Terpenes and Camphors

Terpenes and Camphor.—The importance of these two phenols is in their natural occurrence as ethers in ethereal oils of many plants, e.g., oil of thyme and oil of caraway, and especially in their relationship to the terpenes and camphor, as will be shown later (p. 826, 834). 

Wagner-Meerwein rearrangements occur extremely frequently among branched-chain aliphatic and alkylaryl compounds, and are particularly important in the terpene and camphor series. An example of rearrangement of type a) is that of camphene hydrochloride (1) into isobornyl chloride (2), for which Meerwein and van Emster141 give the following directions ... 

Berthelot s researches on the terpenes and camphor were important from the point of view of classification and he discovered some new compounds. He distinguished d- and /-pinene, calling them australene and terebentene, and d-, /-, and rf/-camphene, the last obtained by removing hydrogen chloride from pinene hydrochloride (bornyl chloride his camphene is now called bornylene). Berthelot used only the hydrochlorides to characterise the various pinenes. He... 

Pine Oil. This oil is obtained by extraction and fractionation or by steam distillation of the wood of Pinuspalustris Mill, and other species. Most of the oil is produced ia the southeastern United States. The composition of the oil depends on the fractions chosen, but the chief constituents are terpene alcohols, mainly terpiaeol. Piae oil finds use as a germicide ia disiafectants and soaps as an ingredient ia iasecticides, deodorants, poHshes, sweepiag compounds, and catde sprays and as raw material for the manufacture of perfumery-grade terpiaeol [8000-41 -7], anethole [104-46-1], fenchone (137), and camphor (35). 

Essential oils are known to have detrimental effects on plants. The inhibitory components have not been identified, but both alde-hydic (benzol-, citrol-, cinnamal-aldehyde) and phenolic (thymol, carvacol, apiol, safrol) constituents are suspected. Muller et al. (104) demonstrated that volatile toxic materials localized in the leaves of Salvia leucophylla, Salvia apiana, and Arthemisia californica inhibited the root growth of cucumber and oat seedlings. They speculated that in the field, toxic substances from the leaves of these plants might be deposited in dew droplets on adjacent annual plants. In a subsequent paper, Muller and Muller (105) reported that the leaves of S. leucophylla contained several volatile terpenes, and growth inhibition was attributed to camphor and cineole. 

Alicyclic hydrocarbons are saturated carbon chains that form ring structures. Naturally occurring alicyclic hydrocarbons are common (Chap. 1). For example, alicyclic hydrocarbons are a major component of crude oil, comprising 20-67 vol.%. Other examples of complex, naturally occurring alicyclic hydrocarbons include camphor (a plant terpene) and cyclohexyl fatty acids (components of microbial lipids). Anthropogenic sources of alicyclic hydrocarbons to the environment include fossil-fuel processing and oil spills, as well as the use of such agrochemicals as the pyrethrin insecticides (Chap. 1, and references therein). 

Terpenes are polymers of the 5-carbon compound isoprene (Figure 1.12) and, as such, generally display properties similar to those of hydrocarbons. Terpenoids are substituted terpenes (i.e. contain additional chemical groups, such as an alcohol, phenols, aldehydes, ketones, etc.). Only a few such substances could be regarded as true drugs. Terpenes, such as limonene, menthol and camphor, form components of various essential oils with pseudo-pharmaceutical uses. A number of these molecules, however, exhibit anti-tumour activity, of which taxol is by far the most important. 

Exercise 30-7 Camphor can be made on an industrial scale from a-pinene (turpentine) by the following reactions, some of which involve carbocation rearrangements of a type particularly prevalent in the bicyclic terpenes and the scourge of the earlier workers in the field trying to determine terpene structures. 

Canadian workers have been exploring methods for the conversion of cheap and readily available camphor into enantiopure cyclopentane derivatives for use in terpene and steroid synthesis, e.g. 1 — 2. In a recent extension of these studies, em/o-3-bromo-4-methylcamphor 3 (0.1 mol) was... 

The biosynthesis of monoterpenoids and camphor has been described by several authors (108-llU). Ruzicka (115,116) proposed a unified biogenetic scheme for terpenes. The biosynthetic building blocks for these terpenes are iso-prene units. The biosynthetically active isoprene units are isopentenyl pyrophosphate [l] and dimethyl allyl pyrophosphate [2] the compounds that are derived from acetate via mevalonic acid (Scheme V). Geranyl pyrophosphate [3] is the C-10 precursor for the terpenes (117). Banthorpe and Baxendale (ll8) confirmed the biosynthetic pathway of (iamphor via acetate mevalonate by conducting degradation study of camphor, biosynthesized from l c labelled mevalonic acid. The biosynthesis of camphor is summarised in Scheme VI. 

The rearrangements of carbocations also play a role in the formation of ter-pene natural products. Menthol and camphor are examples of terpenes derived from plant oils. They are constructed using a common building block, as shown in Section 25-8, and undergo a series of rearrangements in the course of construction to generate the most stable carbocation. 

You will notice that they are all aliphatic compounds with a scattering of double bonds and rings, few functional groups, and an abundance of methyl groups. A better definition (that is, a bio synthetically based definition) arose when it was noticed that all these compounds have 5n carbon atoms. Pinene and camphor are Cto compounds, humulene is Cl5> and phytol is C20 It seemed obvious that terpenes were made from a C5 precursor and the favourite candidate was isoprene (2-methylbuta-l,3-diene) as all these structures can be drawn by joining together 2-, 3-, or 4-isoprene skeletons end to end. Humulene illustrates this idea. 

For both terpene ketones, pulegone and camphor, the A ( C-ASIS) has been estimated by a chemical shift comparison method using t-butylcyclohexane as an additional reference compound [409], By comparing the A (ASIS) of both and nuclei it seems that the carbonyl plane rule is also valid for aromatic solvent-induced NMR chemical shifts of carbonyl compounds [409], Obviously, the particular geometrical arrangement of the aromatic solvent molecules around the carbonyl dipole inhu-ences both the and nuclei in the same way. 

As resolution procedures are often tedious, and asymmetric synthesis provides chiral products with only limited enantiomeric excess, it seems an obvious strategy to use an enantiomerically pure material from the chiral pool to construct chiral ferrocenes by incorporating these compounds in the final product. As such chiral materials, cheap terpenes (menthone, a- and -pinene, and camphor) were chosen. The reaction of ferrocene with carbonyl compounds under acidic conditions is a very convenient way to obtain directly a-ferrocenylalkyl carbocations. The starting materials were therefore converted to aldehydes or their enol ethers (menthone and camphor are too sterically hindered and do not react with ferrocene). Joint dissolution of the aldehydes and ferrocene in trifluoroacetic acid or in the trichloroacetic acid/ fluorosulfonic acid system gives a-ferrocenylalkyl carbocations, which can either... 

Cyclic Terpenes.— The cyclic terpenes and their oxidation derivatives such as pinene, limonene, menthol, terpineol, cineol, carvone, fenchone and camphor are found in a large number of essential oils... 

C5H8, and may be either acyclic or cyclic with one or more benzenoid groups. They are classified as mono-cyclic (dipentene), dicyclic (pinene), or acyclic (myr-cene), according to the molecular structure. Many terpenes exhibit optical activity. Terpene derivatives (camphor, menthol, terpineol, bomeol, geraniol, etc.) are called terpenoids many are alcohols. 

The essential oil of Salvia fructicosa, which contains the terpenes 1,8-cineole, a- and (3-thujone, and camphor, kills herpes simplex virus and several strains of bacteria.266 Antimicrobial peptides of plants and animals267 that resist infection of the host can now be produced in recombinant bacteria.268... 

In plants, terpenes are typically found as a component of the essential oils. The name of this fluid suggests an important characteristic of this class of molecules. If humans have found these oils important enough to refer to them as essential, there is a reasonable chance that they have useful properties. Many terpenes such as menthol (shown in Figure 1) and camphor have medicinal values. You may have used a throat lozenge with menthol to help clear your sinuses the last time you had a cold. 

Oil of Cedar Wood. Volatile oil from wood of J uni perns virginiana L., Cupressaceae, and other species of cedar- Constit. Chiefly cedrene (a terpene), and cedral (cedar camphor). 

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