Monoterpenes classes

The three classes of terpenes have different temperature profiles. These differences can be related to their boiling points, which are related to their molecular weight and their number of carbons [49]. Monoterpenes are highly volatile compounds (limonene bp763... 

Figure 10.4 Temperature profiles for three classes of analytes monoterpenes (limonene) sesquiterpenes [(3 caryophyllene, caryophyllene oxide (caryo. oxide)] and diterpenes [cem brene A, isoincensole acetate (iso. acetate)]. PDMS fibre, sampling time 40 min. Reproduced from S. Hamm, E. Lesellier, J. Bleton, A. Tchapla, J. Chromatogr., A, 1018, 73 83. Copyright 2003 Elsevier Limited...

Monoterpenes are often analyzed by capillary gas chromatography. LC/MS offers a new method for the separation and identification of members of this class of compounds (Figure 4). The separation occurs in a cold liquid phase, which eliminates the risk of thermal decomposition. Spectra obtained from the LC/MS system (Figure 5) can be interpreted by comparison with GC/MS reference spectra. 

Several lines of evidence support the conclusion that allelopathic inhibition of germination and plant growth typically occurs from the joint action of several allelochemicals. Additive or synergistic effects have been shown in bioassays with combinations of monoterpenes, organic acids, and several classes of phenolic compounds. These experiments demonstrate that a specific compound may be present below its threshold for inhibition and still be active in allelopathic interference by its effect in concert with other allelochemicals. 

The factors that influence the diastereoselectivity for the class of monocyclic hydrocarbons, from which monoterpenes are the most studied group, are mainly steric and conformational. Monoterpenes, in general, show relatively low overall syn/anti selectivity, usually attributed to the availability of allylic hydrogen atoms in the right alignment for abstraction. 

Overall, however, the immensity of temperate land corresponds to a most various secondary metabolic production, different from that of tropical land. The most renowned alkaloids belong to the morphine class (Chart 6.2.A1), and, in combination with isoprenoids, to the ergot and triterpene classes (Chart 6.2. A2). Prominent in the peptides are the cyclosporins (the first of which was isolated from a fiingus collected in Norway), streptogramins, and P-lactams (Chart 6.2.P). The isoprenoids are represented by pyrethrin monoterpenes, cedrane sesquiterpenes, ginkgolide and taxane diterpenes, ophiobolane sesterterpenes, and arborane and amyrin-like triterpenes (Chart 6.2.1). In the polyketides, epothilones, recently discovered from Myxobacteria, and the long known rapamycin, are two prominent classes of macrolides (Chart 6.2.FA/PO/C). 

Aromatic monoterpenes which contain a benzene ring like p-cymene 9, car-vacrol 12, thymol 13 and phenylethyl alcohol 14 (Structure 4.4) are common constituents of many essential oils, e.g. oregano (Origanum sp.), thyme (Thymus sp.), savory (Satureja sp.) and rose (Rosa sp.) oils. Another important constituent class of essential oils is phenypropanoids [36]. They are not considered as terpenoids owing to their different biogenetic origins, which will be mentioned later. 

According to the Dictionary of Natural Products (DNP), there are 25 different classes of monoterpenes [37]. 

The biosynthesis of different classes of monoterpenes formed from a-terpinyl cation and respective precursors are illustrated in Fig. 4.3. 

These regular monoterpenes constitute a small class which includes the trienes myrcene 4 and ocimenes (5-7) and the alcohols geraniol 15, nerol 16, citronel-lol 17, linalool 18, etc (Structure 4.5). 

Other important members of this class include oxygenated derivatives such as a-terpineol 32, menthol 33, isopulegol 34 and ds-hexahydrocuminyl alcohol 35, also classified as monoterpene alcohols. 

Research to date focused on isolating insecticidal prototype leads from marine origin has resulted in the report of about 40 active compounds.44 In an attempt to summarize these compounds and their activity margins, they have been categorized into seven classes of chemical structures polyhalogenated monoterpenes, polyhalogenated C15-metabolites, diterpenes, peptides and amino acids, phosphate esters, sulfur-containing derivatives, and macrolides. 

Very recently, the purification and characterisation of an epoxide hydrolase, catalysing the conversion of limonene-1,2-epoxide to limonene-1,2-diol has been described [90]. The enzyme was isolated from Rhodococcus erythropolis DCL14 and is induced when the microorganism is grown on monoterpenes. The authors found evidence that the enzyme, limonene-1,2-epoxide hydrolase is the first member of a new class (the third class) of epoxide hydrolases [91]. 

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. 

Terpenes constitute a large class of fine chemicals widely used in perfumery, food and pharmaceutical chemistry, adhesives. Among the monoterpenes (C qH ), a-pinene, /3-pinene and A -carene fractions are the most used in chemical industry. 

Thus far no sex pheromone has been described in the Phasmida. Some phasmids produce toxic monoterpenes in typical Class III integumentary glands located behind the head (e.g. Happ et al., 1966). The glands exhibit lipid reserves, carboxylic esterases, phosphatases, alcohol dehydrogenase, and a mevalonate kinase, all of which are suggested to be involved in the production of the toxic compounds (Happ et al., 1966). 

Terpenes are a class of naturally occurring olefins found in many plants. They are biosynthetically derived from isoprene, CsHg, and therefore consist of Cs-building blocks and are found in both cyclic and acyclic forms. An important subgroup is the monoterpenes, which consist of two isoprene units and are of the molecular formula Ci0H16. Of the monocyclic monoterpenes, by far the most industrially relevant is limonene, which is abundantly present as a fragrance in various household products. From the bicyclic monoterpenes, a-pinene, p-pinene, camphene, and carene are of industrial relevance and are also mostly used in the fragrance industry [44]. 

Ginger owes its characteristic organoleptic properties to two classes of constituents the odour and the flavour of ginger are determined by the constituents of its steam-volatile oil, while the pungency is determined by non-steam-volatile components, known as the gingerols. The steam-volatile oil comprises mainly of sesquiterpene hydrocarbons, monoterpene... 

The essential oil content during the different stages of leaf growth revealed that the eugenol content in the leaves increased from 38.3 to 95.2% with maturity, while the contents of eugenyl acetate (51.2 to 1.5%) and caryophyllene (6.3 to 0.2%) decreased (Gopalakrishnan and Narayanan, 1988). Clove bud and leaf oil contain various classes of compounds, e.g. monoterpenes, sesquiterpenes, aldehydes and ketones (Vernin et al., 1994), which are indicated in Table 8.8. 

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