Cyclic sesquiterpene

Cyclic sesquiterpenes having the cedrene skeleton have also been isolated. When R = COOH, this stmcture (11) has been named sheUoflc acid. 

They have also carried out the oxidation of selinene by means of ozone and potassium permanganate and have thus been able to establish that the regenerated selinene is not absolutely identical with the natural selinene. They are the first to record the existence of a hemi-cyclic-sesquiterpene this compound is termed pseudo-()3)-selinene. By passing a current of ozone into a solution of natural selinene (pseudo-( 8)-selinene). in acetic acid, there is obtained a diketone, CjjHogOj, which is purified by treatment with permanganate in acetone solution. Its properties are as. follows —... 

As far as we are aware, abscisin II is the only known sesquiterpene possessing the carbon skeleton of structure A. All the other mono-cyclic sesquiterpenes are thought to arise by way of ion D (the example given is bisabolene) or E (the example given is elemol) (21). 

Dicarbonyl coupling (8,483). This Ti-catalyzed coupling offers a useful route to cyclic sesquiterpenes such as humulene (4). The precursor is obtained by coupling a vinylic zirconium compound (1) with the u-allylpalladium complex (2) to give, after deprotection, the keto aldehyde 3 in 84% yield. This product couples to humulene as a single isomer in 60% yield. 

Many cyclic sesquiterpenes of various structural types have been isolated from essential oils. Typical examples are as follows ... 

As in the case of the cyclic monoterpene hydrocarbons, a number of the cyclic sesquiterpenes are used as starting materials in the synthesis of fragrance and flavor substances or for the reconstitution of essential oils. 

Many cyclic sesquiterpene alcohols are key odor components in essential oils, for example, cedrol in cedarwood oil, the vetiverols in vetiver oil, and the santalols in sandalwood oil. Since these alcohols have not yet been synthesized on an industrial scale, they are described under the oil in which they occur (Chapter 3). Some of their derivatives, however, are discussed in this section. 

Sesquiterpenes are formed by the addition of one more isoprene units to a monoterpene molecule, and thus have the molecular formula C15H24 (see also Fig. 4.2). There are linear, branched or cyclic sesquiterpenes. Sesquiterpenes are unsaturated compounds. Cyclic sesquiterpenes may be monocyclic, bicyclic or tricyclic. They are the most diverse group among the volatile terpenoids [2, 3, 7-11, 13,14, 16, 20-24, 37-39, 49]. The DNP treats sesquiterpenoids in 147 different structural types [37]. Various types of sesquiterpenes (69-109) can also be seen in Structure 4.16. 

Recently, germacrene-A (III) has been identified as the alarm pheromone of the sweet clover aphid Therioaphis trifolii (38). This alarm releaser, which has often been proposed as the progenitor of cyclic sesquiterpenes, constitutes the second compound with a germacrane-type skeleton to be identified as an insect pheromone. 

Formation of a cyclic sesquiterpene from farnesyl pyrophosphate. 

Formation of a Cyclic Sesquiterpene from Farnesyl Pyrophosphate Section 28.3 Figure 28.4... 

Non-cyclic sesquiterpenes include the volatiles a- and (3-farnesene (which have alarm pheromone activity) and pleasant odorants from Citrus sinsensis (orange) (Rutaceae), namely a- and [i-sinensal (mandarin peel odour) and nerolidol from orange flower oil (oil of neroli). The epoxide JH III is produced by Cyperus iria (Cyperaceae) and acts critically on insect development. 

Various cyclic sesquiterpenes have also been studied in order to explore the possibility of achieving their microbiological transformations. Very often these were shown to lead to epoxidation processes when one (or several) double bonds were present in the starting substrate (Fig. 16.1-28). 

Japanese chemists have reported a stereoselective synthesis of humulene (9), an 11-membered cyclic sesquiterpene, by intramolecular a-alkylation of a keto group by a jr-allylpalladium complex. The starting material (7) was derived from geranyl acetate. In this case, the efficiency of the cyclization was markedly improved by addition of a diphosphine ligand such as l,3-bis(diphenylphosphino)-... 

Plant sesquiterpenes and other terpenoids aie major detenninants of insect-plant interactions (2i 16V Many insecticidal and antifeedant terpenoids are epoxides including monoteipene Q2, 18. sesquiteipene QQ, 19-23). diteipene (Jl, 24) and triteipene derivatives (25-27) typified by the potent antifeedant azadirachtin (28-30). Most biolo cal effects have been determined with Lepidoptera and non-chrysomelid Coleoptera. Occasionally, the same compound, while normally inhibitory to herbivores, may for adapted insect species or at low concentrations have a stimulatory effect (13). Insects, in turn, synthesize their own defensive (21, and pheromonal (22) terpenoids. Plants may utilize insect pheromones such as the sesquiterpene alarm pheromone, rranj-B-famesene, in their own defense (34. 35). Inhibitory cyclic sesquiterpenes (Table I) and diterpenes (Table II) for insect herbivores have been identified from at least 28 genera of the terpenoid-rich Compositae. These studies were largely confined to extrafloral tissues. 

Table I. Cyclic Sesquiterpenes from Compositae that Deter Insect Herbivores...

Biosynthesis of ( , )-Famesyl Pyrophosphate Interconversion of 2-E-6-E- and 2-Z-6- -Famesyl Pyrophosphate Acyclic Sesquiterpenes Cyclization of Acyclic Precursors The Origin of Major Groups of Cyclic Sesquiterpenes Biological Activity Allelopathy Medicinal Properties... 

The families of cyclic sesquiterpenes are far too numerous, and their biogenesis too complex, to attempt a comprehensive survey in this chapter. The following sections thus represent but a few selected examples intended to describe general principles and to illustrate some of the more elegant applications of modem in vivo tracer techniques. For more comprehensive treatment of this subject the reader is referred to several reviews (Rucker, 1973 Cordell, 1976 Banthorpe and Charlwood, 1977 Cane, 1980). 

Caryoph lenes isomeric cyclic sesquiterpene hy-drocarbc found in many essential oils. a-C. is the same as Humulene (see). 

Proazulenes, azulenogens, hydroazulenes a group of natural cyclic sesquiterpenes, which can be terminally dehydrogenated or dehydrated to Azulenes (see). P. are chiefly compounds of the guaiane type, e.g. guaiol. 

Fig. 107. Synthesis of cyclic sesquiterpenes from 2- rans-6- rans-farnesyl pyrophosphate, part I...

The biosynthesis of cyclic sesquiterpenes probably includes the formation of enzyme-bound intermediates (Figs. 107 and 108). It is accompanied by the shift of double bonds, hydride ions, etc. The diversity of chemical structures of sesquiterpenes is greater than that of mono-, di-, and triterpenes due to the many different types of cyclizations occuring in this group of secondary products. 

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