Terpenoids monoterpene derivatives

Terpenoids are derived from the cytosolic mevalonate pathway or from the plastidial 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway (see also Terpenoid Biosynthesis). Both pathways lead to the formation of the C5 units isopentenyl diphosphate and its allylic isomer dimethylallyl diphosphate, which are the basic terpenoid biosynthesis building blocks (Fig. 1). Although increasing evidence suggests that exchange of intermediates occurs between these compartments, the cytoplasmic mevalonate pathway is generally considered to supply the precursors for the production of sesquiterpenes and triterpenes (including sterols) and to provide precursors for protein prenylation and for ubiquinone and heme-A production in mitochondria. In the plastids, the MEP pathway supplies the precursors for the production of isoprene, monoterpenes, diterpenes (e.g., GAs), and tetraterpenes (e.g., carotenoids). 

Isoprene rule. In the light of the fact that the then known mono- and sesquiterpenes were built up of two or three isoprene units, respectively, O. Wallach in 1887 prtmosed his simple isoprene rule . In 1953 L. Ruzicka formulated his biogenetic isoprene rule which proved to be a milestone in the chemistry of the terpenoids. According to this rule, all terpenoids are derived from acyclic parent structures monoterpenes are formed from geranyl pyrophosphate (GPP, Cio), sesquiterpenes from famesyl pyrophosphate (FPP,... 

Terpenoids are classified by the number of five-carlxMi units they contain like hemiterpenes (C5), monoterpenes (Cio), sesquiterpenes (C15), diterpenes (C20), triterpenes (C30), tetraterpenes (C40), and polyterpenes (more than 8 isoprene units). All terpenoids are derived by repetitive fusion of branched five-carbon units based on isopentane skeleton, i.e., isoprene units. At suitable chemical conditions, isoprene undergoes polymerization to generate numerous terpenoid skeletons. 

Symmetric and asymmetric aliphatic, alicyclic and aromatic ethers and ethers with the oxygen atom bound in the ring can be found in foods. Volatile dialkyl ethers are virtually absent from foods, but some are synthesised and used as flavourings, especially for cosmetic purposes. Odour- and taste-active ethers are mainly terpenoid ethers, derived from monoterpenes and sesquiterpenes, and aromatic ethers. 

Wood is the raw material of the naval stores iadustry (77). Naval stores, so named because of their importance to the wooden ships of past centuries, consist of rosin (diterpene resin acids), turpentine (monoterpene hydrocarbons), and associated chemicals derived from pine (see Terpenoids). These were obtained by wounding the tree to yield pine gum, but the high labor costs have substantially reduced this production in the United States. Another source of rosin and turpentine is through extraction of old pine stumps, but this is a nonrenewable resource and this iadustry is in decline. The most important source of naval stores is spent sulfate pulpiag Hquors from kraft pulpiag of pine. In 1995, U.S. production of rosin from all sources was estimated at under 300,000 metric tons and of turpentine at 70,000 metric tons. Distillation of tall oil provides, in addition to rosin, nearly 128,000 metric tons of tall oil fatty acids annually (78). 

The great diversity of terpenes helps to cormteract tolerance by herbivores. In all, terpenes are not very toxic to vertebrates. Many mammals ingest a significant amount of terpenoids with their diet. Monoterpenes from pine oil added to the diet reduces food intake in red deer, Cervuselaphus, calves (Elliot and Loudon, 1987). The brush-tailed possmn, Trichosurus vulpecula, detoxifies (-l-)-a-pinene to alcohol and carboxylic acid derivatives. 

Essential oils may comprise volatile compounds of terpenoid or non-terpe-noid origin. All of them are hydrocarbons and their oxygenated derivatives. Some may also contain nitrogen or sulphur derivatives. They may exist in the form of alcohols, acids, esters, epoxides, aldehydes, ketones, amines, sulphides, etc. Monoterpenes, sesquiterpenes and even diterpenes constitute the composition of many essential oils. In addition, phenylpropanoids, fatty acids and their esters, or their decomposition products are also encountered as volatiles [1-16, 21-33, 36-38]. 

The diverse, widespread and exceedingly numerous class of natural products that are derived from a common biosynthetic pathway based on mevalonate as parent, are synonymously named terpenoids, terpenes or isoprenoids, with the important subgroup of steroids, sometimes singled out as a class in its own right. Monoterpenes, sesquiterpenes, diterpenes and triterpenes are ubiquitous in terrestrial organisms and play an essential role in life, as we know it. Although the study of terrestrial terpenes dates back to the last century, marine terpenes were not discovered until 1955. 

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... 

Terpenoids occur in abundance in the plant world where they possess many different kinds of functions. Essentially, they are derived from the basic 5-carbon isoprene unit, biosynthetically as isopentenyl pyrophosphate, which is itself derived from acetate via mevalonic acid. They may be classified into diverse groups according to the number of isoprene units, e.g., monoterpenes (Cio), sesquiterpenes (Cu), diterpenes (C2o), triterpenes and steroids (C30),... 

The terpenoid portion of TIAs is derived from secologanin, whose monoterpene precursor geraniol, is produced by the recently discovered Rommer or triose phosphate / pyruvate pathway, responsible for the synthesis of isoprenes like geraniol in the plastids [58-60]. 

Fig. 1.4 Outline of terpenoid biosynthesis from isopentenyl diphosphate (IPP) via dimethylallyl diphosphate (DMAPP), gcranyl diphosphate (GPP), famesyl diphosphate (FPP) and geranylgcranyl diphosphate (GGPP). These reactions are catalyzed by isoprenyl diphosphate synthases and terpene synthases. The major products of the monoterpene, sesquiterpene, and diterpene pathways that constitute the oleoresm of Picea abies are listed. The general precursor IPP is derived either from the plastidial methylerythritol phosphate (MEP) pathway or the cytosolic mevalonaic pathway.

Terpenes are known to play important ecological roles not only in the defense of the tree, but also in bark beetle chemical ecology. Two chapters in this volume (see chapters by Raffa, et al, and Tittiger, et al.) review aspects of terpenoids as chemicals in conifer-bark beetle interactions and isoprenoid formation in bark beetle chemical ecology. While some species of bark beetles may be able to produce monoterpene pheromones de novo, bark beetles can also use host-derived monoterpenes as precursors to their own sex and aggregation pheromones. Bark beetle monoterpene pheromones are often used in a stereospecific manner such that often only one enantiomer is able to serve as a sex or aggregation pheromone. Since both enantiomeric host-derived monoterpene precursors are accepted and chemically modified by the beetle, the exact monoterpene enantiomeric mixture of conifer resin is important to both the beetle s ability to produce the correct pheromone and to the trees ability to escape an attack. ... 

This rapid, usually localized, response also includes biochemical changes, in which monoterpene, diterpene, and sesquiterpene concentrations rise. These terpenes are derived from isoprenoids synthesized via the mevalonate or 1-deoxy-D-xylulose-5-phosphate pathways in the cytosol, endoplasmic reticulum, and plastids. A diverse array of terpenoid synthethases yield the parent compounds, and a number of genes have been characterized. Induction can be elicited by applying methyl jasmonates. ... 

Some monoterpene substrates, such as a-pinene and limonene, are relatively cheap. Potentially, biocatalytic processes based on these preformed compounds seem feasible in view of the many products of industrial importance derived from them. As the specific properties of a terpenoid often depend on its absolute configuration, biotransformation is a valuable tool, as biocatalytic processes have the advantage of being regio-, stereo- and enantioselective. 

Other prominent flavor compounds such as eugenol, phenethyl alcohol, and guaiacol are derived from the shikimic acid pathway [27], The isoprenoids such as P-ionone are probably produced by the degradation of longer terpenoids such as -carotene and lycopene, respectively, whereas monoterpenes such as linalool are formed directly from geranyl diphosphate via the isoprenoid pathway [28]. 

Chrysanthemol from the leaves of Artemisia ludiviciana (Asteraceae) belongs to the cyclopropane monoterpenes Cinerins, jasmolins and pyrethrins (all including derivatives 1 and 11) are esters of /rara-chrysanthemic and pyrethric acid with terpenoid hydroxypentenones such as cinerolone, jasmolone and pyrethrolone. These are the active insecticidal constituents of pyrethrum recovered from dried flowers of several Chrysanthemum species (e.g. Chrysanthemum cinerariaefolium, Asteraceae). Some synthetic esters of chrysanthemic acid are also applied as insecticides. 

Apart from rare terpenoid monoeyelie eyelopentane derivatives sueh as l-aeetyl-4-isopropenyleyelopentene from Eucalyptus globulus (Myrtaeeae), about 200 eyelopentane monoterpenes oeeur as iridoides and seco-iridoides. 4,7-Dimethyleyelo-penta[c]pyran ineorporates the basie skeleton of iridoids, while the C-6-C-7 bond of the eyelopentane rings opens in the seeo-iridoids. 

Early studies indicated that terpenes were composed of five carbon units. By 1860, isoprene, a compound with the empirical formula C5H8, could be derived by pyrolysis of turpentine and rubber. Further, isoprene could be transformed into a C10H16 compound that subsequently decomposed back to the original five-carbon unit. Wallach recognized that terpenes could be subdivided into several groups hemiterpenes, true terpenes, sesquiterpenes, and diterpenes. In 1887, he proposed that monoterpenes (true terpenes) as well as other terpenoids were composed of isoprene units, a concept that became known as the isoprene rule (Wallach, 1887). The structure of isoprene was not solved until much later (Spurgeon and Porter, 1981). 

Sesquiterpenes are the most numerous of all terpenoid compounds. The approximately 5000 known compounds of this type mostly can be grouped into about 30 major skeletal types, but at least 200 less common skeletal types are known (Fig. 21.1). The distribution of sesquiterpenes in plants is essentially the same as monoterpenes. Sesquiterpene hydrocarbons are common essential oil components. Although only a few fungi accumulate monoterpenes, many accumulate sesquiterpenes. Abscisic acid, a plant growth-regulating compound, is synthesized as a sesquiterpene in fungi. Despite the fact that this compound often is considered a sesquiterpene, in higher plants abscisic acid is derived from the breakdown of xanthophylls and should be considered as a tetraterpene derivative see Chapter 26). 

The ketone fenchone and the alcohols a-fenchol (endo) and /8-fenchol (exo) constitute the fenchane family of bicyclic monoterpenes. Fenchanes are of rather limited distribution in the plant kingdom, and interest in this group of compounds stems primarily from the fact that their carbon skeleton appears to be derived by rearrangement of a pinane-type intermediate (Fig. 5). (-l-)-Fenchone is one of the m or terpenoids of the volatile oil of fennel (Foeniculum vulgare), and soluble enzyme preparations from fennel leaves convert GPP and NPP to fenchol in the presence of Mn and, in the pres-... 

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