Terpenes tetraterpenes

The higher terpenes are formed not by successive addition of C5 units but by the coupling of simpler terpenes Thus the triterpenes (C30) are derived from two mole cules of farnesyl pyrophosphate and the tetraterpenes (C40) from two molecules of ger anylgeranyl pyrophosphate These carbon-carbon bond forming processes involve tail to tail couplings and proceed by a more complicated mechanism than that just described... 

Tetraterpenes. Carotenoids make up the most important group of C q terpenes and terpenoids, although not all carotenoids contain 40 carbon atoms. They are widely distributed in plant, marine, and animal life. It has been estimated that nature produces about 100 million t/yr of carotenoids synthetic production amounts to several hundred tons per year (207,208). 

Terpenes are built out of Cs-isoprene units. There is a variety of terpenes ranging from Cs-terpenes which are called hemiterpenes to tetraterpenes with C40 atoms. There are also terpenes with more Cs-units, such as natural rubber, which is a polyterpene. Terpenes are also classified by the number of carbon cycles in the molecule, e.g., monoterpenes can have up to two cyclic systems in one molecule. Examples of some monoterpenes are shown in Scheme 2 myrcene is acyclic,... 

Terpenes are composed of isoprenyl (C-,) units and are conveniently grouped as monoter-penes (skeletal basis CI0 = 2X C-,), sesquiterpenes (G13 = 3X C3), diterpenes (C20 = 4X C-,), triterpenes (C3o = 6X C-,) and tetraterpenes (C40 = 8X G-j. The structures of some representative terpenes are shown in the Appendix (Section 3). Terpenes ultimately derive biosynthetically from acetate (C2) via the activated acetyl thioester (CH3—CO—S—X) acetyl-coenzyme A (acetylCoA CH3-CO-S-C0A) as outlined below (enzymes catalysing key steps being indicated in parentheses). 

Tetraterpene (Section 29.7A) A terpene that contains 40 carbons and eight isoprene units. 

Mono- and sesquiterpenes are found primarily in plants, but the high terpenes occur in both plants and animals, and many have important td logical roles. The triterpene lanosterol, for example, is the precursor fre which all steroid hormones are made the tetraterpene / -carotene is a miij dietary source of vitamin A. 

Isoprene has the chemical formula of C5H8, therefore, each isoprene unit correlates to five carbons present in the skeleton. Terpene classification includes hemiter-penes, monoterpenes, sesquiterpenes, diterpenes, sesterterpenes, and triterpenes, each consisting of one through six isoprene units, respectively [16]. Additional terpene classifications include tetraterpenes, containing eight isoprene units, and polyterpenes, which include isoprene chains. The classification system is further divided into subclasses [15]. 

Oxidation of tetraterpenes yields many compounds, and the cyclic compounds with a trimethylcyclohexane ring are easily associated with degradation of monocyclic and bicyclic carotenes, but allylic compounds are now as conspicuous in their biogenetlc origin. Many such compounds are derived from lycopene, phytoene and phyto-fluene, three very common acyclic carotenes. These allylic compounds are often identified only as terpene aldehydes and ketones and not as carotene degradation products (10). 

Fig. 7.1 Isoprenoid derivatives (a) formation of a terpene from 2 isoprene units (b) structure of lycoprene - a tetraterpene showing the isoprene units.

Terpenes, terpenoMs. T. can formally be considered as polymerization products of the hydrocarbon iso-prene (see isoprenoids). Depending on the number of isoprene units the compounds are classified as mono-terpenes (C,o), sesquiterpenes (C,j), diteipenes(C2o), sesterterpene (C25), triterpenes (C30), tetraterpenes (C40), and polyterpenes. The steroids are derived from a group of triterpenes, the methylsterols. 

The structures of terpenes or terpenoids varies widely and are classified according to various aspects, e.g. number of isoprene units (Cio monoterpenes, C15 sesquiterpenes, C20 diterpenes, C25 sesterterpenes, C30 triterpenes, C40 tetraterpenes) or division in acyclic, mono-, hi-, tri-, tetra-, pentacyclic terpenes. Often terpenes are named by their trivial names, e.g. d-limonene. 

The occurrence of terpenes is ubiquitous. Natural terpenes are found in plants and animals in minute amounts. Especially in higher plants, terpenes characterize the type of plant (chemotaxonomy) mono- and sesquiterpenes in essential oils, sesqui-, di-, triterpenes in balsams and resins, tetraterpenes in pigments and polyterpenes in latexes.Therefore, terpenes are often emitted from natural products such as citrus fruits or trees, e.g. conifers. 

Tetraterpenes terpenes comprising eight isoprene units (C H64). Naturally occurring T. are almost all carotenoids, and the group contains no polycyclic compounds. 

Mono-, sesqui-, di-, and sesterpenes contain the isoprene units linked in a head-to-tail fashion. The triterpenes and carotenoids (tetraterpenes) contain two C15 and C20 units, respectively, linked head to head. Many terpenes are hydrocarbons, but oxygen-containing compounds such as alcohols, aldehydes, or ketones are also found to fall in the class of terpenes. These derivatives are more frequently named as terpenoids. 

As artemisinin has a terpenic structure, its biosynthesis starts in the formation of isopentenyl diphosphate GPP)> as in all the natural terpenoids. In plants, IPP is synthesized either via the mevalonate pathway in the cytosol or via the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway in the plastid. The IPP derived from the mevalonate pathway is generally used in the biosynthesis of sesquiterpenes (such as artemisinin), phytosterols, and triterpenes, and the IPP derived from the non-mevalonate pathway is employed in the biosynthesis of monoterpenes, diterpenes, and tetraterpenes (Fig. 89.15). 

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