Sesquiterpenes are a class of terpenes that consist of three isoprene units and often 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.
Sesquiterpenes are found naturally in plants and insects, as semiochemicals, e.g. defensive agents or pheromones.
Biosynthesis and examples
The reaction of geranyl pyrophosphate with isopentenyl pyrophosphate results in the 15-carbon farnesyl pyrophosphate, which is an intermediate in the biosynthesis of sesquiterpenes such as farnesene.
Cyclic sesquiterpenes are more common than cyclic monoterpenes because of the increased chain length and additional double bond in the sesquiterpene precursors. In addition to common six-membered ring systems such as is found in zingiberene (a constituent of the oil from ginger), cyclization of one end of the chain to the other end can lead to macrocyclic rings such as humulene.
The FPP backbone can be rearranged in several different ways and further decorated with different functional groups, hence the large variety of sesquiterpenoids. Geosmin, the volatile compound that gives an earthy taste and musty odor in drinking water and the characteristic odor on a rainy day, is a sesquiterpenoid, produced by bacteria, especially cyanobacteria, that are present in the soils and water supplies.  Oxidation of farnesene then provides the sesquiterpenoid farnesol.
Dictyophorine A and B
Two sesquiterpenes named dictyophorine A and B were identified from the fungus Phallus indusiatus. These compounds are based on the eudesmane skeleton (a common structure found in plant-derived flavors and fragrances), and they are the first eudesmane derivatives isolated from fungi. The dictyophorines promote the synthesis of nerve growth factor in astroglial cells.
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