Sesquiterpenes consist of three isoprene units and have the molecular formula C₁₅H₂₄. Like monoterpenes, sesquiterpenes may be acyclic or contain rings, including many unique combinations.

Acyclic

When geranyl pyrophosphate reacts with IPP, 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.

Monocyclic

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.

Bicyclic

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.

Tricylic

With the addition of a third ring, the possible structures become increasingly varied. Examples include longifolene and copaene.

Terpenes and terpenoids