TERPENE

'Terpenes' are a large and varied class of hydrocarbons, produced primarily by a wide variety of plants, particularly conifers, though also by some insects such as swallowtail butterflies, which emit terpenes from their osmeterium. They are the major components of resin, and of turpentine produced from resin. The name "terpene" is derived from the word "turpentine".
When terpenes are modified chemically, such as by oxidation or rearrangement of the carbon skeleton, the resulting compounds are generally referred to as terpenoids. Some authors will use the term terpene to include all terpenoids.
Terpenes and terpenoids are the primary constituents of the essential oils of many types of plants and flowers. Essential oils are used widely as natural flavor additives for food, as fragrances in perfumery, in aromatherapy, and in traditional and alternative medicines. Synthetic variations and derivatives of natural terpenes and terpenoids also greatly expand the variety of aromas used in perfumery and flavors used in food additives.

Contents

Structure and biosynthesis

Types

Agri-chemical use

External links

Structure and biosynthesis

Terpenes are derived biosynthetically from units of isoprene, which has the molecular formula C₅H₈. The basic molecular formulas of terpenes are multiples of that, (C₅H₈)_n where n is the number of linked isoprene units. This is called the ''isoprene rule'' or the ''C5 rule''. The isoprene units may be linked together "head to tail" to form linear chains or they may be arranged to form rings. One can consider the isoprene unit as one of nature's preferred building blocks.
Isoprene itself does not undergo the building process, but rather activated forms, isopentenyl pyrophosphate (IPP or also isopentenyl diphosphate) and dimethylallyl pyrophosphate (DMAPP or also dimethylallyl diphosphate), are the components in the biosynthetic pathway. IPP is formed from acetyl-CoA via the intermediacy of mevalonic acid in the HMG-CoA reductase pathway. An alternative, totally unrelated biosynthesis pathway of IPP is known in some bacterial groups and the plastids of plants, the so-called MEP(2-Methyl-D-erythritol-4-phosphate)-pathway, which is initiated from C5-sugars. In both pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase.

Dimethylallyl pyrophosphate

Isopentenyl pyrophosphate

As chains of isoprene units are built up, the resulting terpenes are classified sequentially by size as hemiterpenes, monoterpenes, sesquiterpenes, diterpenes, sesterterpenes, triterpenes, and tetraterpenes.

Types

★ 'Hemiterpenes' consist of ''a single isoprene'' unit. Isoprene itself is considered the only hemiterpene, but oxygen-containing derivatives such as prenol and isovaleric acid are hemiterpenoids.

★ 'Monoterpenes' consist of ''two isoprene'' units and have the molecular formula C₁₀H₁₆. Examples of monoterpenes are: geraniol, limonene and terpineol.

★ 'Sesquiterpenes' consist of ''three isoprene'' units and have the molecular formula C₁₅H₂₄. Examples of sesquiterpenes are: farnesol. The ''sesqui-'' prefix means one and a half.

★ 'Diterpenes' are composed for ''four isoprene'' units and have the molecular formula C₂₀H₃₂. They derive from geranylgeranyl pyrophosphate. Examples of diterpenes are cafestol, kahweol, cembrene and taxadiene (precursor of taxol). Diterpenes also form the basis for biologically important compounds such as retinol, retinal, and phytol.

★ 'Sesterterpenes', terpenes having 25 carbons and ''five isoprene'' units, are rare relative to the other sizes. The ''sester-'' prefix means half to three, i.e. two and a half.

★ 'Triterpenes' consist of ''six isoprene'' units and have the molecular formula C₃₀H₄₈. The linear triterpene squalene, the major constituent of shark liver oil, is derived from the reductive coupling of two molecules of farnesyl pyrophosphate. Squalene is then processed biosynthetically to generate either lanosterol or cycloartenol, the structural precursors to all the steroids.

★ 'Tetraterpenes' contain ''eight isoprene'' units and have the molecular formula C₄₀H₅₆. Biologically important tetraterpenes include the acyclic lycopene, the monocyclic gamma-carotene, and the bicyclic alpha- and beta-carotenes.

★ 'Polyterpenes' consist of long chains of ''many isoprene'' units. Natural rubber consists of polyisoprene in which the double bonds are cis. Some plants produce a polyisoprene with trans double bonds, known as gutta-percha.

Agri-chemical use

Research into terpenes has found that many of them possess qualities that make them ideal active ingredients as part of natural agricultural pesticides.
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External links

★ Institute of Chemistry - terpenes

★ Structures of alpha pinene and beta pinene