关键词: 达玛烷皂苷, Ginsenoside Re, Notoginsenoside R₁, 金催化, 糖苷化

Ginsenoside Re (1) and Notoginsenoside R₁ (2) are two representative dammarane type protopanaxatriol-6,20-O-bisglycosides occurring widely founded in Panaxginseng. Ginsenoside Re (1) showed potent antioxidative, anti-inflammatory and antihyperlipemia activities, and Notoginsenoside R₁ (2) showed potent antioxidative and antiinflammatory activities, so it would be helpful to synthesize these homogeneous natural products in appreciable amounts by accelerating their structure-activity relationship study. As a persistent effort on the chemical syntheses of the diverse ginsenosides in our group, we report herein the efficient syntheses of these two complex natural products. Thus, based on the reactivity sequence of the four hydroxyl groups (i.e., 12-OH > 3-OH > 6-OH >> 20-OH) of the protopanaxatriol aglycon, an orthogonal and efficient protecting group strategy was applied to distinguish these hydroxyl groups. The subsequent installation of the 6,20-O-bisglycosides are challenging, given the poor reactivity of the secondary 6-OH and tertiary 20-OH, moreover, with the latter being labile toward acidic conditions. Taking advantage of the neutral conditions of the Au(I)-catalyzed glycosylation reaction (0.3 equiv. Ph₃PAuNTf₂, 4 Å MS, CH₂Cl₂, r.t.), the glycosylation of the acid-labile 20-hydroxyl group was achieved effectively in a high 84% yield firstly. To be convergent for the syntheses of these two ginsenosides, the 6-O-disaccharide residues were installed in a stepwise manner. For the glycosylation of the 6-OH of protopanaxatriol, a higher loading of the catalyst Ph₃PAuNTf₂ (0.5 equiv.) was employed in order to increase the glycosylation yield while reduce the orthoester formation, thus, the desired 6β-O-glucosides were prepared in satisfactory yields (77%~83%). Both terminal α-L-Rha/β-D-Xyl moieties at the 2' position of 6-O-glc were installed efficiently under 0.2 equiv. Ph₃PAuNTf₂ catalyzing condition (ClCH₂CH₂Cl, 5 Å MS, 40℃) with 86% and 81% yields, respectively. After global deprotection, Ginsenoside Re (1) and Notoginsenoside R₁ (2) were synthesized with the longest 13 linear steps in 5.1% and 4.5% overall yields, respectively.

Key words: dammarane-type saponin, Ginsenoside Re, Notoginsenoside R₁, Au (I)-catalyzed glycosylation