Saponins structural elucidation

Cholestane glycosides furostanol open-chain steroidal glycosides spirostanol stereochemistry steroidal saponins structure elucidation... 

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. 

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 ... 

Oleanane and cycloartane saponins can be isolated and structurally elucidated using similar techniques. A single chromatographic step is rarely sufficient to isolate a pure saponin from an extract. As a general rule, several preparative techniques are required to obtain the pure product. 

The numerous biological activities of saponins, as well as their widespread occurrence in the plants and the hundreds structures described so far, prompted the development of a strategy for their rapid, highly sensitive and, if possible, non degradative, structure elucidation. 

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. 

De Marino, S., Iorizzi, M., Palagiano, E., Zollo, F., and Roussakis, C. Starfish saponins 55. Isolation, structure elucidation and biological activity of the steroid oligoglycosides from an Antarctic starfish of the family Asteriidae, J. Nat. Prod., 61, 1319, 1998. 

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... 

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 last review on the Caryophyllaceae family was published almost two decades ago [1]. Since then, purification and structural elucidation techniques have changed enormously. Many of the earlier chemical studies have been reinvestigated and new findings published. The present review deals with the chemistry of the triterpenoid saponins of Caryophyllaceae family with special emphasis on recent developments in purification techniques and structural study aspects. 

Fig. (5). Structure elucidation strategy of saponins from 2D-NMR approaches.

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... 

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. 

The presence of acids in saponins further complicates the structural elucidation process and in most cases, one chooses to start the study with saponified compounds. It has been observed fortunately, that acyl groups on sugars are more labile than the hindered sugar chain esters at C-23, -24 or -28 and that they can be selectively cleaved with sodium bicarbonate (21) ... 

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