Purification of triterpenoid saponins and 25R/25S‐inokosterone from Achyranthes bidentata Bl. by high‐speed countercurrent chromatography coupled silver nitrate coordination

Abstract

An effective method by high‐speed countercurrent chromatography coordinated with silver nitrate for the preparative separation of sterones and triterpenoid saponins from Achyranthes bidentata Blume was developed. Methyl tert‐butyl ether/n‐butanol/acetonitrile/water (4:2:3:8, v/v/v/v) was selected for 20‐hydroxyecdysone (compound 1), chikusetsusaponin IVa methyl ester (compound 4), 2′‐glycan‐11‐keto‐pigmented saponin V (compound 5), as well as a pair of isomers of 25S‐inokosterone (compound 2) and 25R‐inokosterone (compound 3), which were further purified by silver nitrate coordinated high‐speed countercurrent chromatography. What is more, dichloromethane/methanol/isopropanol/water (6:6:1:4, v/v/v/v) was applied for calenduloside E (compound 6), 3β‐[(O‐β‐d‐glucuronopyranosyl)‐oxy]‐oleana‐11,13‐dien‐28‐oic acid (compound 7), zingibroside R1 (compound 8) and chikusetsusaponin IVa (compound 9). Adding Ag+ to the solvent system resulted in unique selectivity for 25R/25S isomers of inokosterone, which increased the complexing capability and stability of Ag+ coordinated 25S‐inokosterone, as well as the α value between them. These results were further confirmed by the computational calculation of geometry optimization and frontier molecular orbitals assay. Comprehensive mass spectrometry and nuclear magnetic resonance analysis demonstrated the structures of the obtained compounds.