Triterpenoids, structures

Mono- and sesquiterpenoids are of limited use for the identification and classification of aged resins. Due to their volatility, they are rarely found in ancient samples except when they have been conserved in very particular conditions [88,98], On the other hand, the di-and triterpenoids enable us to identify resins thereby identifying their botanical origin [2,99]. Figures 1.1 and 1.2 show the main diterpenoid and triterpenoid structures. 

These novel sulfur alkaloids and their properties are given in Table II. Structural studies have demonstrated for all the known Nuphar alkaloids shown in Tables I and II either the sesquiterpenoid or the triterpenoid structure incorporating the quinolizidine and furan systems, with the tetrahydrothiophcne system in the sulfur-containing alkaloids. 

The methyl resonances of several oleanene derivatives have been assigned using lanthanide shifts and homonuclear INDOR techniques to measure the nuclear Overhauser effects between methyl signals. The assignments of the resonances of a series of 18jSH- and 18aH-oleanenes have been published. The potential use of molecular rotation differences in the identification of triterpenoid structures has been discussed. ... 

At least 450 steroid and triterpenoid alkaloids have been reported (Verpoorte et al., 1991). Most steroidal alkaloids occur in a relatively small number of genera (Cestrum, Cy-phomandra, Lycopersicon, Nicotiana, and Solanum) of the family Solanaceae and of the family Liliaceae (Fritillaria, Veratrum, and Zygadems). Related alkaloids are known from the Apocynaceae (Funtumia, Holarrhena, and Ma-louetia) and Buxaceae. Alkaloids based on triterpenoid structures are much less common. Probably the best known ones are found in plants of the Buxaceae (especially Buxus, Pachysandra, and Sarcococca) (Gross et al., 1985 Roddick, 1980, 1986). 

Elimination of the C-22 hydroxy group from such compounds led to ring contraction and the production of a new triterpenoid structure, for which the name flavicane was proposed. Hence (509) on treatment with phosphoryl chloride in pyridine gave the flavicane (519) (Scheme 67). 

The terpenoid precursor isopentenyl diphosphate, formerly called isopentenyl pyrophosphate and abbreviated IPP, is biosynthesized by two different pathways depending on the organism and the structure of the final product. In animals and higher plants, sesquiterpenoids and triterpenoids arise primarily from the mevalonate pathway, whereas monoterpenoids, diterpenoids, and tetraterpenoids are biosynthesized by the 1-deoxyxylulose 5-phosphate (DXP) pathway. In bacteria,... 

In 2006, Milosavljevic and co-workers64 reported a study of the complete 4H and 13C NMR assignment of a new triterpenoid saponin, leucantho-side-A (13), from Cephalaria leucantha L. In the course of determining the structure and assigning the spectra, the authors made extensive use of the normal ensemble of 2D NMR experiments in use for the characterization of natural product structures HSQC, HMBC, DQF-COSY, TOCSY, and NOESY. The authors supplemented the aforementioned list of experiments with 2D /-resolved, DINE-(Double INEPT-Edited)-HSQC, and INADEQUATE spectra. The authors made no mention of the use of the connectivity information derived from the 1,1-ADEQUATE spectrum in the assembly of the triterpene nucleus of the molecule but reported extensive tabulations of the 1,1-ADEQUATE correlations that were used to sequence and assign the saccharide resonances of the tri- and di-saccharide sub-units, 14 and 15, respectively, linked to the triterpene nucleus. 

Resins older than 40 000 years are considered to be fossil resins. The fossilization of resins begins with polymerisation and forms ambers and copals. Most of the ambers are derived from components of diterpenoid resins with a labdanoid structure other ambers are based on polymers of sesquiterpene hydrocarbons such as cadinene, and may include triterpenoids less common ambers from phenolic resins derive from polymers of styrene. Figure 1.4 shows the skeletal structures of the components which make up the polymers occurring in fossil resins [141]. 

Regert et al. studied [9] a series of 30 Neolithic hafting adhesives from lake dwellings at Chalain (France) using an analytical procedure based on GC/MS analysis involving solvent extraction (dichloromethane) and trimethylsilylation. In the majority of the samples a series of triterpenoid compounds with a lupane structure was clearly identified on the basis of their TMS mass spectra. In particular, the presence of betulin, betulone, lupenone, lupeol and lupa-2,20(29)-dien-28-ol allowed birch bark tar to be identified. In other samples the co-occurrence of other plant biomarkers such as a-amyrin,(3-amyrin... 

The use of HMDS as a derivatization reagent in the analysis of triterpenoid resins has been less explored. The TMS derivatives of triterpenoids bearing hydroxyl groups [a-amyrine, p-amyrine and hop-22(29)-en-3p-ol] have been identified in the triterpenic fraction of Burseraceae resins, thus demonstrating that HMDS combined with Py-GC/MS is effective in the derivatization of triterpenoid compounds [59]. However, the range of structures that can be fully derivatized and detected must be extended and, in order to get comprehensive results comparable with those coming from the well assessed off-line GC/MS procedures, general improvements in the on-line trimethylsilylation-pyrolysis method are needed. 

Saponins are glycosylated secondary metabolites that are widely distributed in the Plant Kingdom.3,4 They are a diverse and chemically complex family of compounds that can be divided into three major groups depending on the structure of the aglycone, which may be a steroid, a steroidal alkaloid, or a triterpenoid. These molecules have been proposed to contribute to plant defense.3 6 Saponins are also exploited as drugs and medicines and for a variety of other purposes.4 Despite the considerable commercial interest in this important group of natural products, little is known about their biosynthesis. This is due in part to the complexity of the molecules, and also to the lack of pathway intermediates for biochemical studies. 

Triterpenoids (C30 compounds) are the most ubiquitous of the terpenoids and are found in both terrestrial and marine flora and fauna (Mahato et al., 1992). Diterpenoids and triterpenoids rarely occur together in the same tissue. In higher plants, triterpenoid resins are found in numerous genera of broad-leaved trees, predominantly but not exclusively tropical (Mills and White, 1994 105). They show considerable diversity in the carbon skeleton (both tetracyclic and pentacyclic structures are found) which occur in nature either in the free state or as glycosides, although many have either a keto or a hydroxyl group at C-3, with possible further functional groups and/or double bonds in the side-chains. 

The chemical composition of birch bark tar is dependent on the temperature at which tar is produced. In producing simulated tars in the laboratory for comparison with an adhesive used to repair a Roman jar from Stan wick, Charters et al. (1993) found that tars prepared at 350 °C displayed an increase in triterpenoid hydrocarbons as well as unresolved components presumably resulting from pyrolysis, although the precise nature of these molecules has not been elucidated. Binder et al. (1990) and Charters et al. (1993) also report the presence of allobetul-2-ene [Structure 7.24] in aged birch bark tars. Since this molecule has not been reported in extracts from fresh birch bark, it could be formed during heating to produce the tar (Regert et al., 2003). 

In this presentation, bioactivity-directed isolation and stmcture elucidation of the active constituents will be given. Structures of the constituents, which form namely terpenoids (sesqni, di- and triterpenoids) were based on spectroscopic tech-niqnes, particnlarly intensive NMR and Mass spectroscopies. 

More than 1500 triterpenoids have been discovered based on 40 skeletal types. Some examples include amyrin (produced by terrestrial plants), cephalosporin (synthesized by marine bacteria), and the steranes. The skeletal structure of the last contains three six-membered carbon rings and one five-membered ring (Figure 22.19a). 

Fujioka T, Kashiwada Y, Kilkuskie RE, Cosentino EM, Balias LM, Jiang JB, Janzen WP, Chen IS, Lee K-H. (1994) Anti-AIDS agents, 11. betulinic acid and platanic acid as anti-HIV principles from syzigium claviflorum, and the anti-HIV activity of structurally related triterpenoids. J Nat Prod 57 243-247. 

Khalid A, Azim MK, Parveen S, et al. Structural basis of acetylchoflnesterase inhibition by triterpenoidal alkaloids, Biochem Biophys Res Commun 331 1528— 1532, 2005. 

The interaction between Drosophila, yeasts and columnar cacti of the Sonoran Desert has been the subject of much recent interest (37) As a coevolved system, perhaps more is known about this interaction than any other. The chemistry of the cacti (70 spp.) has been postulated to play a significant role in the establishment of this system (38 ), but this was based upon reports of a relatively small number of relatively simple alkaloids, and a small number of terpenoid compounds. Only recently, the diversification of plant compounds has been discovered to be much greater (5). In their section on alkaloids in the above work, BajaJ and McLaughlin report the presence of some thirty-five structures. In addition, I have been able to isolate some sixty triterpenoid glycosides (structures were not determined) and more were detected but not Isolated. 

Many glycosidase inhibitors are structurally related to natural substrates, closely resembling carbohydrates. However, several exceptions to this generalization are known. These include such diverse structures as the diterpene andrographolide and its analogues (38, Scheme 12),134 3,21-di-O-acetylcichoridiol (39, a triterpenoid),135 acridone alkaloids, and oriciacridone (40).136... 

A related type of TLC limit test is carried out where the identities of impurities are not completely certain. This type of test is used, for instance, on compounds of natural origin or partly natural origin which may contain a range of compounds related in structure to the test substance which are eo-extracted with the raw starting material. For example, the range of synthetic steroids originate from triterpenoids extracted from plants, which are extensively modified by fermentation and chemical synthesis. 

In an approach to the stereocontrolled creation of the acyclic side-chain of tetracyclic triterpenoids and other natural products, Trost and his colleagues have converted the acyclic starting compound (29) into the cyclopropanoid intermediate (31) via the diazo-ketone (30). The key step in the scheme is the cleavage of the cyclopropane with lithium dimethylcuprate to give (32). The stereochemistry at C-7 is determined by the configuration of the double bond in (29). The c.d. and u.v. spectra of a series of triterpenoid olefins have been measured. The Scott-Wrixon rules can be used to correlate the sign of the c.d. curves with molecular structure. A... 

The structure of benulin (134), a lupene hemiacetal from Bursera arida, has been confirmed by X-ray analysis. An investigation of several Senecio species has resulted in the isolation of a series of lupane derivatives which include the acids (135), (137), and (139), the corresponding aldehydes (136), (138), and (140), the ketone (141), and betulonic acid. Three more triterpenoids (142)—(144) have... 

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