Terpene and Sesquiterpene Alcohols

Acyclic terpene and sesquiterpene alcohols occur in many essential oils. These alcohols were formerly isolated from oils in which they are major components. 

In addition to 75% monoterpene hydrocarbons (Oa-pinene 5-21% /7-pinene 40-70% 3-carene 2-16%) and ca. 10% sesquiterpene hydrocarbons, galbanum oil contains a fairly large number of terpene and sesquiterpene alcohols and their acetates. Minor components, with entirely different structures and low odor threshold values, contribute strongly to the characteristic odor [284a, 515-519]. Examples are as follows ... 

Dehydration see also Phosphoryl chloride-pyridine Thionyl chloride-pyridine). The dehydration of terpene alcohols with acidic reagents usually leads to several isomeric olefins, von RudlofP found that dehydration of terpene and sesquiterpene alcohols with neutral alumina (Woelm 1) to which 1-2% of pyridine or quinoline has been added gives a single or only a few products. The alcohol is heated under reflux at 200-230° for 1-6 hrs. with twice its weight of the treated alumina and the product isolated by steam distillation. 

Acyclic terpene and sesquiterpene alcohols occur in many essential oils. These alcohols were formerly isolated from oils in which they are major components. Currently, large-scale synthesis of terpenoids permits production without the uncertainties associated with isolation from natural sources. However, the odor qualities of synthetic products often differ from those of compounds isolated from natural sources, since the desired natural product often is not separated from small amounts of compounds with similar physical properties but different odor quality. 

Table 28. Separation of terpene and sesquiterpene alcohols by adsorption chromato-graphy on a silica gel G layer using chloroform...

Beaxjn [20], Gbaf and Hoppe [79) and Mehlitz and co-workers [163] have used similarly non-polar solvent combinations. Menthol and isomenthol dinitrobenzoates can be separated with hgroiu (105—120° C)-isopropyl ether (95 + 5) on silica gel G laj -ers under normal conditions [79]. The dinitrobenzoates of other terpene and sesquiterpene alcohols could be clearly differentiated by double development on 500 xm-silica gel G layers using petrol ether-isopropyl ether (94 + 6). Solvent mixtures containing twice as much ether are more suitable for TLC of the dinitrobenzoates of lower alcohols. 

The terpene- and sesquiterpene alcohols, used as food flavouring agents in the food industry, are treated in Chapter J on p. 225 and the sugar alcohols in Chapter X. 

Abundant sources of terpenoids are the essential oils. They consist of a complex mixture of terpenes or sesquiterpenes, alcohols, aldehydes, ketones, acids and esters [21]. 

As a result of acid-catalysed reactions during steam distillation, a number of terpenes and sesquiterpenes [66] of the native cold-pressed oils undergo transformation /9, 67-69]. This applies mainly to the pinenes, sabinene, thujene and some sesquiterpenes. While cold-pressed lime oils contain up to 2.3% of the reactive germacrenes, only traces of germacrene B can be found in distilled oils ]70]. When compared to their educts, the newly formed constiments, mainly alcohols, such as a-terpineol or fenchyl alcohol, possess completely different sensory properties. Additionally, cyclisations and hydratisations of aldehydes result in a reduced presence of these constituents in distilled products. Many compounds which characterise the flavour of distilled lime are formed during production. The extremely different composition of cold-pressed and distilled oils accounts for their completely different flavour profile and they, therefore, have to be considered as a reaction flavouring. 

In the process of extraction with low boiling organic solvents, an important consideration is the presence of residual solvent in the extract. Also, extraction with organic solvents does not totally eliminate thermal degradation since the solvent has to be removed by distillation. Owusu-Yaw et al. (20) extracted cold-pressed Valencia orange oil with aqueous ethyl alcohol to remove the terpenes and sesquiterpenes. The ratio 1 3 of oilisolvent resulted in sesquiterpeneless oils with a low terpene content. However, this ratio also gave low oil recoveries. 

Occurrence In the outer parts of birch bark Betula alba, Betulaceae) in amounts of up to 24% dry weight and in other barks together with saponins and sesquiterpene alcohols such as betulenol. B. is a triterpene alcohol with the lupane skeleton (cf. figure under tri-terpenes) and was one of the first natural products to be investigated (first mentioned in 1788 ). 

Most essential oils are complex mixtures of terpenic and sesquiterpenic hydrocarbons and their oxygenated terpenoid and sesquiterpenoid derivatives (alcohols, aldehydes, ketones, esters, and occasionally carboxylic acids), as well as aromatic (benzenoid) compounds such as phenols, phenolic ethers, and aromatic esters. So-called terpeneless and sesquiterpeneless essential oils are commonly used in the avor industry. Many terpenes are bitter in taste, and many, particularly the terpenic hydrocarbons, are poorly soluble or even completely insoluble in water-ethanol mixtures. Since the hydrocarbons rarely contribute aitything of importance to their avoring properties, their removal is a commercial necessity. They are removed by the so called washing process, a method used mostly for the treatment of citrus oils. This process takes advantage of the different polarities of individual essential oil constituents. The essential oil is added to a carefully selected solvent (usually a water-ethanol solution) and the mixture partitioned by prolonged stirring. This removes some of the more polar oil constituents into the water-ethanol phase (e.g., the solvent phase). Since... 

The position of these substances in the chromatogram is determined by the polarity of the parent compound. Menthofuran migrates just behind guaiazulene [182 a, 247]. The terpene and sesquiterpene epoxides follow, lying in the upper part of the ester zone on silica gel G layers (Table 23) (cf. also [155]). in agreement with El-Deeb [55]. The carbonyl and alcohol oxides follow with lower hRf-values. This sequence holds even at — 9° C using Freon (Frigen 21) as solvent [255]. 

Esters mainly derived from cyclic terpene alcohols, especially the acetates, are common fragrance and flavor components. Menthanyl, menthenyl, bicyclic bornyl acetates and a few acetates of sesquiterpene alcohols are extensively used in perfume and aroma compositions. 

Higher oxygenated terpenes include the sesquiterpene alcohol, famesol, which has a lily-of-the-valley odor and occurs in ambrette-seed oil. On acid dehydration it gives a-farnesene (Table 30-1) under some conditions, and bis-abolene (a component of oil of bergamot) under others ... 

Turmeric owes its aromatic taste and smell to the oil present in the rhizome. Analysis of the oil, obtained by steam distillation of the powdered rhizome, followed by fractional distillation and derivatization, shows that the components are a mixture of predominantly sesquiterpene ketones and alcohols (Kelkar and Rao, 1933). The residue on steam distillation yields mainly sesquiterpene alcohols. Besides these major components, they have also identified a mixture of low-boiling terpenes, c/-sabinene, a-phellandrene, cineole, borneol and the higher-boiling sesquiterpene, zingiberene, in substantial amounts (25%). 

Pure essential oils are mixtures of more than 200 components, normally mixtures of terpenes or phenyl-propanic derivatives, in which the chemical and structural differences between compounds are minimal. They can be essentially classified into two groups A volatile fraction, constituting 90-95% of the oil in weight, containing the monoterpene and sesquiterpene hydrocarbons, as well as their oxygenated derivatives along with aliphatic aldehydes, alcohols, and esters and a nonvolatile residue that comprises 1-10% of the oil, containing hydrocarbons, fatty acids, sterols, carotenoids, waxes, and flavonoids. 

Tea tree oil is derived from leaves of Melaleuca alternifolia and M. linearifolia (Myrtaceae), native to the east coast of New South Wales. To meet the Australian standard for tea tree oil the terpinen-4-ol content must be at least 30% while that of the oxidised terpene 1,8 dneole must not exceed 15% (Williams et al. 1988). The chemo-types of tea tree oil from natural stands is genetically determined, so that all commercial plantations are started from seed known to be of the right chemotype. Other terpene constituents of tea tree oil are the alcohol a-terpineol, hydrocarbons a-pinene and (3-pinene, p-cymene and y-terpinene, and sesquiterpenes including the unique compound viridflorene (Penoel 1990). Terpenin-4-ol is also a major ingredient in marjoram—Marjorana hortensis (Lamiaceae). 

Other terpene alcohols include geraniol, citronellol from rose oil, Rosa gallica (Rosaceae), and scented geraniums Pelargonium spp. (Geraniaceae). Nerol is a stereoisomer of geraniol. Borneol is found in rosemary oil—Rosmarinus officinalis (Lamiaceae). Santalol from heartwood of sandalwood—Santalum album, S. spicatum (Santa-laceae)—is a sesquiterpene alcohol. 

As widely acknowledged, the composition of essential oils is mainly represented by mono- and sesquiterpene hydrocarbons and their oxygenated (hydroxyl and carbonyl) derivatives, along with aliphatic aldehydes, alcohols, and esters. Terpenes can be considered as the most structurally varied class of plant natural products, derived from the repetitive fusion of branched five-carbon units (isoprene units) [7]. In this respect, analytical methods applied in the characterization of essential oils have to account for a great number of molecular species. Moreover, it is also of great importance to highlight that an essential oil chemical profile is closely related to the extraction procedure employed and, hence, the choice of an appropriate extraction method becomes crucial. On the basis... 

Mono- and sesquiterpenes are the main constituents of the essential oils, while the other terpenes are constituents of balsams, resins, waxes, and rubber. Oleoresin is a roughly equal mixture of turpentine (85 % Cio-monoterpenes and 15 % C15- sesquiterpenes) and rosin (C2o-diterpene) that acts in many conifer species as a toxic material to invading insects and their pathogenic fungi [12]. A number of angiosperm species have inducible terpenoid defensive compounds (phytoalexins) [13]. These include both sesquiterpenoid and diterpenoid t)pes. Isoprenoid units are also found within the framework of other natural molecules. Thus, indole alkaloids, several quinones (vitamin K), alcohols (vitamin E, vitamin A formed from p-carotene), phenols, and isoprenoid alcohols (also known as terpenols or polyprenols) also contain terpenoid fragments. The biogenesis. 

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