Terpenes

INTRODUCTION TO TERPENES

Useful links:

http://en.wikipedia.org/wiki/Terpene
http://en.wikipedia.org/wiki/Osmeterium
http://www.chemie.fu-berlin.de/chemistry/oc/terpene/terpene_en.html
http://www.chem.qmul.ac.uk/iubmb/enzyme/glossary/pinene.html
http://www.nlm.nih.gov/cgi/mesh/2011/MB_cgi?mode=&term=Terpenes
http://www.nlm.nih.gov/cgi/mesh/2011/MB_cgi?mode=&term=Xanthophylls&field=entry

A. CHEMISTRY OF TERPENES

What are terpenes?

Terpenes are a large and diverse class of strong smelling organic compounds produced by plants and insects. Terpenes are found in abundance in the oil of plants and flowers. They have distinctive flavours, odours and colours. They are responsible for the odour of the pine trees, pine wood and pine furniture. They are responsible for the red colour in tomatoes.

What is the building block of terpenes?

Terpenes have isoprene units as a common structural feature.

What are terpenes vs. terpenoids?

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. Terpenoids are also known as isoprenoids.

What are the types of terpenes?

Terpenes are classified by the number of isoprene units in the molecule; a prefix in the name indicates the number of terpene units needed to assemble the molecule.

(i) From Institute of Chemistry, FU Berlin, Germany:

Terpenes are widespread in nature, mainly in plants as constituents of essential oils. Many terpenes are hydrocarbons, but oxygen-containing compounds such as alcohols, aldehydes or ketones (terpenoids) are also found. Their building block is the hydrocarbon isoprene, CH₂=C(CH₃)-CH=CH₂ (isoprene rule, Wallach 1887). Terpene hydrocarbons therefore have molecular formulas (C₅H₈)_n, they are classified according to the number of isoprene units:

number of isoprene units 

monoterpenes  2 

sesquiterpenes 3 

diterpenes       4 

triterpenes       6 

tetraterpenes   8

Examples of monoterpenes are: pinene, nerol, citral, camphor, menthol, limonene.
Examples of sesquiterpenes are: nerolidol, farnesol.
Examples of diterpenes are: phytol, vitamin A1.
Squalene is a triterpene.
Carotene (provitamin A1) is a tetraterpene.

• Pinene: alpha- and beta-pinene are bicyclic terpenes, they are the major components of wood turpentine which can be obtained from the resinous sap of pine trees (Pinus) by steam distillation.

• Nerol, a monoterpene, is a colorless liquid that has a rose scent. It is a constituent of attar of roses, oil of orange blossoms and oil of lavender.

• Citral a (trans form) and citral b (cis form) are terpene aldehydes, colorless liquids, components of many essential oils that have a strong lemon and verbena odor.

• Camphor, a bicyclic terpene ketone, is a tough gummy volatile fragrant crystalline compound obtained from the wood and bark of the camphor tree (Cinnamomum camphora, Taiwan) by steam distillation.

• Phytol, an acyclic diterpene alcohol, is a colorless, high-boiling oil. Esterified, it is part of chlorophyll.

• beta-Carotene, provitamin A1, is a tetraterpene, forms orange to red crystals and occurs in the chromoplasts of plants and in the fatty tissues of plant-eating animals.

• Terpenes:

(ii) From Wikipedia:

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 are derived from the molecular formula C₁₀H₁₆. Examples of monoterpenes are: geraniol, limonene (from orange peel) and terpineol.

Sesquiterpenes consist of three isoprene units and are derived from the molecular formula C₁₅H₂₄. Examples of sesquiterpenes are: humulene, farnesenes, farnesol. (The sesqui- prefix means one and a half.)

Diterpenes are composed of 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. They are known to be antimicrobial and antiinflammatory.

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.) An example of a sesterterpene is geranylfarnesol.

Triterpenes consist of six isoprene units and are derived from 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.

Sesquarterpenes are composed of seven isoprene units and are derived from the molecular formula C₃₅H₅₆. Sesquartepenes are typically microbial in their origin. Examples of sesquarterpenes are ferrugicadiol and tetraprenylcurcumene.

Tetraterpenes contain eight isoprene units and are derived from the molecular formula C₄₀H₆₄. 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.

Norisoprenoids, such as the C13-norisoprenoids 3-oxo-α-ionol present in Muscat of Alexandria leaves and 7,8-dihydroionone derivatives, such as megastigmane-3,9-diol and 3-oxo-7,8-dihydro-α-ionol found in Shiraz leaves (both grapes in the species Vitis vinifera) or wine (responsible for some of the spice notes in Chardonnay), can be produced by fungal peroxydases or glycosidases.

B. BIOLOGY OF TERPENES

Where are terpenes found?

1. Terpenes and terpenoids are the primary constituents of the essential oils of many types of plants (particularly conifers) and flowers. 
2. Terpenes are released by trees more actively in warmer weather, acting as a natural form of cloud seeding. The clouds reflect sunlight, allowing the forest to regulate its temperature.
3. Turpenes are the major components of resin, and of turpentine produced from resin. The name "terpene" is derived from the word "turpentine". 
4. Terpenes are produced by some insects such as termites. Terpenes are used by termites of the Nasutitermitinae family to attack enemy insects, through the use of a specialized mechanism called a fontanellar gun.
5. Terpenes are produced by caterpillars of the swallowtail butterflies, which emit terpenes from their osmeteria. They are often strong smelling and thus may have had a protective function.

Conifer resin is a source of terpenes.

The osmeterium is a fleshy organ found in the prothoracic segment of larvae (caterpillars) of Swallowtail butterflies including Birdwings. This organ emits odorous compounds believed to be pheromones (insect hormones). Normally hidden, this forked structure can be everted when the caterpillar is threatened, and used to emit a foul-smelling secretion containing terpenes.

Caterpillar with bright orange osmeterium (fleshy feelers) extruded.

Osmeterium of a caterpillar.

Close-up of osmeterium.

C. INDUSTRIES INVOLVING TERPENES

What are the industrial uses of terpenes?

1. Essential oils are used as fragrances in perfumery.
2. Essential oils are used in traditional and alternative medicines such as aromatherapy. 
3. Synthetic variations and derivatives of natural terpenes and terpenoids also greatly expand the variety of aromas used in perfumery - lavender oil, rose oil, etc
4. Natural rubber consists of polyisoprene in which the double bonds are cis. 
5. Some plants produce a polyisoprene with trans double bonds, known as gutta-percha (getah perca).
6. Research into terpenes has found that many of them possess qualities that make them ideal active ingredients as part of natural agricultural pesticides.

What are the uses of terpenes in food?

1. Essential oils are used widely as natural flavour additives for food. 
2. Synthetic variations and derivatives of natural terpenes and terpenoids also greatly expand the variety of flavors used in food additives - peppermint oil, banana oil, etc.
3. The linear triterpene squalene, is the major constituent of shark liver oil.
4. Tetraterpenes include the acyclic lycopene found in tomatoes, and the bicyclic beta-carotenes found in carrots. 
5. Norisoprenoids, such as the C13-norisoprenoids 3-oxo-α-ionol present in Muscat of Alexandria leaves and 7,8-dihydroionone derivatives, such as megastigmane-3,9-diol and 3-oxo-7,8-dihydro-α-ionol found in Shiraz leaves (both grapes in the species Vitis vinifera). These leaves are used to wrap food to make steamed dumplings in Mediterranean cuisine. They are healthy alternatives.

D. HEALTH & MEDICAL ASPECTS OF TERPENES

What are the health applications of terpenes?

1. Vitamin A is an example of a terpene. 
2. In addition to their roles as end-products in many organisms, terpenes are major biosynthetic building blocks within nearly every living creature. Steroids, for example, are derivatives of the triterpene squalene. Squalene is an intermediate in cholesterol synthesis.  
3. Diterpenes form retinol, retinal, and phytol, which are known to be antimicrobial and antiinflammatory.
4. Grapeseed oil is a therapeutic oil.
5. Terpin hydrate is a derivative of turpentine. An expectorant and humectant, it is commonly used in the treatment of acute or chronic bronchitis and related conditions.
6. Xanthophyll carotenoids are oxygenated forms of carotenoids. They are usually derived from alpha- and beta-carotene.