The Study On The Efficient Preparation Of Ginsenoside Rg5 Assisted By Molecular Sieve And The Stability Of Related Saponins

Rare ginsenosides Rg5 and 20(S,R)-Rg3 are one of the main active components isolated from processed ginseng,showing more significant pharmacological activities in anti-cancer,anti-inflammatory and anti-depression than the main ginsenosides.Therefore,in order to improve the pharmacological efficacy of ginsenosides,many researchers devote themselves to transforming the main ginsenosides into secondary ginsenosides with stronger activity.In this study,we explore and optimize ginsenoside Rg5’s efficient preparation conditions with molecular sieve,and study ginsenoside Rg5’s generation mechanism and stability in depth.Rare ginsenoside Rg5 was prepared from PPD saponins by hydrochloric acid catalysis with molecular sieve’s assistance.The experimental results show that Rg5’s yield can reach up to its highest percentage 57.46%when the dosage of molecular sieve is 11.7 mg,the dosage of acid is 0.2 mol/L,the reaction temperature is 65℃,the reaction time is 75 min,and the stereoselectivity of 20(S)-Rg3 is highly selectively generated which yield is 16.3%,de%92.64 at the same time.The results of molecular sieve action mechanism showed that during the transformation of PPD saponins catalyzed by acid,the protons and PPD saponins in the acid catalyst were adsorbed by molecular sieve through hydrogen bond,and the rare ginsenoside Rg5 and 20(S)-Rg3were generated on the surface of molecular sieve with high acid concentration,and the rare ginsenoside Rg5 was generated due to the decreased hydrogen bond ability.In this way,the decomposition of rare ginsenoside Rg5 by inorganic strong acid can be effectively avoided and the yield of ginsenoside Rg5 can be improved.In this paper,we used high performance liquid chromatography(HPLC)and density function theory(DFT)to study the generation mechanism of rare ginsenoside Rg5 in experimental and theoretical aspects.The generation mechanism is greatly affected by the solvent,when the water is used as the solvent,the protopanaxadiol saponins are converted directly to Rg5 by desugarizing;however,in the organic solvent,the Rg5 is obtained not only directly desugarized from the protopanaxadiol saponins,some of Rg5 is got via dehydration from Rg3 which is produced from the protopanaxadiol saponins.The DFT’s theoretical calculation shows the activation energy of carbocation generation derived from ginsenoside Rb1 and Rg3 is 1.81 mol/L and 7.57 mol/L,respectively,which further proves that Rb1 is more prone to E1eliminate than Rg3.In this paper,the generation mechanism of ginsenoside Rg5 is systematically studied via a combination of experiments and theoretical calculations for the first time.The research results clarify two different ways to generate Rg5,provide a new idea for the research on the generation mechanism of such rare ginsenosides and theoretical evidence for the efficient preparation of Rg5.Using ginsenoside Rg5,20(S)-Rg3,20(R)-Rg3 as raw materials,we separately investigate the effects of air,light,no air,light,ultraviolet,solvent(methanol,ethanol,water)on the stability of ginsenoside.The results showed that ginsenoside Rg5,20(S)-Rg3,20(R)-Rg3 had higher stability in methanol,ethanol and short time UV irradiation.In the air,light,no air and no light conditions stability is weak;and very weak stability in water.The main decomposition products of ginsenoside Rg5 in water were separated and characterized.Thus,the structure of compound 4 was established as(3β,12β,20R,22E,24R)-3,12,20,24,25-pentahydroxydammar-22-ene-3-O-β-_D-glucopyranosyl-(1→2)-β-_D-glucopyranoside;the structure of compound 5 was established as(3β,12β,20S,22E,24R)-3,12,20,24,25-pentahydroxydammar-22-ene-3-O-β-_D-glucopyranosyl-(1→2)-β-_D-glucopyranoside;the structure of compound 6 was established as(3β,12β,20S,22E,24S)-3,12,20,24,25-pentahydroxydammar-22-ene-3-O-β-_D-glucopyranosyl-(1→2)-β-_D-glucopyranoside.The study of the preparation and formation mechanism of ginsenoside Rg5assisted by molecular sieves can provide theoretical and technical support for the efficient preparation and low-cost preparation of ginsenoside Rg5.Meanwhile,the study of the stability of ginsenoside Rg5 has important reference value for its storage,pharmacological activity research and drug development.