Bisabolene, structure

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). 

Bisabolene-type sesquiterpenes, e.g. a-bisabolene 74 (Structure 4.20), are widely distributed in nature. This sesquiterpene hydrocarbon is a constituent of bergamot, myrrh and a wide variety of essential oils. Its oxygenated derivatives a-bisabolol [6-methyl-2-(4-methyl-3-cyclohexen-l-yl)-5-hepten-2-ol] 75 and -bisabolol [4-methyl-l-(6-methylhept-5-en-2-yl)cyclohex-3-enol] 76 are found abundantly in chamomile. 

Bisabolene A cyclic structure (with a carbon ring in the molecule) found in myrrh oil and German chamomile. 

Two new sesquiterpenoids isolated from the roots of Ptilostemon chamaepeuce have been assigned structures (38) and (39) on the basis of their spectroscopic properties and chemical behaviour.18 The isomeric y-bisabolenes (40a and b), postulated... 

Stereospecific syntheses of both E) and Z)-o -bisabolenes, (73) and (74) respectively, have been carried out (Scheme 9) and the spectral data for each diastereoisomer have been recorded/ The enantiomers of each have also been made starting from (+)- and (-)-limonene and as a result the j8-bisabolene present in the essential oil of Opoponax has been shown to be the (+)-(5,Z)-isomer, thus correcting a previous report. As might be expected the odour characteristics of the ( )- and (Z)-isomers are subtly different. Both dia-stereoisomeric racemic a-bisabolols (75) and (76) (only one enantiomer of each is shown) have been prepared from the two isoxazolidines (77) and (78) which, in turn, were derived from intramolecular cyclization of the nitrones of (6E)- and (6Z)-farnesal respectivelyFrom this work it is suggested that natural (-)-a-bisabolol must be (75) in contradiction to a recent report which held that (76) is the correct structure of the natural isomer. Further work will be needed to resolve this question. 

The biosynthesis of (Z)-y-bisabolene (55b) and paniculide B (86) in callus cultures of Andrographic paniculata " is described in Chapter 6. A new compound (87) structurally similar to paniculide (86) has recently been isolated from a Senecio species (c/. p.72). 

In 1985, Kinghorn and coworkers isolated an extremely sweet bisabolene-type sesquiterpene from an Aztec herb Lippia dulcis,121 This plant was known to the Aztecs as tzonpelic xihuitl (sweet herb), and was described in a book written between 1570-1576 by a Spanish physician F. Hernandez.121 Kinghorn named the sesquiterpene (+)-hernandulcin, showed it to be more than 1000 times as sweet as sucrose, and elucidated its structure as 56 (Figure 2.46) including the (6R, l. R )-relative stereochemistry. 

A number of bisabolane sesquiterpenes, including ) -bisabolene (51), iso-bisabolene (52), and cryptomerion (53) have been synthesised by standard procedures. The structure of cryptomerone (54) has been established largely on the basis of n.m.r. spectra. A short and efficient route to nuciferal (55) has been described by Biichi and Wiiest. The same authors have also synthesised bilabanone (56) starting from (-l-)-carvone. ... 

Hi) Bisabolanes (compounds reported— 4) A family of four bisabolene sesquiterpenes, isolated from Barbadian P. americana, was reported by Miller et al. in 1995. The gorgonian extracts yielded the new compound (—)-curcuhydroquinone-1-monoacetate (219), and the known compounds (—)-curcuhydroquinone (200), (—)-curcuphenol acetate (220), and (—)-curcuquinone (221). The structures of all compounds were confirmed by the use of HMQC and HMBC NMR experiments. 

Further non-stereospecific syntheses of isobisabolene (54) and )3-bisabolene (48) have been reported by Vig and co-workers. The structural and stereochemical assignments of delobanone (55 R = H) and its acetoxy-derivative (55 R = OAc) have been carried out and, furthermore, this study has demonstrated that crypto-merion has the absolute stereochemistry depicted by (56). Two recent additions... 

This a-domain is common to all class I terpene synthases and is the only domain in bacterial and fungal class I enzymes such as the pentalenene synthase from Streptomyces ey oliatus or the trichodiene synthase from Fusarium sporotri-chioides (Fig. 87.14a) [191,194]. Plant hemi-, mono-, and sesquiterpene synthases exhibit a second helical p-domain that resembles a barrel structure. An example of this class is given by the (+)-5-cadinene synthase from G. arboreum (Fig. 87.14b) [195]. In plant diterpene cyclases and in the exceptional case of the ( )-a-bisabolene synthase from A. grandis (Fig. 87.14c), a third helical y-domain with a barrel-like structure is present [197, 200, 202]. The p- and y-domains in the plant enzymes are nonfunctional but are required for correct enzyme folding. 

Fig. 87.14 Three different structure types for class I terpene synthases, (a) The a-type pentalenene synthase from S. exfoliatus (IPSl), (b) a(3-type (+)-5-cadinene synthase from G. arboreum (3G4D), and (c) a(3y-type ( )-a-bisabolene synthase from A. grandis (3SAE)...

The number of sesquiterpenes is high (over 150 compounds). There are compounds with aliphatic (farnesenes), monocyclic (humulenes, bisabolenes, elem-enes), bicyclic (cadinenes, muurolenes, selinenes, caryophyllenes, germacrenes), tricyclic (copaenes, ylangenes, longifolenes.longicyclenes), and tetracyclic (sati-vene) structures. 

FIGURE 7.4 Crystal structure of (a) bisabolene synthase active center bound to farnesyl diphosphate inhibitor (b) aristolochene synthase cocrystallized with FDP (metal coordination and hydrogen bonds are shown as dashed lines magnesium anions and water molecules are shown as spheres) and the proposed catalytic mechanisms for each cyclization Images created with PyMOL. (See insert for color representation of the figure.)... 

The sesquiterpene (C15) molecules include structures like hydrocarbons () -bisabolene, j3-caryophylene), alcohols (famesol, carotol, patchoulol), ketones (nootkatone) and aldehydes (anisaldehyde, valerenal). 

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