Structure elucidation of saponins

The distribution and structural elucidation of saponins have been reviewed. Glucuronides with a free carboxy-group and carbohydrate residues at C-2 and C-4 are readily cleaved with acetic anhydride in refluxing pyridine to give the genuine aglycone (see Vol. 7, p. 145). Details of the mass spectra of a series of permethylated oleanane saponins have been discussed. 

Van Setten DC, Zomer G, Van DeWerken G, Wiertz EJHJ, Leeflang BR, Kamerling JP (2000) Ion Trap Multiple-Stage Tandem Mass Spectrometry as a Pre-NMR Tool in the Structure Elucidation of Saponins. Phytochem Anal 11 190... 

The structure elucidation of saponins requires identification of the different building blocks (namely genins, monosaccharide units, substituents), determination of the sequence of the component monosaccharides in the carbohydrate moiety, the way in which monsaccharide units are attached to one another, the anomeric configuration of each glycosidically linked monosaccharide unit and the location of the carbohydrate moiety (or moieties) in the aglycone. 

Saponins constitute an important class of secondary metabolites from plant and animal domains (1-3). They display a broad spectrum of biological activities that have raised the interest of phytochemists for the past forty years. With molecular weights ranging from 600 to 2000 Daltons, their structures are complex. It was only in the seventies that, with the use of chemical degradation techniques, their precise structures were established (4-5). Since then, the field has benefitted from the recent progress in instrumentation. The purpose of this article is to describe the current techniques in use for the structural elucidation of saponins. 

R152 S. C. B. Gnoatto and G. Gosmann, Three-Dimensional Structural Elucidation of Saponins , in Strategies for the Determination of Carbohydrates Structure and Conformation, eds. H. Verli and J. A. Guimaraes, Transworld Research Network, Trivandrum, India, 2010, p. 133. 

The structural elucidation of the ginsenosides was challenging, but thanks to the dedicated work of Prof. Shibata (Tokyo University)2 and of Prof. Tanaka (Hiroshima University)3 their full stereochemistry is known. The ginsenosides are saponins of the dammaran class and are of two different types ... 

Thus, the rapid and unambiguous structure elucidation of complex saponins and their NMR full assignments can be performed with a combination of ID and 2D NMR techniques [136]. Since all the experiments can be run in an automated sequence, all the necessary information can be obtained during a week end acquisition. Moreover, the use of a 600 MHz spectrometer gives the necessary sensitivity to work with very low amounts of samples (few milligrams are enough), that, in natural product chemistry, often is a limiting factor. 

The conventional method of structure elucidation of steroidal saponins starts with acid hydrolysis of the homogeneous saponin leading to identification of the aglycone and the individual monosaccharide constituents separately. The stmcture of the sugar moieties of the glycosides is then determined by identification of the monosaccharides (obtained on acid hydrolysis) by PC, GLC (alditol acetates/TMS derivatives), and HPLC (comparison with authentic samples). Sometimes microhydrolysis is used to identify the monosaccharide constituents (56). The method has been applied to the identification of monosaccharide... 

The first step in the structure elucidation of a saponin is to obtain the ID fl and 13C-NMR spectra. Saponins are usually investigated as deuterium exchanged samples and the most commonly used solvent is pyridine-r/5, although the use of mcthanol-<74 or DMSO-Jg has been reported in the literature. Hydroxylic protons can be exchanged by adding few drops of DzO when required. 

Plock A, Beyer G, Hiller K, Griindemann E, Krause E, Nimtz M, Wray V (2001) Application of MS and NMR to the Structure Elucidation of Complex Sugar Moieties of Natural Products Exemplified by the Steroidal Saponin from Yucca filamentosa L. Phytochemistry 57 489... 

In early 1990s, Frechet et al. reported the isolation and structure elucidation of four new triterpenoid saponins from the roots of G. paniculata and G. arrostii (Fig. 24) [76], the roots being imported from Hebei, P. R. China. The... 

Adapted from Hostettmann et aL, 1991 Fig (3). Structural elucidation of naturally-derived saponins. 

Crystallography may allow the complete structural elucidation of intact saponins. This technique is extremely powerful but suffers the drawback that it requires a pure, crystallized sample. This method has been used in the determination of aglycone stereochemistry, but has found little application to intact saponins because of their non-crystallinity and contamination with related molecules [10]. 

From the fruits of Gleditsia japonica, which have been used in Japanese and Chinese folk medicine as a diuretic and expectorant, seven saponins (gleditsiasaponins B, C, Dj, D2, E, G and I) had been isolated. The structural elucidation of these saponins have been carried out by the chemical degradations, enzymatic hydrolysis and C-NMR spectra, and characterized as 3, 28-0-bisglycosides of echinocystic acid acylated with monoterpene carboxylic acid [46-49]. 

In our previous papers, we reported the identification and structure elucidation of twenty-four known saponins and fourteen new saponins named vina-ginsenosides -R1 — R14 which were isolated from the underground part of P. vietnamensis. We now summarize the result of our further study on the saponin composition of Vietnamese ginseng, which led to the isolation and structure elucidation of two known saponins, pseudo-ginsenoside-RCi (1) and gypenoside-IX (2), and ten new dammarane saponins named vina-ginsenosides-Rl5 — R24 (3 -12, respectively). 

The ethanol extract from the fruit of Randia siamensis after separation by column chromatography yielded four components ursolic acid (1), pseudoginsenoside-RPi (2), pseudoginsenoside-RTi (3) and siamenoside (4). 1 is a conunonly occurring triterpenoid acid while 2 and 3 are saponins which were first isolated from Panax (Araliaceae) species and were identified by Tanaka et al. (198S). The major component isolated in our study, oleanolic acid glycoside 3, has since been reported to be present in additional species (Morita et al., 1986 Shukla and Thakur, 1988 Shukla, 1989 Ida et al., 1994 Sakai et al., 1994). In the structural elucidation of the four components from R. siamensis, NMR spectroscopy was foimd to be... 

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