Sesquiterpenes diverse structures

Like GDP in monoterpenes, FDP in sesquiterpenes can give rise to linear or cyclic compounds. Due to the increased complexity of FDP translated to the length of the side chain and the multiple unsaturation compared to GDP, the number of possible cyclization modes is also increased and a large number of mono-, bi-, and tricyclic diverse structures can be formed catalyzed by specific enzymes collectively termed as... 

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. 

The terpenoids form a large and structurally diverse family of natural products derived from C5 isoprene units (Figure 5.1) joined in a head-to-tail fashion. Typical structures contain carbon skeletons represented by (Cs) , and are classified as hemiterpenes (C5), monoterpenes (C10), sesquiterpenes (C15), diterpenes (C2o), sesterterpenes (C25), triterpenes (C30) and tetraterpenes (C40) (Figure 5.2). Higher polymers are encountered in materials such as rubber. Isoprene itself (Figure 5.1) had been characterized as a decomposition product from various natural cyclic hydrocarbons, and was suggested as the fundamental building block for these compounds, also referred to as isoprenoids . Isoprene is produced naturally but is not involved in the formation of... 

A number of natural sesquiterpenes like hirsutene 90 or corioline 91 have as their common structural unit a system of linearly fused five-membered rings (Scheme 3.21). The standard pathway of the retrosynthetic analysis of this system involves the search for strategic bonds in one of the rings. A, B, or C, disconnection of which would lead to the simplification of the target molecule and eventually to simple cyclopentane derivatives as available starting materials. As a result, diverse synthetic plans were devised and successfully employed in numerous synthetic studies in this field(see also the set of syntheses described in Section 2.23.2). 

The class of natural products under focus of this work, the sesquiterpene lactones (STLs) represent one prime example, as a very large group of secondary metabolites of high diversity, with respect to chemical structure as well as biological activity. The first section of this contribution ( Structural Diversity of Sesquiterpene Lactones ) therefore focuses on a general description and discussion of STL structure in terms of chemical and biological diversity. 

Finally it should not remain unmentioned, that several STLs of other structural types have been found which contain a cyclic peroxide structure. The STL database (see Structural diversity of sesquiterpene lactones ) contains 88 entries with peroxide moieties of which 24 contain this structure as part of a ring system. Of these, 21 are not derived from seco-cadinane but belong to the guaianolide and xanthanolide series. It remains to be shown whether these compounds possess anti-protozoal activity in a similar way as the artemisininoids. 

Using these methods in combination with gas chromatography-mass spectrometry (GC-MS), we were able to detect the emission of a number of monoterpenes as well as a large group of sesquiterpenes from whole Arabidopsis Columbia plants (Fig. 1.4). In total, 3 monoterpenes (P-myrcene, linalool, and limonene) and over 20 sesquiterpene hydrocarbons were detected with E- -caryophyllene as the predominant terpene volatile. The sesquiterpene volatiles showed a high structural diversity including acyclic, mono-, di- and tricyclic compounds. All monoterpenes and 19 sesquiterpenes were identified with certainty by mass spectra and comparison with authentic standards. 

The terpenoids, which are composed of the five-carbon isoprenoids, constitute the largest family of natural products with over 22,000 individual compounds in this class having been described. The terpenoids (hemiterpenes, monoterpenes, sesquiterpenes, diterpenes, triterpenes, tetraterpenes, polyterpenes, and the like) play diverse functional roles in plants as hormones, photosynthetic pigments, electron carriers, mediators of polysaccharide assembly, structural components of membranes, and defense compounds. Many compounds used by man including resins, latex, waxes, and oils contain plant terpenoids. 

Sesquiterpenes (sesquiterpenoids). A structurally highly diverse class of terpenoids with 15 carbon atoms skeleton derived biosynthetically from famesyl pyrophosphate (FPP) ( famesol, isoprene rule, ter-penes). More than 70 different ring systems are formed by enzyme-catalyzed cyclization of the linear parent structure these cyclic structures can be further modified by 1,2- and 1,3-hydride shifts, renewed cycliza-tions, hydroxylations, and other subsequent reactions. S. are widely distributed in plants, fiingi, and animals but are less common in bacteria. Specific biosynthetic routes are often characteristic for certain organisms. Thus, basidiomycetes preferentially use humulene as the basis for the syntheses of protoilludanes, illu-danes, lactaranes, hirsutanes, and related S. skeletons. Individual S. systems are also known for liverworts and marine organisms. In addition, liverworts often contain the optical antipodes of S. known from plants. 

Basically, the same phylogenetic conclusions that can be derived from studies of the structural types and distribution of monoterpenes are true for sesquiterpenes. Despite their diversity, these compounds have proved to be of limited value in establishing the phylogeny of higher plants. 

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