Bicyclic monoterpene alcohol

Although cyclic terpene alcohols occur widely in nature, few have the physiological properties that make them important fragrance or flavor materials. Exceptions are a-terpineol and (—)-menthol, the latter because of its cooling/refreshing effect. Of the bicyclic monoterpene alcohols, bomeol deserves mention. 

An important precursor of (-)-perrillyl aldehyde (periUal) is (-)-perillyl alcohol. Fenchol (also known as a-fenchyl alcohol or fenchan-2-ol, 8-20) is a bicyclic monoterpenic alcohol occurring as a minor component in citrus, fennel and sage essential oils. Another common compound is the bicyclic alcohol thujan-4-ol (sabinene hydrate), which occurs at a high level in the marjoram essential oil (see Table 8.32, later). Borneol (bornan-2-ol) is a component of camphor oils Cinnamomum camphora, Lauraceae). Trivial and systematic names of selected monoterpenic alcohols are listed in Table 8.6. 

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... 

The ketone fenchone and the alcohols a-fenchol (endo) and /8-fenchol (exo) constitute the fenchane family of bicyclic monoterpenes. Fenchanes are of rather limited distribution in the plant kingdom, and interest in this group of compounds stems primarily from the fact that their carbon skeleton appears to be derived by rearrangement of a pinane-type intermediate (Fig. 5). (-l-)-Fenchone is one of the m or terpenoids of the volatile oil of fennel (Foeniculum vulgare), and soluble enzyme preparations from fennel leaves convert GPP and NPP to fenchol in the presence of Mn and, in the pres-... 

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. 

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... 

Thujane monoterpenes are based on the bicyclic (G3 C5) monoterpene thujane and include umbellone (thujan-2-one) and the neuroactives a-thujone and 3-thujone (thujan-3-one isomers) that can cause convulsions. Thujones are GABA(A) receptor antagonists and are the active constituents in oil of wormwood from Artemisia absinthium (Asteraceae) used in the alcoholic drink absinthe that was eventually banned because of its deleterious neurotoxic effects. 

Monoterpenes (Cio) are the simplest members of the terpene series. They result from the condensation of two isoprene units and may be acyclic, monocyclic, bicyclic or tricyclic. The monoterpenes can have another functional moiety like alcohol (geraniol, linalool, menthol, bomeol), aldehyde (geranial, cihonellal), ketone (menthone, carvone, thujone), ester (bomyl acetate, linalyl acetate), ether (1,8-cineol) and phenol (thymol, carvacol). In the case of optically active molecules, the proportions of the enantiomers vary largely from one species to another. 

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