Sesquiterpenes are a class of terpenes that consist of three isoprene units and have the molecular formula C15H24. Like monoterpenes, sesquiterpenes may be acyclic or contain rings, including many unique combinations. Biochemical modifications such as oxidation or rearrangement produce the related sesquiterpenoids.
Biosynthetically, when geranyl pyrophosphate reacts with isopentenyl pyrophosphate, the result is the 15-carbon farnesyl pyrophosphate which is an intermediate in the biosynthesis of sesquiterpenes such as farnesene. Oxidation can then provide sesquiterpenoids such as farnesol.
With the increased chain length and additional double bond, the number of possible ways that cyclization can occur is also increased and there exist a wide variety of cyclic sesquiterpenes. In addition to common six-membered ring systems such as is found in zingiberene, a consitituent of the oil from ginger, cyclization of one end of the chain to the other end can lead to macrocyclic rings such as humulene.
In addition to common six-membered rings such as in the cadinenes, one classic bicyclic sesquiterpene is caryophyllene, from the oil of cloves, which has a nine-membered ring and cyclobutane ring. Additional unsaturation provides aromatic bicyclic sesquiterpenoids such as vetivazulene and guaiazulene.