Seeking saponins in Panax ginseng plants, the Russian research group headed by Professor Elyakov isolated from methanolic extracts of ginseng roots a series of sugar-linked compounds which they named "panaxosides"; panaxosides A and B were reported by Elyakov et al. in 1962 and C, D, E and F by Elyakov et al. in 1964. The Russian chemists noted that on hydrolysis the saponin glycosides panaxosides A, B, and C were based on the aglycone panaxatriol and that the panaxosides D, E and F formed a separate group based on the aglycone panaxadiol. In addition Elyakov's group were able to demonstrate that the sidechains of monosaccharide molecules were different. Thus panaxoside A possessed 3 glucose units, panaxoside B 2 glucose and 1 rhamnose units, panaxoside C 3 glucose and 1 rhamnose units, panaxoside D 4 glucose units, panaxoside E 4 glucose and 1 arabinose units, and panaxoside F 6 glucose units.
At the same time, in Japan, a research group headed by Professor Shibata had isolated and described similar ginseng saponins which they preferred to name "ginsenosides" and this terminology is now universally accepted. Fujita et al., (1962), studying methanolic extracts of P. ginseng, P. japonicum and P. quinquefolium roots, differentiated the saponins and their aglycones and sapogenins obtained by hydrolysis using hot methanolic hydrochloric acid. Further work (Shibata et al., 1963a, 1963b) established the structure of the sapogenin panaxadiol as a dammarane-type tetracyclic triterpene.
(5-1) Dammarane (5-2) Panaxadiol
Dammarane (5-1) is the name that was applied to the foundation triterpenoid structure of dammarenediol, a compound extracted from the resins of various tree species of Agathis, Balanocarpus, Hopea and Shorea, genera of the family
Dipterocarpaceae occurring in East Asia. East Indian dammar resin was used for technical processes such as varnishes for the preservation of oil paintings and microscope slide preparation. Shibata established the relationship of the structures of dammar resin triterpene and the ginseng triterpenes.
Panaxadiol (5-2), C30H52O3, yielded the characteristic red-violet Liebermann-Burchard reaction of a triterpenoid structure and a negative tetranitromethane reaction coupled with no ultraviolet absorption peak at 210 nm indicated that it was not a normal pentacyclic oleanane structure. Further chemical reactions and ultraviolet, infrared and mass spectral observations established that panaxadiol was indeed a compound with a molecular ion peak (M+) at m/z 460 confirming the formula C30H52O3 and peaks at m/z 127 and m/z 341 were consistent with the presence of a trimethyltetrahydropyrane ring structure. In addition secondary hydroxyl groups were present at C-3 and C-12 and the trimethyltetrahydropyrane group occurred at C-20.
Shibata's team repeated Kotake's (1930) observations, obtaining panaxadiol by mineral acid hydrolysis and a chlorine-containing compound (5-3) directly by crude saponin hydrolysis with concentrated hydrochloric acid at room temperature. Dechlorination of the latter compound with potassium tert-butoxide produced an unsaturated compound that was converted by a mild hydrolysis process using methanolic 0.7 per cent sulphuric acid to reveal the true sapogenin which was designated as a prosapogenin (Shibata et al., 1963c).
This prosapogenin was identified chemically as protopanaxadiol (Tanaka et al., 1964; Shibata et al., 1966)) and was shown to be an open chain compound with a free alcohol group and a terminal vinyl group; cyclisation of the C-17 side chain under strong acid hydrolysis conditions resulted in the substituted pyran ring system of panaxadiol. Further studies of the acid-catalysed isomerisation of dammarane-type triterpenes and of the mild hydrolysis of pure ginsenosides undertaken by the same team proved that the normal naturally occurring protopanaxadiol was the 20(Sj-epimer (12^-hydroxydammarenediol II)(5-4) and not the previously assumed 20(R)-epimer (12^-hydroxydammarenediol I)(5-5) (Tanaka et al., 1964, 1966, 1967, 1972).
(5-4) 20(/i)-Protopajiaxadiol (5-5) 20(S)-Protopanaxadio1
( 12ß-Ilydroxydammarenedio! I) ( 12ß-Hydroxydammarenediol II)
Shibata et al. (1965) noted that hydrolysis of the pure ginsenoside Rg i using dilute mineral acid produced glucose and a crystalline compound named panaxatriol (5-6) which was shewn by spectral data analysis to be 6-a-hydroxypanaxadiol. Using methods applied to the study of panaxadiol, Shibata's team demonstrated that the true sapogenin of ginsenoside Rg1 was 20 (S)-protopanaxatriol (5-7) and that panaxadiol and its homologue panaxatriol, not being the true aglycones, could be regarded as artefacts.
Ginsenosides based on the sapogenin (20S)-protopanaxadiol possess sugar moieties attached at the C-3 OH and C-20 OH positions while ginsenosides based on the sapogenin (20S)-protopanaxatriol have sugar moieties attached at C-6 OH only as in ginsenosides Rf, Rg2 and Rh1 or at C-6 OH and C-20 OH e.g. ginsenosides Re, Rg1 and 20-gluco-Rf.
One further sapogenin was known to occur in some ginsenosides. As early as 1961 the German group of Horhammer, Wagner and Loy had isolated and identified from P. ginseng roots a pentacyclic, oleanane-type (5-8) triterpene compound named oleanolic acid (5-9) which occurs widespread in the plant kingdom e.g. in Olea europaea L., the olive tree, Thymus vulgaris L., common thyme, etc. Its saponin, ginsenoside-Ro, occurs commonly in ginseng species.
(5-8) Oleanane ring system (5-9) Oieanolic acid
The dammarene saponins are hydrophilic but oleanolic acid, the aglycone of ginsenoside Ro, is lipophilic. The glycosides of ginseng are formed by the addition of sugar units to the above aglycone units, the different compounds varying according to the number, nature and occurrence of sugars such as arabinose, glucose, rhamnose and xylose. As the number of sugar units is small and variable and mixed sugars can occur the saponins are also referred to as triterpene-oligosides.
The overall yield of glycosides in Panax ginseng roots varies between 0.5 and 4.0 per cent and is age-dependent. Mild acid hydrolysis of the mixed ginsenosides revealed three principal structural types based on:-
a) the tetracyclic, dammarene, triterpenoid sapogenin (20-S)-protopanaxadiol, b) the tetracyclic, dammarene, triterpenoid sapogenin (20-S)-protopanaxatriol, c) the pentacyclic, triterpene oleanolic acid.
The international confusion concerning the nomenclature of the ginsenosides was resolved by using the designation Rx where the capital R refers to the root and the lower case x=o, ax, a2, bx, b2, c, d, e, f, gx, g2, g3, hx, h2, etc., relating to the relative positions of the separated neutral saponin spots on thin-layer chromatograms, ginsenosides Ro and Ra being the least polar (Shibata et al, 1965). In a similar manner new ginsenosides from the leaves were designated Fx, the capital F indicating "folia" meaning "leaves".
The chemical characteristics and physical properties of the commoner saponins were soon established and widely published (Table 5.1).
During the 1960's the Japanese group including Iida, Shibata and Tanaka had elucidated the characteristics and detailed chemical structures and configurations of the nine then-known ginsenosides. Using better thin layer chromatography (TLC) methods, Sanada et al. (1974) isolated 13 ginsenosides
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