C15-sesquiterpenes

The use in coal analysis of gas chromatography-mass spectroscopy as well as pyrolysis-mass spectrometry and pyrolysis gas chromatography-mass spectroscopy has enabled low-molecular-weight benzenes, phenols, and naphthalenes to be identified as well as the C27 and C29-C30 hopanes and C15 sesquiterpenes (Gallegos, 1978 Smith and Smoot, 1990 Blanc et al., 1991). 

CIO monoterpenes C15 sesquiterpenes C20 diterpenes C30 triterpenes C27 steroids C40 tetraterpenes C n) polyterpenes Saponins Cucurbitacins Terpenoid alkaloids... 

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. 

Mono- and sesquiterpenes are the main constituents of the essential oils, while the other terpenes are constituents of balsams, resins, waxes, and rubber. Oleoresin is a roughly equal mixture of turpentine (85 % Cio-monoterpenes and 15 % C15- sesquiterpenes) and rosin (C2o-diterpene) that acts in many conifer species as a toxic material to invading insects and their pathogenic fungi [12]. A number of angiosperm species have inducible terpenoid defensive compounds (phytoalexins) [13]. These include both sesquiterpenoid and diterpenoid t)pes. Isoprenoid units are also found within the framework of other natural molecules. Thus, indole alkaloids, several quinones (vitamin K), alcohols (vitamin E, vitamin A formed from p-carotene), phenols, and isoprenoid alcohols (also known as terpenols or polyprenols) also contain terpenoid fragments. The biogenesis. 

The multiples of isoprene units in a terpene serve to classify it Cio, monoterpenes C15, sesquiterpenes C20, diterpenes C30, tri-terpenes to which the steroids are closely related C25, sesterpenes. It is a curious thing to note that the isoprene rule, sound and powerfully useful not least for structure elucidation, has unnatural foundations. Isoprene (4.1) is not a naturally occurring compound. Instead the clay from which the terpenes are so orderly fashioned is isopentenyl pyrophosphate (4.14) and dimethylallyl pyrophosphate (4.15). The latter is seen in many natural compounds, e.g. (4.11) as a prenyl ... 

Geranyl diphosphate is converted to farnesyl diphosphate, a C15 sesquiterpene, by addition of a five-carbon unit. Farnesyl diphosphate is the starting material for all sesquiterpenes and diterpenes. 

More than half of the reported secondary metabolites from macroalgae are isoprenoids. Terpenes, steroids, carotenoids, prenylated quinines, and hydroqui-nones make up the isoprenoid class, which is understood to derive from either the classical mevalonate pathway, or the mevalonate-independent pathway (Stratmann et al. 1992). Melavonic acid (MVA) (Fig. 1.2) is the first committed metabolite of the terpene pathway. Dimethylallyl (dl meth al lal) pyrophosphate (DMAPP) (Fig. 1.3) and its isomer isopentenyl pyrophosphate (IPP, Fig. 1.3) are intermediates of the MVA pathway and exist in nearly all life forms (Humphrey and Beale 2006). Geranyl (ja ran al) (C10) and famesyl (C15) units are generated by head-to-tail (Fig. 1.3) condensation of two (for C10) or three (for C15) 5-carbon DMA-like isoprene units, identifiable in final products by the characteristic fish-tail repeating units, as traced over the structure of a sesquiterpene in Fig. 1.3 (Humphrey and Beale 2006). Additional IPP condensation with famesyl pyrophosphate (FPP)... 

Acyclic terpene (Cjo) and sesquiterpene (C15) hydrocarbons find little use in flavor and fragrance compositions. They are relatively unstable and some have a slightly aggressive odor due to their highly unsaturated structure. 

Most of the compounds shown in Figure 22-4 are derived from the C15 famesyl diphosphate. There are more than 300 known cyclic structures among these sesquiterpenes, and many sesquiterpene synthases have been characterized.91/91a Aristolochene is formed by the action of a 38-kDa cyclase that has been isolated from species of Penicillium and Aspergillus,92"4 Notice that the synthesis must involve two cyclization steps and migration of a methyl group. Three-dimensional structures are known for at least two terpene synthases,95 96 and comparison of gene sequences suggests that many others have similar structures. 

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... 

Figure 10.7 All terpenes are derived from allylic diphosphates which are polymers of repeating isopentyl units (IPP) put together by the action of prenyltransferases. In plants, IPP can be derived from the mevalonate biosynthetic pathway (a cytoplasmic pathway) or the methyl erythritol phosphate pathway (a plastidic pathway). Monoterpenes are then derived from the CIO precursor geranyl diphosphate (GPP), sesquiterpenes from the C15 precursor famesyl diphosphate (FPP), and diterpenes from the C20 precursor geranylgeranyl diphosphate (GGPP) by the action of terpene synthases or cyclases, which divert carbon into the specific branch pathways.

Though terpenes are made from C5 units, they are classified in C10 units. The monoterpenes are the C10 compounds, the sesquiterpenes (sesqui is Latin for one-and-a-hatf) are the C15 compounds, the diterpenes are the C20 compounds, and so on. 

First, we will take up cyclopropyl group formation by the rearrangement of carbon skeletons via cationic intermediates encountered in various mono- and sesquiterpenes, and also examine the illudin biosynthesis where contraction of a cyclobutyl cation to a cyclopropane has been invoked. We will then discuss the head-to-head condensation of isoprenoid alcohols at the C15 or C20 level to generate the cyclopropyl intermediates, presqualene pyrophosphate and prephytoene pyrophosphate, on the way to the C30 and C40 polyene hydrocarbons, squalene and phytoene respectively. Conversion of 2,3-oxidosqualene via common intermediate protosterol cation to cycloartenol or lanosterol represents an important pathway in which the angular methyl group participates in the three-membered ring formation. The cyclopropanation outcome of this process has been carefully studied. 

Antibacterial bisabolane sesquiterpenes and tricyclic sesquiterpenes, the latter consisting of an irregularly built C15 backbone, were also isolated from Leontopodium alpinum [36]. These compounds showed significant antibacterial activity towards Enterococcus faecium. 

The C5 trimer famesyl pyrophosphate (35), in addition to serving as a pregenitor of steroids via squalene (114), is also the pregenitor of the C15 compounds known as sesquiterpenes. It has been suggested that famesyl pyrophosphate (88) similarly serves as the carbon backbone of alkaloids such as deoxynupharidine (120) from JShtpharjaponicum (Nymphaceae) (water lilies) and dendrobine (121) from Dendrobium nobile Undl (Orchidaceae) (Table 11). 

The C30 hydrocarbon squalene is present at about 200-500 ppm in crude palm oil sesquiterpene (C15) and diterpene (C20) hydrocarbons are present at lower levels. Abdul Gapor and Hazrina (2000) reported squalene as high as 979 ppm in some crude oils and 791 ppm in refined oils. These levels are generally higher than that of other vegetable oils, with the exception of olive oil. Palm fatty acid distillate has 5000 ppm to 8000 ppm of squalene. Crude palm oil also has 10-80 ppm of ubiquinone 10 (Hazura et al. 1996). 

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