Diverse rare ginsenosides derived from ginsenoside Re in aqueous methanol solution <i>via</i> heterogeneous catalysis and identified by HPLC-MS

Rare ginsenosides, known for their significant pharmacological effects, are found in only trace amounts in natural ginseng, making it necessary to produce them through transformation processes. In this study, ginsenoside Re was chemically transformed into 30 rare ginsenosides using a novel heterogeneous catalyst HSiW@MeSi in aqueous methanol solution. The HSiW@MeSi catalyst was synthesized by the incorporation of silicotungstic acid (H₄SiW₁₂O₄₀, HSiW) into a mesoporous silica (MeSi) carrier. The resulting rare ginsenosides, which included six pairs of isomers, three sets of four isomers and one set of six isomers, were separated and identified using high-performance liquid chromatography coupled with multistage tandem mass spectrometry through characteristic neutral loss, product ions, and chromatographic retention times. The transformation pathways involved deglycosylation, epimerization, elimination, addition, and cyclization reactions. Water and methanol molecules competitively participated in the reaction, forming 8 hydroxylated and 14 methoxylated products at the C-20(21) or C-24(25) position, respectively. Notably, the HSiW@MeSi catalyst could be recycled and maintained an 83.3 ± 0.3% transformation rate after three cycles. This study represents the successful chemical transformation to produce protopanaxatriol-type rare ginsenosides featuring methoxyl groups at either the C-20(21) or C-24(25) positions. It highlights the potential of heteropolyacid-based heterogeneous transformation strategies in the generation of structurally diverse rare ginsenosides and demonstrates the expanded utility of HPLC-MS in the structural identification of these compounds.