Cycloartane saponins

Astragalus genus is one of the richest source of cycloartane saponins. Oleanane type saponins are also found in Astragalus sp., but their occurrence is limited to structures common to Leguminosae. Fig (3) shows the naturally occurring cycloartanes and oleananes which have been isolated from different Astragalus species. 

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. 

Table 3. Cycloartane saponins found in Astragalus species... 

The hydrolytic studies are usually performed under acidic conditions, the major disadvantage with cycloartane saponins being cyclopropane ring opening and rearrangement. 

Cycloartane saponins seemingly contribute to the immunomodulatory properties of Astragalus, in fact, curculigosaponin G from Curculigo orchioides was reported to significantly promote the proliferation of spleen lymphocytes in mice compared to controls, without a marked influence on antibody formation [167]. 

In a recent article by Verotta et al. [286] the structures of new cycloartane saponins from A. sieberi have been elucidated by the combination of ID- and 2D- gradient-enhanced NMR techniques H- C GHSQC, 1H-13C GHMBC, besides the DQC-COSY, E-COSY, ROESY, ID- and 2D- TOCSY experiments performed at 600 MHz. 

Several other activities are reported for Astragalus spp. the cycloartane saponins from A. sieversienus show an hypocholesterolemic action [219], while astramembrannin I, 87, and astrasieversianin I, 92 produce antileukocytopenic and anti-stress effects in mice [319]. Aqueous extract of A. membranaceus root causes significant stimulation of melanocyte proliferation using the sulphorhodamine B (SRB) test. This result supports the use of these extracts in traditional treatments of vitiligo [191]. 

Table 2. Astragalus species studied for their cycloartane and oleanane saponins... 

Cycloartanes are found in the plants in free states as well as in the glycosidic form. Over than 100 saponins, mainly derivatives of the 20(R),24(S) form of 90, 19 cyclolanost-24-ene-30-ol (cycloastragenol), named astragalosides or astraversianins, and more rarely the 20(S), 24(R) form (cyclogalegenin), named cyclogaleginosides and sieberosides have been so far isolated (Table 3). Some species also contain trihydroxyolean-12-ene saponins (Table 4). 

The side chain m. Astragalus cycloartane sapogenins and saponins may be of different type (Table 1) side chain with acyclic construction, or with tetrahydrofuranic ring at C-17 (20,24-epoxy cycloartane, the most rapresentative side chain as in cycloastragenol, 18), or with a tetrahydropyranic ring (20,25-epoxy and 16,23-epoxy cycloartane derivatives) and with two condenced epoxy ring (16P,23 16a,24-diepoxycycloartane compounds). All these chains influence the signals of the carbons and protons involved and that of the vicinal atoms [147,219]. 

About 130 saponins have been described in Astragalus spp. untill now. Table (2). D-xylose (xyl), D-glucose (g c), L-rhamnose (rha), L-arabinose (ara) represent the carbohydrate components of these glycosides pentoses are more frequent then hexoses. Apiose (apio), in the furanosic form, occurs only in cycloaraloside C, named also astrailienin A [140,61]. Glucuronic acid has been found only in pentacyclic triterpene saponins (as in soyasaponins). Astragalus saponins of cycloartane series... 

Interestingly novel sweet-tasting saponins based on the cycloartane (from Abrus precatorius), oleanane (from Periandra dulcis), secodammarane (from Pterocarya paliurus) and steroid saponin types (from Polypodium glycyrrhiza) have been reported [226, 227]. These compounds which have been demonstrated safe were rated by a human taste panel as about 50 and 100 times sweeter than 2% sucrose respectively [227]. ... 

Among cycloartane glycosides from Astragalus Turkish species, only astragaloside I (88) was able to stimulate NF-kB expression in macrophages. This implies that they are critical structural features responsible for macrophage activation by saponins [71]. 

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