Sesquiterpenes isoprene units

Although the term terpene once referred only to hydrocarbons current usage includes functionally substituted derivatives as well grouped together under the general term isoprenoids Figure 26 6 (page 1086) presents the structural formulas for a number of representative examples The isoprene units m some of these are relatively easy to identify The three isoprene units m the sesquiterpene farnesol, for example are mdi cated as follows m color They are joined m a head to tail fashion... 

Terpenes are characterized as being made up of units of isoprene in a head-to-tail orientation. This isoprene concept, invented to aid in the stmcture deterrnination of terpenes found in natural products, was especially useful for elucidation of stmctures of more complex sesquiterpenes, diterpenes, and polyterpenes. The hydrocarbon, myrcene, and the terpene alcohol, a-terpineol, can be considered as being made up of two isoprene units in such a head-to-tail orientation (1). 

Sesquiterpenes are formed by the head-to-tad arrangement of three isoprene units (15 carbon atoms) there are, however, many exceptions to the rule. Because of the complexity and diversity of the substances produced in nature, it is not surprising that there are many examples of skeletal rearrangements, migrations of methyl groups, and even loss of carbon atoms to produce norsesquiterpenoids. 

Terpenes (Section 26.7) Compounds that can be analyzed as clusters of isoprene units. Terpenes with 10 carbons are classified as monoterpenes, those with 15 are sesquiterpenes, those with 20 are diterpenes, and those with 30 are triterpenes. 

Terpenes (and terpenoids) are further classified according to the number of 5-carbon units they contain. Thus, monoterpenes are 10-carbon substances biosynthesized from two isoprene units, sesquiterpenes are 15-carbon molecules from three isoprene units, diterpenes are 20-carbon substances from four isoprene units, and so on. Monoterpenes and sesquiterpenes are found primarily in plants, but the higher terpenoids occur in both plants and animals, and many have important biological roles. The triterpenoid lanosterol, for example, is the precursor from which all steroid hormones are made. 

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)... 

From their carbon skeletons terpenes can be seen as isoprene (C5) oligomers. Terpenes are classified according to their number of isoprene units as monoterpenes (C10), sesquiterpenes (Q5), diterpenes (C2o), etc. /1-Carotene (1) is a tetrater-pene. 

Isoprene metabolism in plants is very complex. Plants can synthesize many types of aromatic substances and volatile oils from isoprenoids. Examples include menthol (1= 2 ), camphor (1 = 2), and citronellal (1 = 2). These Cio compounds are also called monoterpenes. Similarly, compounds consisting of three isoprene units (1 = 3) are termed sesquiterpenes, and the steroids (1 = 6) are called triterpenes. 

The farnesols and nerolidols are sesquiterpene analogs of geraniol-nerol and linalool. These compounds are formed by extending one of the methyl groups in the 7-position of the corresponding monoterpene with an isoprene unit. Because these compounds have an extra double bond, they also have an additional possibility for cis-trans isomerism. Thus, there are four stereoisomers of farnesol and two of nerolidol. 

Sesquiterpenes are formed by the addition of one more isoprene units to a monoterpene molecule, and thus have the molecular formula C15H24 (see also Fig. 4.2). There are linear, branched or cyclic sesquiterpenes. Sesquiterpenes are unsaturated compounds. Cyclic sesquiterpenes may be monocyclic, bicyclic or tricyclic. They are the most diverse group among the volatile terpenoids [2, 3, 7-11, 13,14, 16, 20-24, 37-39, 49]. The DNP treats sesquiterpenoids in 147 different structural types [37]. Various types of sesquiterpenes (69-109) can also be seen in Structure 4.16. 

Head-to-tail rearrangement of four isoprene units results in the formation of diterpenes (C20H32), as seen also in Fig. 4.2. Diterpenes are generally found in resins, e.g. pimaric acid and abietic acid. Some diterpenoids are also constituents of essential oils, e.g. phytol [3, 7-14, 37, 52, 53]. Like sesquiterpenes, diterpenes are heavier than monoterpenes therefore, they require more energy to go to the vapour phase. For this reason, longer distillation times are necessary for their recovery. The DNP lists 118 different structural types for diterpenoids [37]. Important diterpenes found in essential oils will be detailed. Some representatives of volatile diterpenes are as in Structure 4.32. 

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. 

Sesquiterpenes, 15-carbon-containing terpenoids, are composed of three isoprene units, and found abundantly in plants, e.g. artemisinin from... 

Locate the isoprene units in each of the monoterpenes, sesquiterpenes, and diterpenes shown in Figure 26.6. (In some cases there are two equally correct arrangements.)... 

Sesquiterpenes and their derivatives are found together with monoterpenes in many essential oils. Many of them are important flavour and perfume compounds, some are of considerable importance for pharmaceutical applications [131]. As sesquiterpenoids contain one more isoprene unit than monoterpenes, a greater variety of structures is possible which is... 

We have investigated 4-bromo-3-methyl-3-buten-l-ol as a four-carbon-chain extender that incorporates an isoprene unit. First experiments suggest this reagent will be useful for preparing sesquiterpenes from myrcene. 

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... 

Know the meaning of terpene, isoprene unit, monoterpene, sesquiterpene, diterpene triterpene, steroid. 

Sesqui means half as much again, so sesquiterpenes have a molecular formula one and a half times a monoterpene they are made up of three isoprene units. The molecular formula is C15H24 (Fig. 3.3). 

Carefully circle the isoprene units in the following terpenes, and label each compound as a monoterpene, sesquiterpene,... 

Adam, K.R and Zapp, J. (1998) Biosynthesis of the isoprene units of chamomile sesquiterpenes. Phytochemistry, 48, 953-9. 

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