Tetraterpene

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... 

Section 26 16 Carotenoids are tetraterpenes They have 40 carbons and numerous dou ble bonds Many of the double bonds are conjugated causing carotenes to absorb visible light and be brightly colored They are often plant pigments... 

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). 

More than 600 different carotenoids from natural sources have been isolated and characterized. Physical properties and natural functions and actions of carotenoids are determined by their chemical properties, and these properties are defined by their molecular structures. Carotenoids consist of 40 carbon atoms (tetraterpenes) with conjugated double bonds. They consist of eight isoprenoid units j oined in such a manner that the arrangement of isoprenoid units is reversed at the center of the molecule so that the two central methyl groups are in a 1,6-position and the remaining nonterminal methyl groups are in a 1,5-position relationship. They can be acyclic or cyclic (mono- or bi-, alicyclic or aryl). Whereas green leaves contain unesterified hydroxy carotenoids, most carotenoids in ripe fruit are esterified with fatty acids. However, those of a few... 

The biosynthesis of monoterpenes, the major components of peppermint essential oils, can be divided into four stages (Fig. 9.4). Stage 1 includes the formation of isopentenyl diphosphate (IPP) and dimethylallyl alcohol (DMAPP). In plants, two separate pathways are utilized for the synthesis of these universal C5 intermediates, with the cytosolic mevalonate pathway being responsible for the formation of sterols and certain sesquiterpenes, and the plastidial mevalonate-independent pathway being involved in the biosynthesis of isoprene, monoterpenes, certain sesquiterpenes, diterpenes, tetraterpenes, as well as the side chains of chlorophyll and plastoquinone.16 In peppermint oil gland secretory cells, however, the mevalonate pathway is blocked and the biosynthesis of monoterpenoid essential... 

Chart 8.3.12 Sesterterpenes, triterpenes, steroids, tetraterpenes, and polyprenyls (skeletons) typical of both marine and land organisms (for S and HseeC 8.3.11). 

Scheme 23.16 Microbial pathways from triterpene and tetraterpene (carotenoid) precursors to valuable flavour and fragrance compounds. 1 Carotenoid-cleaving peroxidase-containing supernatant of certain fungal cultures, e.g. Lepista irina 2 Serratia liquefaciens, Botrytis sp.

In similar fashions, the core pathway up to C25 compounds (five isoprene units) is formed by sequential addition of C5 moieties derived from IPP to a starter unit derived from DMAPP. Thus, sesquiterpenes are formed form the precursor 2E, hS-farnesyl pyrophosphate (FPP), and diterpenes from 2E, 6E, IO -geranylgeranyl pyrophosphate (GGPP). The parents of triterpenes and tetraterpenes are formed by reductive coupling of two FPPs or GGPPs, respectively. Rubbers and other polyisoprenoids are produced from repeated additions of C5 units to the starter unit GGPP. 

One of the simplest and most significant of the diterpenes is phytol, a reduced form of geranylgeraniol, which constitutes the lipophilic side-chain of the chlorophylls. Phytol also forms a part of vitamin E (tocopherols) and K molecules. Vitamin A is also a 20-carbon-containing compound, and can be regarded as a diterpene. However, vitamin A is formed from a cleavage of a tetraterpene. Among the medicinally important diterpenes, paclitaxel, isolated from Taxus brevifolia (family Taxaceae), is one of the most successful anticancer drugs of modern time. 

The terpenoids form a large and structurally diverse family of natural products derived from C5 isoprene units (Figure 5.1) joined in a head-to-tail fashion. Typical structures contain carbon skeletons represented by (Cs) , and are classified as hemiterpenes (C5), monoterpenes (C10), sesquiterpenes (C15), diterpenes (C2o), sesterterpenes (C25), triterpenes (C30) and tetraterpenes (C40) (Figure 5.2). Higher polymers are encountered in materials such as rubber. Isoprene itself (Figure 5.1) had been characterized as a decomposition product from various natural cyclic hydrocarbons, and was suggested as the fundamental building block for these compounds, also referred to as isoprenoids . Isoprene is produced naturally but is not involved in the formation of... 

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