Stereoscopic Differences in the Identification, Bioactivity, and Metabolism of C-20 and C-24 Epimeric Ginseng Saponins

Ginseng, the roots and/or rhizomes of Panax spp.(Araliaceae), has been used as a popular herbal medicine in East Asia for at least two millennia. As a functional food and healthenhancing supplement, ginseng has been shown to have a wide range of pharmacological effects on cognition and blood circulation as well as antioxidant, antitumor, and anti-fatigue effects. The main active properties of ginseng are considered to be the triterpene saponins, often referred to as ginsenosides, which are the basis for their wide-ranging pharmacological effects. Four of these glycosides, including protopanaxadiol, protopanaxatriol, ocotillol, and oleanolic acid, are the most common saponins found in ginseng. Compared to other ginsenosides, the C-20 chimeric ginsenosides, including Rg3, Rh2, Rg2, Rh1, PF11, C-20, and C-24, as well as epimeric ocotillol-type saponins and their derivatives exhibit significant, steric differences in biological activity and metabolism. 20(R)-ginseng saponins, one class of important rare ginsenosides, have antitumor, antioxidative, antifatigue, neuroprotective and osteoclastogenesis inhibitory effects. However, 20(R)- ginsenosides are rare in natural products and are usually prepared from 20(S)-isomers through chemical differential isomerization and microbial transformation. The C20 configuration of 20(R)-ginseng saponins is usually determined by ¹³C NMR and X-ray single-crystal diffraction. There are regular differences in the chemical shift values of some of the carbons of the 20(S)- and 20(R)-epimers, including C-17, C-21, and C-22. Owing to their chemical structure and pharmacological and stereoselective properties, 20(R)-ginseng saponins have attracted a great deal of attention in recent years. Herein, the stereoscopic differences in the identification, bioactivity, and metabolism of C-20 and C-24 epimeric ginseng saponins are summarized.