Sesquiterpenes are a class of terpene

Terpene

Terpenes are a large and diverse class of organic compounds, produced by a variety of plants, particularly conifers, though also by some insects such as termites or swallowtail butterflies, which emit terpenes from their osmeterium. They are often strong smelling and thus may have had a protective...

s that consist of three isoprene

Isoprene

Isoprene , or 2-methyl-1,3-butadiene, is a common organic compound with the formula CH2=CCH=CH2. Under standard conditions it is a colorless liquid...

 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

Redox

Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 or rearrangement

Rearrangement reaction

A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. Often a substituent moves from one atom to another atom in the same molecule...

 produce the related sesquiterpenoids.

Sesquiterpenes are found naturally in plants and insects, as semiochemical

Semiochemical

A semiochemical is a generic term used for a chemical substance or mixture that carries a message. These chemicals acts as messengers within or between species...

s, e.g. defensive agents or pheromones.

Acyclic

When geranyl pyrophosphate

Geranyl pyrophosphate

Geranyl pyrophosphate is an intermediate in the HMG-CoA reductase pathway used by organisms in the biosynthesis of farnesyl pyrophosphate, geranylgeranyl pyrophosphate, cholesterol, terpenes and terpenoids....

 reacts with isopentenyl pyrophosphate

Isopentenyl pyrophosphate

Isopentenyl pyrophosphate is an intermediate in the classical, HMG-CoA reductase pathway used by organisms in the biosynthesis of terpenes and terpenoids. IPP is formed from acetyl-CoA via mevalonic acid...

, the result is the 15-carbon farnesyl pyrophosphate

Farnesyl pyrophosphate

Farnesyl pyrophosphate is an intermediate in the HMG-CoA reductase pathway used by organisms in the biosynthesis of terpenes, terpenoids, and sterols...

, which is an intermediate in the biosynthesis of sesquiterpenes such as farnesene

Farnesene

The term farnesene refers to a set of six closely related chemical compounds which all 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...

. Oxidation can then provide sesquiterpenoids such as farnesol

Farnesol

Farnesol is a natural organic compound which is an acyclic sesquiterpene alcohol found as a colorless liquid. It is insoluble in water, but miscible with oils...

.

Monocyclic

With the increased chain length and additional double bond, the number of possible ways that cyclization can occur is also increased, and there exists a wide variety of cyclic sesquiterpenes. In addition to common six-membered ring systems such as is found in zingiberene

Zingiberene

Zingiberene is a monocyclic sesquiterpene that is the predominant constituent of the oil of ginger , from which it gets its name....

, a constituent of the oil from ginger

Ginger

Ginger is the rhizome of the plant Zingiber officinale, consumed as a delicacy, medicine, or spice. It lends its name to its genus and family . Other notable members of this plant family are turmeric, cardamom, and galangal....

, cyclization of one end of the chain to the other end can lead to macrocyclic rings such as humulene

Humulene

Humulene, also known as α-humulene or α-caryophyllene, is a naturally occurring monocyclic sesquiterpene, which is a terpenoid consisting of 3 isoprene units. It is found in the essential oils of Humulus lupulus from which it derives its name. It is an isomer of β-caryophyllene, and the two are...

.

Bicyclic

In addition to common six-membered rings such as in the cadinene

Cadinene

Cadinene is the trivial chemical name of a number of isomeric hydrocarbons that occur in a wide variety of essential oil-producing plants. The name is derived from that of the Cade juniper , the wood of which yields an oil from which cadinene isomers were first isolated.Chemically, the cadinenes...

s, one classic bicyclic sesquiterpene is caryophyllene

Caryophyllene

Caryophyllene , or -β-caryophyllene, is a natural bicyclic sesquiterpene that is a constituent of many essential oils, especially clove oil, the oil from the stems and flowers of Syzygium aromaticum , the essential oil of hemp Cannabis sativa, rosemary Rosmarinus oficinalis, and hops...

, from the oil of clove

Clove

Cloves are the aromatic dried flower buds of a tree in the family Myrtaceae. Cloves are native to the Maluku islands in Indonesia and used as a spice in cuisines all over the world...

s, which has a nine-membered ring and cyclobutane

Cyclobutane

Cyclobutane is an organic compound with the formula 4. Cyclobutane is a colourless gas and commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes...

 ring. Additional unsaturation provides aromatic bicyclic sesquiterpenoids such as vetivazulene

Vetivazulene

Vetivazulene is an azulene derivate obtained from vetiver oil. It is a bicyclic sesquiterpene and an isomer of guaiazulene....

 and guaiazulene

Guaiazulene

Guaiazulene, also azulon or 1,4-dimethyl-7-isopropylazulene, is a dark blue crystalline hydrocarbon. A derivative of azulene, guaiazulene is a bicyclic sesquiterpene that is a constituent of some essential oils, mainly oil of guaiac and chamomile oil, which also serve as its commercial sources....

.

Tricyclic

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

Longifolene

Longifolene is the common chemical name of a naturally occurring, oily liquid hydrocarbon found primarily in the high-boiling fraction of certain pine resins. The name is derived from that of a pine species from which the compound was isolated, Pinus longifolia Chemically, longifolene is a...

, copaene

Copaene

Copaene, or more precisely, α-copaene, is the common chemical name of an oily liquid hydrocarbon that is found in a number of essential oil-producing plants. The name is derived from that of the resin-producing tropical copaiba tree, Copaifera langsdorfii, from which the compound was first...

 and the alcohol patchoulol

Patchoulol

Patchoulol or patchouli alcohol is a terpene extracted from Patchouli. The -optical isomer is one of the organic compounds responsible for the typical patchouli scent. Patchoulol is also used in the synthesis of the chemotherapy drug Taxol....

.