Acyclic monoterpene hydrocarbons

Myosin see Muscle proteins Myrcene a triply unsaturated acyclic monoterpene hydrocarbon. M is a pleasant smelling liquid, M, 136.24, b.p.,2 55-56°C, p 0.8013, n 1.470. It is a component of many essential oils, and it is prepared for the perfumery industry by pyrolysis of p-pinene (from oil of turpentine). M. is al used commercially for the preparation of isomeric acyclic monoterpene alcohols and their acetates... 

Monoterpenoids are responsible for fragrances and flavors of many plants and thus their products are used in perfumery and as spices. To date over 1,500 monoterpenoids are known, and these constitute acyclic, monocyclic, and bicyclic monoterpenoids (32), which occur in nature as hydrocarbons, alcohols, aldehydes, and carboxylic acids and their esters. Several acyclic monoterpenoid hydrocarbons are known, and these include trienes such as 3-myrcene (Cl), a-myrcene (C2), (Z)-a-ocimene (C3), ( )-a-ocimene (C4), (Z)-3-ocimene (C5), and ( )-P-ocimene (C6). 3-Myrcene and 3-ocimene are constituents of basil (Oci-mum basilicum, Labiatae) and bay (Pimenta acris, Myrtaceae), pettitgrain (Citrus vulgaris, Rutaceae) leaves, strobiles of hops (Humulus lupulus, Cannabaceae), and several other essential oils. Unsaturated acyclic monoterpene alcohol constituents of plants and their derived aldehydes play a signihcant role in the perfume industry. Some common acyclic monoterpene alcohols and aldehydes include geraniol (C7), linalool (C8) (a... 

Molecular structures of monoterpenoids are present as three major types acyclic, monocyclic and bicyclic. Oxygenated derivatives of acyclic monoterpenes are more widespread in nature than acyclic monoterpenes themselves. Such derivatives are the monoterpene alcohols citronellol and geraniol, or the monoterpene aldehydes citronellal and geranial. Mononocyclic monoterpene hydrocarbons are exemplified by limonene, which is a major component of orange and lemon peel oils, and p-phellandrene, which is emitted by conifer trees under biotic stress. These compounds are relatively common in nature. Bicyclic monoterpenes are represented by the aforementioned a-pinene, one of the most abundant and important monoterpenes. 

Monoterpenes occur in plants in various structural forms some are cyclic while the others are acyclic. They also contain various types of functional group, and depending on their functional groups they can be classified as simple hydrocarbons, alcohols, ketones, aldehydes, acids or phenols. Some examples are cited below. 

The monoterpenes are subdivided into three groups acyclic, monocyclic and bicyclic (there is only one tricyclic terpene tricyclene). Each group contains hydrocarbon terpenes, terpene alcohols, terpene aldehydes, ketones, oxides etc... 

Isoprene (2-methylbnta-l,3-diene), a branched diunsaturated Cj hydrocarbon, is the bnilding block of a large family of open-chain and cyclic isoprenoids and terpenoids (Fig. 4.8). Essential oils of higher plants are enriched in monoterpenes (Cj ) with two isoprene nnits. Farnesol, an unsaturated alcohol, is an example of a sesquiterpene with three isoprene units. The acyclic diterpene phytol is probably the most abnndant isoprenoid on Earth. It occnrs esterified to chlorophyll a and some bacteriochlorophylls and is. 

Monoterpene biosynthesis has been studied in conifers using labeled precursors such as carbon dioxide, acetate and mevalonate (63,64). Specifically labeled precursors have been employed to deduce mechanistic features of a-plnene (65,66) and 3-carene (67,68) biosynthesis in pine species. Glelzes and co-workers (69) have argued, by way of time-course studies, that the initial formation of acyclic hydrocarbons (oclmene, myrcene) from C02 in Pinus pinaster needles indicated that these olefins serve as precursors to cyclic monoterpenes (a-plnene, 8-pinene) by a reversible protonatlon mechanism. These suggestions, however, are without precedent, and run counter to direct evidence demonstrating that the cyclizatlon of geranyl pyrophosphate occurs without the involvement of free intermediates (17). 

Using these methods in combination with gas chromatography-mass spectrometry (GC-MS), we were able to detect the emission of a number of monoterpenes as well as a large group of sesquiterpenes from whole Arabidopsis Columbia plants (Fig. 1.4). In total, 3 monoterpenes (P-myrcene, linalool, and limonene) and over 20 sesquiterpene hydrocarbons were detected with E- -caryophyllene as the predominant terpene volatile. The sesquiterpene volatiles showed a high structural diversity including acyclic, mono-, di- and tricyclic compounds. All monoterpenes and 19 sesquiterpenes were identified with certainty by mass spectra and comparison with authentic standards. 

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