Other Sesquiterpene Skeletons

It appears worthwhile to discuss the reactions of longifolene in terms of the above four categories, even though some reactions of longifolene are known to lead to products of more than one type. In such cases, the reaction has been discussed under only one head or both the heads, depending on whether one pathway is dominant or not. Besides these four categories of reactions, some other pertinent transformations of longifolene and its derivatives are discussed under two additional heads (i) Conversions into other sesquiterpene skeletons, and (ii) Miscellaneous transformations. 

Non-celastraceous sesquiterpene pyridine alkaloids are those compounds which have been isolated from other plants not belonging to the Celestraceae family. Rotundine, for example, has been isolated from Cyperus rotun-dus (Cypraceae). This is a structurally interesting alkaloid because it has a sesquiterpene skeleton containing a cyclopentane ring attached to the pyridine ring . 

Weyerstahl P., Marshall H., Splittgeiser V., Wolf D., 1,7-Cyclogermacra-l(10),4-dien-15-al, a Sesquiterpene with a Novel Skeleton, and Other Sesquiterpenes from Haitian Vetiver Oil. Flavour and Fragrance J., 2000, 15, 61-83. 

ABSTRACT This article reviews the literature published in the last decade dealing with the transformation of a-santonin into bioactive or potentially bioactive sesquiterpenes. A number of syntheses starting from 8a-hydroxysantonin (artemisin) have also been included. Special emphasis has been placed on synthesized products that show biological activity. Major advances in this field include the application of new reagents and methodologies for the structural modification of the santonin skeleton and functionality, and its transformation into other sesquiterpenes, especially sesquiterpene lactones. 

A new sesquiterpene skeleton has been identified in an isocyanide isolated from Phillidia varicosa, and the name pupukeanane (after the locality where the molluscs were obtained) has been suggested for this carbon framework. X-Ray analysis of the crystalline phenylthiourea derived from 9-isocyanopupukeanane (4) established the structure. The structure of stemarin tosylate reveals a new diterpene skeleton (5). Crystal structures reported for various other bridged carbocyclics are reported in other, more appropriate, chapters of this volume. 

Many diterpenes isolated from Dictyotales have carbon skeletons which are prenylated forms of sesquiterpenic skeletons already known in the terrestrial environment, and which can be distinguished from each other by... 

Four bicydic carbon skeletons containing a cyclopropane have been charaderized in Octocorallia. These are only sparsely distributed among the various orders and genera (Figure 20.42) and have very few representatives (usually fewer than five). Two of them are original isoprenologs of atypical sesquiterpenic skeletons and two others are derived directly from the cembrane skeleton. 

Abscisin II is a plant hormone which accelerates (in interaction with other factors) the abscission of young fruit of cotton. It can accelerate leaf senescence and abscission, inhibit flowering, and induce dormancy. It has no activity as an auxin or a gibberellin but counteracts the action of these hormones. Abscisin II was isolated from the acid fraction of an acetone extract by chromatographic procedures guided by an abscission bioassay. Its structure was determined from elemental analysis, mass spectrum, and infrared, ultraviolet, and nuclear magnetic resonance spectra. Comparisons of these with relevant spectra of isophorone and sorbic acid derivatives confirmed that abscisin II is 3-methyl-5-(1-hydroxy-4-oxo-2, 6, 6-trimethyl-2-cyclohexen-l-yl)-c s, trans-2, 4-pen-tadienoic acid. This carbon skeleton is shown to be unique among the known sesquiterpenes. 

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). 

In the l960s, Kropp showed that fused bicyclic dienones structurally related to santonin could potentially serve as synthetically useful precursors to either spirocyclic skeletons or hydroazulenones. One of these cases, 91, was successfully used by Marshall and Johnson as the starting point in an elegant synthesis of the spirocyclic sesquiterpene )6-vetivone (Scheme 23). More recently, a variety of bicyclic cyclohexadienones have been studied by Caine and coworkers. For example, bicyclic dienone 92 could be photochemically rearranged to the oxygenated bicyclo[4.3.0]nonenone system 93, along with other rearrangement products, via acetic acid solvolysis of the cyclopropyl ketone intermediate. ... 

Marine sponges of the genus Dysidea are a rich source of structurally unique and biologically active compounds, including spiro-sesquiterpenes such as spirodysin and dehydroherbadysinolide, furanosesquiterpenes based on the furodysinin and furodysin skeletons, brominated diphenyl ethers, polychlorinated alkaloids and other compounds [99-103],... 

Trichothecenes from Pusarium, Stachybotrys, and Trichoderma, among others. Note Trichothecenes include sesquiterpenes with a trichothecane skeleton, olefinic groups at C-9 and C-10, and epoxies at C-12 and C-13. Macrocyclic trichothecenes have a carbon chain between C-4 and C-15 in an ester or ether linkage (e.g., T-2 toxin, DON, satratoxins G and H verrucarins B and J, trichoverrins A and B)). 

---