Monocyclic monoterpene hydrocarbon

Monocyclic monoterpene hydrocarbons occur in many essential oils and their by-products. They have relatively weak odours, although some add dryness and green notes to the oils containing them. This is particularly so for lime and petitgrain. D-Limonene (1) occurs in citrus oils whereas the L-isomer is found in pine. If limonene or other terpenes break down during processing to produce isoprene, then racemic limonene, dipentene, is found in the product as a result of the Diels-Alder reaction. Terpinolene (2) is the dehydration product of a-terpineol and so it is often present as an artefact. a-Phellandrene (3) occurs in eucalyptus oil. Since it is a 1,3-diene, it is an obvious precursor for Diels-Alder reactions and a number of speciality ingredients are... 

Monocyclic monoterpenic hydrocarbons are derived predominantly from the optically active hydrocarbon 4-isopropyl-1-methylcyclohexane, known as p-menthane (8-2). An exception isp-cymene also known as cymene (l-isopropyl-4-methylbenzene, 8-3), which is an aromatic hydrocarbon. Cymene is a common component of many essential oils, especially the essential oils of cumin (the seed of the herb Cuminum cyminum of the parsley family Apiaceae) and common thyme Thymus vulgaris, from the mint family Lamiaceae) Hsted in Table 8.32 (see later). 

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. 

Monoterpenic hydrocarbons found in foods are hnear (acychc), monocyclic, bicyclic and tricyclic compounds. Linear monoterpenes are mainly present in fruits and essential oils. Examples of common hydrocarbons are myrcene (fl-myrcene, 8-1) and ocimene (P-ocimene, 8-1), which occurs in two stereoisomers, as traws-isomer (E)-ocimene and as cis-isomer (Z)-ocimene. 

The gaschromatographical analysis of the volatile compounds in comparison with the volatile pattern synthesized by whole daffodil flowers (Fig. 1) revealed that the compounds formed in vitro were monoterpene hydrocarbons plus linalool, mainproducts being myrcene (36.4 o), ocimene (18 o) and linalool (22 9 o). Moreover the bicyclic a-pinene (2.8 0 and the monocyclic limonene (l o) were formed. The low portion in cyclic monoterpenes is in good agreement with the natural pattern. Monoterpene ketones as well as sesquiterpenes which both occur in the natural pattern, were not obtained in chromoplast stroma incubations. On the other hand, linalool was not present in the in vivo pattern. 

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. 

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... 

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... 

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