Sesquiterpenes, structural chemistry

Kodama, H., T. Fujimori, and K. Kato. Structure determination of new sesquiterpene glycosides from flue-cured tobacco by two dimensional NMR. Proc 26th Symposium on the Chemistry of Natural Products, Kyoto, Japan 1983 26 1-8. 

The sesquiterpenes, diterpenes, and poly terpenes can be considered most simply from a structural point of view as products composed of multiple units of isoprene. Structures currently assigned to some of the sesquiterpenes may have to be revised as the study of terpene chemistry advances, since many of these have been based on the validity of the isoprene rule for terpene structures and exceptions to this rule are becoming known (37). 

Bathydoris hodgsoni can be found in the deep waters of the Weddell Sea. This nudibranch elaborates the drimane sesquiterpene hodgsonal (Structure 7.78) and sequesters it in its mantle tissues.50 164The nudibranch Tritoniella belli collected from McMurdo Sound is the only documented example of an Antarctic mollusc sequestering defensive chemistry from its diet.44 Among the glyceride esters (Structures 7.79-7.81) isolated from this species, chimyl alcohol (Structure 7.79) can also be found in one of its prey items, the stoloniferan coral Clavularia frankliniana. Sequestration of these defensive chemicals is opportunistic, however, and other, still undescribed metabolites are more commonly associated with defense in this mollusc.49... 

Often used in concert with mass spectrometry, NMR spectroscopy remains an invaluable structural diagnostic tool of particular importance to tricyclic natural products chemistry. For example, the tricyclic phenolic compound moracin P (3) was one of eight previously known compounds identified together with two new isoprenoid-substituted flavanones in isolates of the root bark of mulberry trees <89H(29)807>. In a series of studies of 6-7-5 tricyclic natural products, ID and 2D H and l3C NMR spectroscopy were employed extensively in the structure determination of sesquiterpene lactones (4)—(8) found among the aerial natural products of toxic plants (85P1378,90P551, 90P3875). 

Over the last two decades discoveries in the field of marine natural products chemistry have yielded a remarkable array of novel secondary metabolites. These have included structure classes arising from a variety of biosynthetic pathways, some of which are unique to the marine environment. One class of marine metabolite that has received a considerable amount of attention, both through the abundance of structural variants and the biological properties ascribed to specific examples, is that involving the mixed biogenesis of a sesquiterpene unit with a quinol or a quinone. This review sets out to present a comprehensive survey of the known examples of this structure class, 1 to 108 (1-52). 

The present volume reflects these developments, and there is a growing emphasis on bioactive natural products. Articles in this volume include those on structure-activity relationships of highly sweet natural products, chemical constituents of cchinodenns, diterpenoids from Rabdosia and Eremophila sp., structural studies on saponins, marine sesquiterpene quinoncs and antimicrobial activity of amphibian venoms. The reviews on bioactive metabolites of Phomopsis, cardenolide detection by ELISA, xenocoumacins and bioactive dihydroisocoumarins, CD studies of carbohydrate-molybdate complexes, oncogene function inhibitors from microbial secondary metabolites and Gelsemium and Lupin alkaloids present frontier developments in several areas of natural product chemistry. It is hoped that the present volume, which contains articles by eminent authorities in each field, will be received with the same enthusiasm as the previous volumes of this series. 

Isoprene rule. In the light of the fact that the then known mono- and sesquiterpenes were built up of two or three isoprene units, respectively, O. Wallach in 1887 prtmosed his simple isoprene rule . In 1953 L. Ruzicka formulated his biogenetic isoprene rule which proved to be a milestone in the chemistry of the terpenoids. According to this rule, all terpenoids are derived from acyclic parent structures monoterpenes are formed from geranyl pyrophosphate (GPP, Cio), sesquiterpenes from famesyl pyrophosphate (FPP,... 

Analogous processes involving cyclizations and rearrangements of carbocations derived from farnesyl diphosphate produce a rich variety of structural types in the sesquiterpene series. We will have more to say about the chemistry of higher terpenes, especially the triterpenes, later in this chapter. For the moment, however, let s return to smaller molecules to complete the picture of how isoprenoid compounds arise from acetate. 

The structure of the frequently occurring bicyclic sesquiterpene 6-caryophyllene was for many years a matter of doubt. After numerous investigations W. Treibs (1952) has been able to isolate the crystalline caryophyllene epoxide from the autoxidation products of clove oil and F. Sorm et al. (1950) suggested caryophyllene to have a 4- and 9-membered ring on bases of infrared (IR) investigations. This suggestion was later confirmed by the Fnglish chemist D. H. R. Barton (Barton and Lindsay, 1951), who was awarded the Nobel Prize in Chemistry in 1969. 

Schmidt TJ (2006) Structure-activity relationships of sesquiterpene lactones. In Atta-ur R (ed) Studies in natural products chemistry, vol 33 Bioactive Natural Products (Part M),... 

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