Acyclic Monoterpenoid

FIGURE 5.16 Some of the more common terpenoid hydrocarbons.

b-pinene (76), and 3-carene (77) are all major constituents of turpentine from a wide range of pines, spruces, and firs. The pinenes are often found in other oils, 3-carene less so. Like the pinenes, cam-phene (89) is widespread in nature.

Simple hydrolysis of geranyl pyrophosphate gives geraniol, (E)-3,7-dimethylocta-2,6-dienol (63). This is often accompanied in nature by its geometric isomer, nerol (90). Synthetic material is usually a mixture of the two isomers and when interconversion is possible, the equilibrium mixture comprises about 60% geraniol (63) and 40% nerol (90). The name geraniol is often used to describe a mixture of geraniol and nerol. When specifying the geometry of these alcohols it is better to use the modern (E)/(Z) nomenclature as the terms cis and trans are somewhat ambiguous in this case and earlier literature is not consistent in their use. Both isomers occur in a wide range of essential oils, geraniol (63) being particularly widespread. The oil of Monarda fistulosa contains over 90% geraniol (63) and the level in palmarosa is over 80%. Geranium contains about 50% and citronella and lemongrass each contain about 30%. The richest natural sources of nerol include rose, palmarosa, citronella, and davana although its level in these is usually only in the 10-15% range. Citronella and related species are used commercially as sources of geraniol but the price is much higher than that of synthetic material. Citronellol (91) is a dihydrogeraniol and occurs widely in nature in both enantiomeric forms. Rose, geranium, and citronella are the oils with the highest levels of citronellol. Geraniol, nerol, and citronellol, together with 2-phenylethanol, are known as the rose alcohols because of their occurrence in rose oils and also because they are the key materials responsible for the rose odor character. Esters (the acetates in particular) of all these alcohols are also commonly encountered in essential oils (Figure 5.17).

Allylic hydrolysis of geranyl pyrophosphate produces linalool (92). Like geraniol, linalool occurs widely in nature. The richest source is Ho leaf, the oil of which can contain well over 90% linalool. Other rich sources include linaloe, rosewood, coriander, freesia, and honeysuckle. Its acetate is also frequently encountered and is a significant contributor to the odors of lavender and citrus leaf oils.

Figure 5.18 shows a selection of cyclic monoterpenoid alcohols. a-Terpineol (74) is found in many essential oils as is its acetate. The isomeric terpinen-4-ol (93) is an important component of Ti tree oil but its acetate, surprisingly, is more widely occurring, being found in herbs such as marjoram and rosemary. l-Menthol (94) is found in various mints and is responsible for the cooling effect of oils containing it. There are eight stereoisomers of the menthol structure, l-menthol is the commonest in nature and also has the strongest cooling effect. The cooling effect makes menthol and mint oils valuable commodities, the two most important sources being cornmint (Mentha arvensis) and peppermint (Mentha piperita). Isopulegol (95) occurs in some species including Eucalyptus citriodora and citronella. Borneol (endo-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ol) (79) and esters thereof, particularly the acetate, occur in many essential oils. Isoborneol (exo-1,7,7-trimethylbicyclo[2.2.1]heptan-2-ol) (96) is less common; however, isoborneol and its esters are found in quite a number of oils. Thymol (97), being a phenol, possesses antimicrobial properties, and oils, such as thyme and basil, which find appropriate use in herbal remedies. It is also found in various Ocimum and Monarda species.

Three monoterpenoid ethers are shown in Figure 5.19. 1,8-Cineole (98), more commonly referred to simply as cineole, comprises up to 95% of the oil of Eucalyptus globulus and about 40-50% of

FIGURE 5.17 Key acyclic monoterpenoid alcohols.

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