Monoterpenes in essential oils

Shellie R, Mondello L, Dugo G, Marriott P, Enantioselective gas chromatographic analysis of monoterpenes in essential oils of the family Myrtaceae, Flavour Fragr J 19 582-585, 2004. 

The knowledge of enantiomeric composition of monoterpenes in essential oils is important from a scientific as well as from a practical point of view. It can give information related to chemotaxonomy, biogenesis and biosynthetic relations between compounds. It can also be useful in quality control genuines of essential oils. In some cases it can also answer the questions of applied extraction techniques and geographic origin of the extracted plant. 

The composition of monoterpenes in essential oils in the coniferous can be useful in the genetic and taxonomic studies. Monoterpenes are used as genetic markers of clones and varietes. They can be applied for the identification of clones, hybrids and subspecies. 

K. Kempe, M. Kohnen (1994) 6 C-values of monoterpenes in essential oils as a mirror of their biosynthesis. Isotopenpraxis Environ. Health Stud. 30, 205-211... 

Croteau, R., Catabolism of monoterpenes in essential oil plants, in Flavors and Fragrances A World Perspective (B. M. Lawrence, B. D. Mookherjee, and B. J. Willis, eds.), 65-84, Elsevier, Amsterdam, 1988. 

The sesquiterpenes found in essential oils have low volatilities compared with monoterpenes and so are isolated mainly by steam distillation or extraction, but some are also isolated by distillation or crystallization. Most of the sesquiterpene alcohols are heavy viscous Hquids and many crystallize when they are of high enough purity. Sesquiterpene alcohols are important in perfume bases for their odor value and their fixative properties as well. They are valuable as carriers of woody, balsamic, or heavy oriental perfume notes. 

Many of the monoterpenes found in essential oils of plants also occur as pheromonal substances in insects (45,57-60) and are often involved in plant-insect interactions. Some compounds found both in plants and insects are the monoterpenes citronellal, cit-ronellol, geraniol, myrcene, citral, 6-phellandrene, limonene, 2-terpinolene, a-pinene, 6-pinene, 1,8-cineole, and verbenone. 

Some volatile iridoid monoterpenes with biological activity are also found in essential oils and in insect pheromonal and defensive substances. Eisner ( ) found that 17 species of insects were repelled by the iridoid monoterpene nepetalactone. Lacewings (Chrysopa septempunctata) are attracted by the leaves and fruits of Actinidia polyqama (Actinidiaceae) which contain a series of volatile iridoid monoterpenes (67). 

Monoterpenes are formed from two attached isoprene 3 units 2,6-dimethyloc-tane as the simplest skeleton. Thus, they can be acyclic or linear like -myrcene 4, ( )-/l-ocimene 5, (Z)-/l-ocimene 6, and allo-ocimene 7 (Structure 4.3). Or they can be cyclic, meaning ring-forming, such as in the simplest form like p-menthane 8 or p-cymene 9. Monocyclic 8, 9, bicyclic (5-3-carene 10 and tricyclic tricyclene 11 type monoterpenes are found in essential oils [1-4, 6-14, 16-23,38,39, 42, 47, 48]. 

Pinane monoterpenes are bicyclic monoterpenes resulting from intramolecular rearrangement of the a-terpinyl cation yielding the [3.1.1] bicyclic system (Fig. 4.3). a-Pinene 45 and -pinene 46 (Structure 4.11) are the main constituents of turpentine oil from pines. They occur widely in essential oils. 

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. 

Esters of benzenoid and monoterpenic acids and alcohols as well as unsaturated carboxylic acids such as tiglic acid 142 and angelic acid 143 (Structure 4.43) are found in essential oils [60, 61]. 

Kreck M Scharrer A Bilke S Mosandl A (2002) Enantioselective analysis of monoterpene compounds in essential oils by stir bar sorptive extraction (SBSE)-enantio-MDGC-MS. Flavour Fragrance J 17 32... 

Konig, W.A., Kruger, A., Icheln, D. and Runge, T. (1992) Enantiomeric composition of the chiral constituents in essential oils. Part 1 monoterpene hydrocarbons, journal of High Resolution Chromatography 15, 184-189. 

Most essential oils are liquids with constituents that exhibit a range of molecular sizes. Monoterpenes have molecular formulae C1QH1G, while sesquiterpenes have the formula C15H24. Although quite large molecules are present in essential oils, the oils do not contain any macromolecules. 

Myreene is an example of a monoterpene and is found in essential oils of bay, verbena, pine and juniper, and in many others. 

SUMMARY In essential oils, most constituents are terpenes and terpenoid molecules. The method of extraction can influence the terpene content. In citrus oils extracted by expression (squeezing or pressing the plant material), the terpenes present are similar to those found in the living plant tissue for example, orange, lemon, mandarin and grapefruit essential oils may be made up of up to 90% of the monoterpene limonene. When extraction is by steam distillation the action of the hot water and steam on thermolabile (heat-sensitive) molecules present in the plant is responsible for the formation of the bulk of the terpene content. Solvent extraction often produces absolutes that are very low in terpenes or do not contain them at all, when compared to a distillation of the same material, for example as found in lavender and rose products. 

The MEP pathway is the starting material of major plant terpenoids of economic value e.g., monoterpenes from essential oils, diterpenoids with potent biologic activity (taxol, ginkgolides), vitamins (tocopherol), or vitamin precursors (carotenoids). The first attempts to overexpress some of its genes (especially dxs or dxr) led to enhanced carotenoid production in tomato (18) or monoterpenes in mint (19). 

Monoterpenic hydrocarbons sometimes occur in essential oils in considerable amounts. They are often used as a starting material for a synthesis of flavor or fragrance compounds although they themselves contribute relatively little to the fragrance and aroma. 

Solution (a) Monoterpenes are compounds which have the general formula of CioHie They can be found in essential oils, resins, and many other natural substances. They are usually made up of isoprene units (2-methyl-l,3-butadiene), joined in a head-to-tail manner. This information tells one that citral has at least one olefinic bond. 

C,gH,4, Mr 134.22, bp. ca. 30 C (30 Pa). Conjugated unsaturat acyclic monoterpene widely distributed in essential oils, e. g., with a content of20-40% in the essential oil of Cosmos bipinnatus (Asteraceae). 

Acyclic monoterpenes geraniol in essential oils, perfume products and luxury foods, production from other terpene products, e.g. beta-pinene acyclic, double unsaturated alcohol, several possible reactions, occurrence as esters, typicjd rose odor... 

Monocyclic monoterpenes p-menthane in essential oils, e.g. eucalyptus fruits typical odor (fennel)... 

Bicyclic monoterpenes pinenes in essential oils and conifer products, industrial production and use, e.g. in fragrance and flavor industry typical odor (turpentine), softening agent... 

Acetylenic compounds that occur in essential oils probably contribute to the overall taste-odor properties in several spices and herbs derived from the Apiaceae. Compound (8) (Fig. 3.13), in association with monoterpenes, contributes to the flavor of caraway, Carum carvi. 

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