Convergent synthesis of glycoalkaloids solasonine and its saponin derivative.

We present a practical and convergent synthesis of glycoalkaloids solasonine 1 and its saponin derivative 2, incorporating a {3-O-α-L-rhamnopyranosyl-(1→2)-O-[ß-D-glucopyranosyl-(1→3)]-ß-D-galactopyranoside} moiety. The key features of this strategy include the following: (1) AuCl3-tBuCN cooperative catalysis enabling 1,2-trans stereoselective glycosidation of 2-branched trisaccharide trichloroacetimidate donors with steroidal aglycons, in the absence of neighboring group participation; (2) "cyanide effect" mediated regioselective benzoylation of the 4- and 6-hydroxyl groups of galactopyranosyl disaccharide; and (3) an effective approach to prevent orthoester byproduct formation.

Synthesis and cytotoxicity evaluation of d- and l-sugar-containing mono- and bidesmosidic ursane-type saponins.

Ursolic acid and uvaol are naturally occurring triterpenoids that exhibit a broad spectrum of pharmacological activities, including cytotoxicity. However, a primary challenge in the development of ursane-type pentacyclic triterpenoids for pharmacological use is their poor aqueous solubility, which can impede their effectiveness as therapeutics agents. In this study, we present the facile synthesis of ursolic acid monodesmosides and uvaol bidesmosides, incorporating naturally occurring and water-soluble pentoses and deoxyhexose sugar moieties of opposite d- and l-configurations at the C3 or C3/C28 positions of the ursane core. The twenty synthetic saponins were evaluated in vitro for their cytotoxicity against lung carcinoma (A549) and colorectal adenocarcinoma (DLD-1) cell lines. Notably, all the bidesmosidic uvaol saponins were shown to be cytotoxic as compared to their non-cytotoxic parent triterpenoid. For each series of ursane-type saponins, the most active compounds were 3-O-α-l-arabinopyranosyl ursolic acid (3h) and 3,28-di-O-α-l-rhamnopyranosyl uvaol (4f), showing IC50 values in the low micromolar range against A549 and DLD-1 cancer lines.

Discovery of a New Anti-Inflammatory Agent from Anemoside B4 Derivatives and Its Therapeutic Effect on Colitis by Targeting Pyruvate Carboxylase.

Anemoside B4 (AB4), a triterpenoidal saponin from Pulsatilla chinensis, shows significant anti-inflammatory activity, and may be used for treating inflammatory bowel disease (IBD). Nevertheless, its application is limited due to its high molecular weight and pronounced water solubility. To discover new effective agents for treating IBD, we synthesized 28 AB4 derivatives and evaluated their cytotoxic and anti-inflammatory activities in vitro. Among them, A3-6 exhibited significantly superior anti-inflammatory activity compared to AB4. It showed a significant improvement in the symptoms of DSS-induced colitis in mice, with a notably lower oral effective dose compared to AB4. Furthermore, we discovered that A3-6 bound with pyruvate carboxylase (PC), then inhibited PC activity, reprogramming macrophage function, and alleviated colitis. These findings indicate that A3-6 is a promising therapeutic candidate for colitis, and PC may be a potential new target for treating colitis.

Synthesis and Structural Revision of a Natural Tetrasaccharide from Starfish Asterias rollestoni Bell.

Starfish provide important saponins with diverse bioactivities as the secondary metabolites, among which 2-O-glycosylated glycosides are commonly found. Preparation of those 1,2-trans 2-O-glycosylated glycosides usually relies on 2-O-acyl participation requiring the selective installation and cleavage of 2-O-acyl groups. A convergent synthesis using 2-O-glycosylated oligosaccharide donors would be more straightforward but also pose greater challenges. Herein, we report a convergent synthesis of a distinctive tetrasaccharide isolated from starfish Asterias rollestoni Bell. Dual 2-(diphenylphosphinoyl)acetyl (DPPA) groups at O3 and O4 on galactose moiety led to high ß-selectivities (ß/α=12/1 or ß only) in the challenging [2+2] glycosylation, giving the desired tetrasaccharides in >90 % yields from the 2-O-glycosylated disaccharide donors. These synthetic studies have also unambiguously revised the structure of these natural tetrasaccharides. This work would facilitate further studies on new inhibitors of α-glucosidase as hypoglycemic drugs.

Efficient synthesis of diverse C-3 monodesmosidic saponins by a continuous microfluidic glycosylation/batch deprotection method.

Triterpene and steroid saponins have various pharmacological activities but the synthesis of C-3 monodesmosidic saponins remains challenging. Herein, a series of C-3 glycosyl monodesmosidic saponins was synthesized via the microfluidic glycosylation of triterpenoids or steroids at the C-3 position, without the formation of orthoester byproducts, and subsequent deprotection of the benzoyl (Bz) group. This microfluidic glycosylation/batch deprotection sequence enabled the efficient synthesis of C-3 saponins with fewer purification steps and a shorter reaction time than conventional batch synthesis and stepwise microfluidic glycosylation. Furthermore, this system minimized the consumption of the imidate donor. Using this reaction system, 18 different C-3 saponins and 13 different C-28-benzyl-C-3 saponins, including 8 new compounds, were synthesized from various sugars and triterpenes or steroids. Our synthetic approach is expected to be suitable for further expanding the C-3 saponin library for pharmacological studies.

Synthesis and antitumor activity evaluation of oleanolic acid saponins bearing an acetylated l-arabinose moiety.

A series of oleanolic acid derivatives bearing acetyl-substituted l-arabinose moiety has been synthesized and screened in vitro for cytotoxicity against ten cancer cell lines and four normal cell lines. The antiproliferative evaluation indicated that synthetic derivatives showed excellent selectivity, as they were toxic against only A431 cell line. Among them, the compound 6 possesses the best inhibitory activity. A series of pharmacology experiments showed that compound 6 significantly induced A431 cells apoptosis and cell cycle arrest, which could serve as a promising lead candidate for further study.

Facile Synthesis of Saikosaponins.

Saikosaponin A (SSa) and D (SSd) are typical oleanane-type saponins featuring a unique 13,28-epoxy-ether moiety at D ring of the aglycones, which exhibit a wide range of biological and pharmacological activities. Herein, we report the first synthesis of saikosaponin A/D (1-2) and their natural congeners, including prosaikosaponin F (3), G (4), saikosaponin Y (5), prosaikogenin (6), and clinoposaponin I (7). The present synthesis features ready preparation of the aglycones of high oxidation state from oleanolic acid, regioselective glycosylation to construct the ß-(1→3)-linked disaccharide fragment, and efficient gold(I)-catalyzed glycosylation to install the glycans on to the aglycones.

Discovery and structural optimization of 3-O-ß-chacotriosyl oleanane-type triterpenoids as potent entry inhibitors of SARS-CoV-2 virus infections.

Currently, SARS-CoV-2 virus is an emerging pathogen that has posed a serious threat to public health worldwide. However, no agents have been approved to treat SARS-CoV-2 infections to date, underscoring the great need for effective and practical therapies for SARS-CoV-2 outbreaks. We reported that a focused screen of OA saponins identified 3-O-ß-chacotriosyl OA benzyl ester 2 as a novel small molecule inhibitor of SARS-CoV-2 virus entry, via binding to SARS-CoV-2 glycoprotein (S). We performed structure-activity relationship profiling of 2 and discovered C-17-COOH of OA was an important modification site that improved both inhibitor potency toward SARS-CoV-2 and selectivity index. Then optimization from hit to lead resulted in a potent fusion inhibitor 12f displaying strong inhibition against infectious SARS-CoV-2 with an IC50 value of 0.97 µM in vitro. Mechanism studies confirmed that inhibition of SARS-CoV-2 viral entry of 12f was mediated by the direct interaction with SARS-CoV-2 S2 subunit to block membrane fusion. These 3-O-ß-chacotriosyl OA amide saponins are suitable for further optimization as SARS-CoV-2 entry inhibitors with the potential to be developed as therapeutic agents for the treatment of SARS-CoV-2 virus infections.

Structures/cytotoxicity/selectivity relationship of natural steroidal saponins against GSCs and primary mechanism of tribulosaponin A.

Glioblastoma multiform (GBM) is the highly aggressive brain tumor with poor prognosis. Glioma stem cells (GSCs), small population of cancer cells that exist in GBM tissues, resistant to chemotherapy and radiotherapy and usually driving GBM recurrence, have been developed as effective therapeutic target. Steroidal saponins are one of important resources for anti-tumor agent and may be benefited to selectively clear GSCs. In this report, total of 97 natural steroidal saponins were investigated the relationship among structures/cytotoxicity/selectivity against GSCs, glioma cell lines and human untransformed cells, and revealed that tribulosaponin A was the most potent compound. Further investigation suggested that tribulosaponin A up-regulated the expression of NCF1 and NOX1 to accumulate ROS for triggering apoptosis in GSCs, but not in untransformed cells, and it was further supported by the assay that N-acetyl-l-cysteine (NAC) clearing ROS delayed GSCs apoptosis. Besides, tribulosaponin A damaged GSCs recapturing tumor spheres formation.

Synthesis, Modification and Biological Activity of Diosgenyl ß-d-Glycosaminosides: An Overview.

Saponins are a structurally diverse class of natural glycosides that possess a broad spectrum of biological activities. They are composed of hydrophilic carbohydrate moiety and hydrophobic triterpenoid or steroid aglycon. Naturally occurring diosgenyl glycosides are the most abundant steroid saponins, and many of them exhibit various pharmacological properties. Herein, we present an overview of semisynthetic saponins syntheses-diosgenyl ß-d-glycosaminosides (d-gluco and d-galacto). These glycosides possess a 2-amino group, which creates great possibilities for further modifications. A wide group of glycosyl donors, different N-protecting groups and various reaction conditions used for their synthesis are presented. In addition, this paper demonstrates the possibilities of chemical modifications of diosgenyl ß-d-glycosaminosides, associated with functionalisation of the amino group. These provide N-acyl, N-alkyl, N,N-dialkyl, N-cinnamoyl, 2-ureido and 2-thiosemicarbazonyl derivatives of diosgenyl ß-d-glycosaminosides, for which the results of biological activity tests (antifungal, antibacterial, anti-cancer and hemolytic) are presented.

Biological and Pharmacological Effects of Synthetic Saponins.

Saponins are amphiphilic molecules consisting of carbohydrate and either triterpenoid or steroid aglycone moieties and are noted for their multiple biological activities-Fungicidal, antimicrobial, antiviral, anti-inflammatory, anticancer, antioxidant and immunomodulatory effects have all been observed. Saponins from natural sources have long been used in herbal and traditional medicines; however, the isolation of complexed saponins from nature is difficult and laborious, due to the scarce amount and structure heterogeneity. Chemical synthesis is considered a powerful tool to expand the structural diversity of saponin, leading to the discovery of promising compounds. This review focuses on recent developments in the structure optimization and biological evaluation of synthetic triterpenoid and steroid saponin derivatives. By summarizing the structure-activity relationship (SAR) results, we hope to provide the direction for future development of saponin-based bioactive compounds.

Purified Gymnemic Acids from Gymnema inodorum Tea Inhibit 3T3-L1 Cell Differentiation into Adipocytes.

Gymnema inodorum (GI) is an indigenous medicinal plant and functional food in Thailand that has recently helped to reduce plasma glucose levels in healthy humans. It is renowned for the medicinal properties of gymnemic acid and its ability to suppress glucose absorption. However, the effects of gymnemic acids on adipogenesis that contribute to the accumulation of adipose tissues associated with obesity remain unknown. The present study aimed to determine the effects of gymnemic acids derived from GI tea on adipogenesis. We purified and identified GiA-7 and stephanosides C and B from GI tea that inhibited adipocyte differentiation in 3T3-L1 cells. These compounds also suppressed the expression of peroxisome proliferator-activated receptor gamma (Pparγ)-dependent genes, indicating that they inhibit lipid accumulation and the early stage of 3T3-L1 preadipocyte differentiation. Only GiA-7 induced the expression of uncoupling protein 1 (Ucp1) and pparγ coactivator 1 alpha (Pgc1α), suggesting that GiA-7 induces mitochondrial activity and beige-like adipocytes. This is the first finding of stephanosides C and B in Gymnema inodorum. Our results suggested that GiA-7 and stephanosides C and B from GI tea could help to prevent obesity.

Preparing molecularly imprinted nanoparticles of saponins via cooperative imprinting strategy.

Saponin is an important class of natural products with various pharmacological activities. The selective separation of saponins is an essential step before further analysis. Molecular imprinting has been an effective strategy for preparing antibody mimics. However, a facile and efficient imprinting strategy for saponins is still lacking owing to their amphiphilic nature. Herein, we have prepared the saponins imprinted nanoparticles via cooperative imprinting strategy. This new strategy relies on the combination of various non-covalent interactions (hydrophobic and hydrogen bonding) and covalent boronate affinity interactions. The obtained imprinted nanoparticles could rebind specific saponins from complex matrices with good selectivity, superb tolerance to interference, and fast binding equilibrium. This method was verified to be versatile and facile. Thus, this strategy could greatly facilitate the preparation of imprinted nanoparticles for the specific recognition of saponins.

Structural Effect on Adjuvanticity of Saponins.

We have prepared a number of saponin-based vaccine adjuvant candidates. These unnatural saponins have a different terminal-functionalized side chain incorporated into the glucuronic acid unit that is attached to a triterpenoid core at its C3 position. The semisynthetic saponin adjuvants have shown significantly different immunostimulatory activities, suggesting that the structure of the side chain, triterpenoid core, and oligosaccharide domain together orchestrate saponin adjuvant's potentiation of immune responses. Among these new adjuvant candidates, VSA-2 (5b), a derivative of Momordica saponin (MS) II, showed consistent enhancement of immunoglobulin G2a (IgG2a) production when it was in formulation with either ovalbumin or recombinant hemagglutinin B (rHagB) antigen. With rHagB antigen, it induced a significantly higher IgG2a response than the positive control GPI-0100, a well-studied semisynthetic saponin adjuvant mixture derived from Quillaja saponaria Molina saponins, known for its ability to induce a balanced Th1/Th2 immunity. These results confirm that Momordica saponins are a viable natural source to provide potent saponin adjuvants after simple chemical derivatization and identify VSA-2 (5b) as another MS-based promising immunostimulant lead owing to its distinctive ability in potentiating the IgG2a response.

Production of antifungal saponins in an airlift bioreactor with a cell line transformed from Solanum chrysotrichum and its activity against strawberry phytopathogens.

Biotechnology through plant cell cultures in bioreactors is a tool that allows increasing the production of secondary metabolites of commercial interest. The hydrodynamic characterization, in addition to the transfer (OTR) and uptake (OUR) of oxygen through the dynamic method with different aeration rate, were used to see their influence on the production of biomass and saponins. The culture poisoning technique was used to determine the antifungal activity of the SC-2 and SC-3 saponins in vitro. Likewise, the shear or hydrodynamic stress of 273.6 mN/m2 were calculated based on the Reynolds Number. The oxygen supply (OTR) was always greater than the demand (OUR) for all the aeration rate evaluated. Dry weight values of 8.6 gDW/L and a concentration of 2.7 mg/L and 187.3 mg/L of the saponins SC-2 and SC-3 respectively were obtained with an air flow of 0.1 vvm. In addition, it was possible to inhibit the growth of phytopathogenic fungi in vitro by up to 93%, while in vivo it was possible to reduce the infections of strawberry seeds inoculated with phytopathogens, obtaining up to 94% of germinated seeds. This information will facilitate the rational operation of the bioreactor culture system that produces secondary metabolites.

Exploiting structure-activity relationships of QS-21 in the design and synthesis of streamlined saponin vaccine adjuvants.

We report the design, synthesis, immunological evaluation, and conformational analysis of new saponin variants as promising vaccine adjuvants. These studies have provided expedient synthetic access to streamlined adjuvant-active saponins and yielded molecular-level insights into saponin conformation that correlated with their in vivo adjuvant activities.

Development of Chemical Probes for Functional Analysis of Anticancer Saponin OSW-1.

Chemical probe-based approaches have proven powerful in recent years in the target identification studies of natural products. OSW-1 is a saponin class of natural products with highly potent and selective cytotoxicity against various cancer cell lines. Understanding its mechanism of action is important for the development of anticancer drugs with potentially novel target pathways. This account reviews recent progress in the development of OSW-1 derived probes for exploring the mechanism of its action. The key to the probe development is a judicious choice of functionalization sites and a selective functionalization strategy. The types of probes include fluorescent probes for cellular imaging analysis and affinity probes for target identification analysis.

Oleanolic acid 3-glucoside, a synthetic oleanane-type saponin, alleviates methylmercury toxicity in vitro and in vivo.

Methylmercury (MeHg) is one of the most toxic environmental pollutants, presenting a serious health hazard worldwide. In this study, we examined the potential of derivatives of oleanolic acid (OA), such as OA 3-glucoside, OA 28-glucoside, and OA 3,28-diglucoside, to mitigate MeHg toxicity in vitro and in vivo. We found that OA 3-glucoside suppressed the cellular MeHg uptake by 63.4% compared with that of the control and improved the cell viability from 75.4% to 107.9% upon exposure to cytotoxic MeHg in Caco-2 cells. To verify the anti-MeHg activity of OA 3-glucoside, mice were orally administered MeHg (0, 1.0, or 5.0 mg kg-1·d-1), with or without OA 3-glucoside, and then mercury accumulation was measured in various organs of the mice. The mice co-treated with MeHg and OA 3-glucoside showed significantly lower mercury content in organs such as the cerebrum, cerebellum, liver, kidney, and spleen, with 83.1%, 68.7%, 71.7%, 82.1%, and 18.2% of those in the OA 3-glucoside-untreated group, respectively. This suggested OA 3-glucoside had the potential as an anti-MeHg compound, owing to its ability to suppress the distribution of MeHg into organs. Supporting this hypothesis, the mice treated with MeHg and OA 3-glucoside showed a tendency to survive one day longer than the control mice. Our findings suggest OA 3-glucoside administration alleviates the toxicity of MeHg by suppressing MeHg accumulation in organs.

Synthesis and Evaluation of a QS-17/18-Based Vaccine Adjuvant.

We have synthesized a QS-17/18 analogue (7) and evaluated its adjuvant activity in the formulation with rHagB antigen. Compound 7 and QS-21 analogues 5 and 6 are presumably the major components of GPI-0100, a widely used complex mixture of semisynthetic derivatives of Quillaja saponaria (QS) Molina saponins. The QS-17/18 analogue 7 shows an adjuvant activity profile similar to that of GPI-0100, potentiating mixed Th-1/Th-2 immune responses, which is different from those of QS-21 analogues 5 and 6 that probably only induce a Th2-like immunity. The combination of QS-17/18 and QS-21 analogues does not show a synergistic effect. These results suggest that QS-17/18 analogue 7 might be the active component of GPI-0100 responsible for its immunostimulant property. Therefore, compound 7 can not only be a structurally defined alternative to GPI-0100 but also provide a valuable clue for rational design of new QS-based vaccine adjuvants with better adjuvant properties.

Chemical synthesis of marine saponins.

Covering: 1989-2017 Saponins are characteristic metabolites of starfish and sea cucumbers, and occasionally are also found in sponges, soft coral, and small fish. These steroid or triterpenoid glycosides often show remarkable biological and pharmacological activities, such as antifungal, antifouling, shark repellent, antitumor and anti-inflammatory activities. Over one thousand marine saponins have been characterized; the majority of them can be categorized into three major structural types, i.e., asterosaponins, polyhydroxysteroid glycosides, and holostane glycosides. Thus far, only 12 marine saponins have been synthesized; those representing the major types were successfully synthesized recently. The syntheses involve preparation of the aglycones from the terrestrial steroid or triterpene materials, installation of the carbohydrate units, and manipulation of the protecting groups. Herein, we provide a comprehensive review on these syntheses.