-(Z,E)-Farnesene has been isolated from the oil of perilla. Both isomers are also insect pheremones.

β-Farnesene has one naturally occurring isomer. The (E)-isomer is a constituent of various essential oils. It is also released by aphids as an alarm pheromone upon death to warn away other aphids. Several plants, including the wild potato have been shown to synthesize this pheromone as a natural insect repellent.