Limonene from isoprene

Monoterpenes, 10-carbon-containing terpenoids, are composed of two isoprene units, and found abundantly in plants, e.g. (+)-limonene from lemon oil, and (—)-linalool from rose oil. Many monoterpenes are the constituents of plant volatile oils or essential oils. These compounds are particularly important as flavouring agents in pharmaceutical, confectionery and perfume products. However, a number of monoterpenes show various types of bioactivity and are used in medicinal preparations. For example, camphor is used in liniments against rheumatic pain, menthol is used in ointments and liniments as a remedy against itching, bitter-orange peel is used as an aromatic bitter tonic and as a remedy for poor appetite and thymol and carvacrol are used in bactericidal preparations. 

Terpenes are a class of naturally occurring olefins found in many plants. They are biosynthetically derived from isoprene, CsHg, and therefore consist of Cs-building blocks and are found in both cyclic and acyclic forms. An important subgroup is the monoterpenes, which consist of two isoprene units and are of the molecular formula Ci0H16. Of the monocyclic monoterpenes, by far the most industrially relevant is limonene, which is abundantly present as a fragrance in various household products. From the bicyclic monoterpenes, a-pinene, p-pinene, camphene, and carene are of industrial relevance and are also mostly used in the fragrance industry [44]. 

As part of a general investigation of terpenoid synthesis from isoprene units it has been shown that condensation of limonene (46) with senecioyl chloride in the presence of stannic chloride, followed by dehydrochlorination, provides a new simple synthetic route to a mixture of cis- (47) and frans-atlantones (48).23a Using acetyl chloride instead of senecioyl chloride in the synthetic sequence provides a C12 trisnor-sesquiterpenoid (49) which co-occurs with trans-atlantone in the essential oil of Cedrus atlantica.23b... 

Respiratory irritant mixtures can arise from environmental chemical reactions. For example, ozone reacts rapidly with terpenes under environmental ambient conditions to produce aldehydes, ketones, and carboxylic acids. Several studies that have been carried out demonstrated that reaction of ozone with a-pinene, c/-limonene, and isoprene produce low level concentrations (at or below NOEL levels) of oxidation products and that along with residual ozone and terpenes act as respiratory irritants. 1012 Table 17.3 lists the species typically contained in these mixtures along with their K values. As can be seen, the mixtures contain lipophiles (residual terpenes) and hydrophiles (the reaction products). Similar results have also been reported for environmental reaction of terpenes with ozone and nitrogen dioxide. 9 ... 

Synthesis of cn-Atlantone from d-Limonene. The total synthesis of a-atlantone from isoprene shown in Figure 6.22, gave positive proof of the structure as far as atom connectivity is concerned. However, what it did not achieve was to demonstrate the absolute stereochemistry of the product as the Diels-Alder reaction was not stereoselective. In order to establish the absolute stereochemistry, Cookson carried out a partial synthesis of a-atlantone from d-limonene.6 4 This is shown in Figure 6.24. 

Most of the hydrocarbons emitted by plants are terpenes, which are produced by conifers (evergreen trees and shrubs such as pine and cypress), plants of the genus Myrtus, and trees and shrubs of the genus Citrus. Three of the most common terpenes emitted by trees are a-pinene from pine trees, limonene from citrus trees and fruit, and isoprene (2-methyl-l,3-butadiene), a hemiterpene, from the Hevea brasilinesis tree, the main source of isoprene from which natural rubber is synthesized (see Figure 7.7). In many areas, terpenes from plants, which are highly reactive with atmospheric oxidants including ozone and hydroxyl ladicaL constitute the main source of hydrocarbons... 

Hydrocarbon resins comprise a range of low-molecular-weight products (M < 3000) used as adhesives, hot-melt coatings, tackifying agents, inks, and additives in rubber. These include products based on monomers derived from petroleum as well as plant sources. The petroleum-derived products include polymers produced from various alkenes, isoprene, piperylene, styrene, a-methylstyrene, vinyltuolene, and dicyclopentadiene. The plant-derived products include polyterpenes obtained by the polymerization of dipentene, limonene,... 

Component 1 in Singapore buildings was correlated with compounds associated with humans and their activities. Human effluents have been reported to contain isoprene (Ellin et al, 1974) while tetrachloroethylene is a VOC found in dry-cleaned clothes worn by building occupants (Wallace, Pellizzari and Wendel, 1991) or from the use of consumer products (Sack et al., 1992). Tetradecane, benzaldehyde, o-xylene, naphthalene are emissions from dry process photocopiers (Leovic et al., 1996). Component 2 with high loadings ofn-decane, n- undecane, toluene, styrene, n-nonane, 1,2,4-trimethyl benzene probably reflects the emissions of carpets and vinyl floorings (Yu and Crump, 1998). Component 3 was primarily correlated with heptane and methylcyclopentane, which could be due to the emissions of water-based paints. Finally, component 4 was associated with 2-methylpentane, hexane, cyclohexane, methylcyclohexane and limonene, which is reflective of the emissions of air fresheners and cleaning products (Sack et al., 1992). 

The epoxidation of terpenic substrates is of interest in the flavor and fragrance industry [55,56]. Terpenes are derivatives of isoprene, which has formula CsHg (2-methyl-frans-butadiene). There are tens of examples of terpenes, including limonene, a-pinene, geraniol, citronellol, myrcene, ocimene, camphene, a-terpin-eol, menthol, and isopugelol. Limonene is an abundant monoterpene extracted from citrus oil, which can be epoxidized to obtain fragrances, perfumes, and... 

It was thus shown that the CsHs]" ions produced by the decomposition of limonene have a similar structure to that of isoprene (a) and slightly different from those of the isomers (b, c, d and e) (Table 10, Fig. 30). 

Although isoprene itself has been implicated as part of a strategy some plants have for coping with heat and apparently contributes to the bluish haze hanging over some wooded hills in summer, condensation products of isoprene, for example, limonene abound. S-(-)-Limonene (boiling point [bp] 175-177°C) can be obtained from citrus (e.g.. Citrus limon). Considered simply as an achiral, unconjugated... 

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