Monoterpenes chemical structures

Chemical structure-activity relationships suggested that phenolic monoter-penes (thymol, methyleugenol) seemed to be the most active, followed by alcohols (terpineol) and other oxigenated monoterpenes (1,8-cineole) [225, 229, 230]. Within the monoterpenes, -pinene was more active than a-pinene [226], and a-pinene was more active than caryophyllene and myrcene [234]. 

Research to date focused on isolating insecticidal prototype leads from marine origin has resulted in the report of about 40 active compounds.44 In an attempt to summarize these compounds and their activity margins, they have been categorized into seven classes of chemical structures polyhalogenated monoterpenes, polyhalogenated C15-metabolites, diterpenes, peptides and amino acids, phosphate esters, sulfur-containing derivatives, and macrolides. 

The major monoterpene hydrocarbons present in pepper oil are a- and (3-pinenes, sabinene and limonene. Chemical structures of major aroma compounds are illustrated in Fig. 2.1. 

Terpenes are are a class of compounds whose chemical structures are based on a number of isoprene units , derived from the hydrocarbon CH2=C(CH3)-CH=CH2 they may themselves be hydrocarbons, but may also contain alcohol (OH), aldehyde/ketone (CO) and carboxylic acid (COOH) groups. Monoterpenes are Cm compounds derived from two isoprene units, sesquiterpenes (Cu) are derived from three isoprene units, diterpenes (C20) from four and tritepenes (C30) from six. Terpenes are widespread in plants, where they are largely responsible for the odor, and they are the major constituents of plant-derived essential oUs . Among the best known terpenes are 3-pinene (turpentine), camphor, menthol and citroneUal (aU monoterpenes) and farnesol (a sesquiterpene that is a constituent of the essential oils of many plants). Certain terpenes have important biological roles vitamin A, for example, is a diterpene, and steroid hormones have a structure related to triterpenes (and are biosynthesized by a similar route). 

Figure 2.2 Chemical structures of the most common turpentine monoterpene components.

Hydroformylation, which yields aldehydes, seems to be of particular value for the production of aroma compounds [1]. One important reason is associated with the chemical structure of starting compounds and products. Thus, in several cases pleasantly smelling olefins, mainly terpenes, are found in nature. Such terpenes (preferentially monoterpenes and some sesquiterpenes) with their sometimes multiple C=C-double bonds constitute one of the most important feedstock in flavor and fragrance industry. 

Fig. 1 Chemical structures of (co-)polymerizable monoterpenes (a) myrcene, (b) a-ocimene, (c) alloocimene, (d) citronellol, (e) geraniol, (f) linalool, (g) limonene (dipentene), (h) phellandrene, (i) a-terpineol, (j) a-pinene, and (k) P-pinene...

Among the complex mixtures that comprise a perfume, the fragrance chemicals they contain can be classified in different families according to their chemical structure, where it is usual to find the five-carbon isoprene unit in most of them, giving them the names of terpenes. So, one can find monoterpene hydrocarbons (e.g. Umonene), sesquiterpene hydrocarbons (e.g. a-famesene), alcohols (e.g. ai-3-hexenol), monoterpene alcohols (e.g. linalool), sesquiterpene alcohols (e.g. famesol), phenols (e.g. eugenol), aldehydes (e.g. 2,6-nonadienal), terpene aldehydes (e.g. citral), ketones (e.g. cyclohexanone), terpene ketones (e.g. jS-ionone), lactones (e.g. y-undecalactone), esters (e.g. methyl salicylate), terpene esters (e.g. linalyl acetate), and oxides (e.g. eucalyptol), etc. Some examples are depicted in Figure 6.1.1. 

The use of volatile chemicals as systematic markers has the obvious advantage of lending itself to quantification through GLC. In many, if not most, of the cases discussed below, qualitative differences in monoterpene profiles would not have been sufficient to allow distinctions to be made between taxa, or even between individuals within a population. This is true because most conifers synthesize many of the same monoterpenes, although often in vastly different relative concentrations. It is these quantitative differences that have been constructively used in the following examples. Structures of the terpenes commonly studied are presented in Fig. 3.7. 

Structures and 13C chemical shifts (in ppm) of selected bicyclic monoterpenes are collected in Table 5.4. 

Despite their low cost and abundant availability, the applications of monoterpenes as chiral synthons or building blocks for synthesis of chiral fine chemicals on an industrial scale have lagged far behind amino acids and carbohydrates. Most of the work in this area is related to multi-step total synthesis of complex natural products in laboratory scale. With the structures of new drug candidates in the research and development pipeline of pharmaceutical companies getting bigger and more complicated, the application of more sophisticated chiral building blocks such as the terpenes will... 

All the optically active terpenes mentioned in this chapter are commercially available in bulk (>kg) quantities and are fairly inexpensive. Although many of them are isolated from natural sources, they can also be produced economically by synthetic methods. Actually, two thirds of these monoterpenes sold in the market today are manufactured by synthetic or semi-synthetic routes. These optically active molecules usually possess simple carbocyclic rings with one or two stereo-genic centers and have modest functionality for convenient structural manipulations. These unique features render them attractive as chiral pool materials for synthesis of optically active fine chemicals or pharmaceuticals. Industrial applications of these terpenes as chiral auxiliaries, chiral synthons, and chiral reagents have increased significantly in recent years. The expansion of the chiral pool into terpenes will continue with the increase in complexity and chirality of new drug candidates in the research and development pipeline of pharmaceutical companies. 

The direct use of biomass components may be subject to specialty and fine chemicals or pharmaceuticals. Processes are well established and mainly comprise extraction and simple transformation of biomass components. Examples are monoterpenes or fatty alcohols. They can be summarized as products with a very similar molecular structure compared to the deployed biomass. 

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