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.

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.

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.

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.

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.

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.

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.

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.

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.

Mechanism of enhanced oral absorption of akebia saponin D by a self-nanoemulsifying drug delivery system loaded with phospholipid complex.

Akebia saponin D (ASD) exhibits a variety of pharmacological activities, such as anti-osteoporosis, neuroprotection, hepatoprotection, but has poor oral bioavailability. A self-nanoemulsifying drug delivery system loaded with akebia saponin D - phospholipid complex (APC-SNEDDS) (composition: Peceol: Cremophor® EL: Transcutol HP: ASD: phospholipid; ratio: 10:45:45:51:12.3, w:w:w:w:w) was first developed to improve the oral absorption of saponins and it was found to significantly enhance ASD's oral bioavailability by 4.3 - fold (p < .01). This study was conducted to elucidate the mechanism of enhanced oral absorption of ASD by the drug delivery system of APC-SNEDDS. The aggregation morphology and particle size of ASD and APC-SNEDDS prepared in aqueous solutions were determined by transmission electron microscope and particle size analyzer, respectively. Stability of ASD and APC-SNEDDS in gastrointestinal luminal contents and mucosa homogenates were also explored. The differences of in situ intestinal permeability of ASD and APC-SNEDDS were compared. APC-SNEDDS reduced the aggregation size from 389 ± 7 nm (ASD) to 148 ± 3 nm (APC-SNEDDS). APC-SNEDDS increased the remaining drug in large intestine luminal contents from 47 ± 1% (ASD) to 83 ± 1% (APC-SNEDDS) during 4 h incubation. APC-SNEDDS provided an 11-fold increase in Ka value and an 11-fold increase in Peff value compared to ASD. In summary, APC-SNEDDS improved ASD's oral bioavailability mainly by increasing membrane permeability, destroying self-micelles and inhibiting the intestinal metabolism.

Synthesis, antitumor activity evaluation and mechanistic study of novel hederacolchiside A1 derivatives bearing an aryl triazole moiety.

In an attempt to arrive at a more potent antitumor agent than the parent natural saponin hederacolchiside A1, 23 hederacolchiside A1 derivatives (4a-4w) were synthesized via Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition and screened in vitro for cytotoxicity against six human cancer cell lines. The structure-activity relationship of these compounds was elucidated, and the biological screening results showed that most of the compounds exhibited moderate to high levels of antitumor activities against the tested cell lines and some of them displayed more potent inhibitory activities compared with hederacolchiside A1. Compound 4f showed a 2- to 7-fold more potent activity than hederacolchiside A1. The mechanistic study of 4f revealed that this compound can induce cell apoptosis in HepG2 cells via mitochondrial-mediated intrinsic pathways.

Synthesis and structure-activity relationship of theasapogenol galactosides against Magnaporthe oryzae.

Camellia oleifera is expected to provide alternative aglycone to synthesize some saponins similar to that from Schima superba with inhibitory activity against Magnaporthe oryzae. Eight theasapogenol galactosides were synthesized via protection of adjacent hydroxyl groups by a benzylidene for regioselective glycosylation in the multi-hydroxyl sapogenin. Water soluble galactose chain connected far from liposoluble end was a key group in inhibiting the growth of M. oryzea unless theasapogenol was modified by two galactosyl groups or by one galactosyl group and one benzylidene group. The amphoteric characteristics of saponin such as saccharide group number, distance between bipolar groups play an important role in inhibiting mycelium growth of M. oryzae.

Studies on anti-angiogenesis of ginsenoside structure modification HRG in vitro.

This study investigates the anti-angiogenic effect of 3ß, 12ß, 20(S)-trihydroxy dammarane-3-O-ß-d-glucopyranosyl(1-2)-ß-d-glucopyranoside(HRG), a new chemical compound obtained by structure modification on Ginseng saponins Rg3, associated with the regulation of matrix metalloproteinases(MMPs) and its upstream signal-regulated molecule of vascular endothelial growth factor(VEGF) and basic fibroblast growth factor(b-FGF) in vitro, which plays an critical role in angiogenesis during the process of carcinoma. In our study, to investigate the anti-angiogenesis effect of HRG in HUVECs, we utilized cell proliferation assay, tube formation assay, wound-healing assay, Semi-quantitative reverse transcription PCR, and Western blot assay. Our results demonstrated that HRG plays a major role in the regulation of proliferation, migration and tube formation of HUVECs by suppressing the expression of VEGF and b-FGF in both transcriptional and post-transcriptional levels. In addition, the expression of MMP-2 and MMP-9, which were related to the ECM degradation, were down-regulated after administration of HRG as well. Overall, our results revealed that HRG strongly inhibited the process of angiogenesis and shows better effectiveness than Rg3.

Development of Improved Vaccine Adjuvants Based on the Saponin Natural Product QS-21 through Chemical Synthesis.

Vaccines based on molecular subunit antigens are increasingly being investigated due to their improved safety and more precise targeting compared to classical whole-pathogen vaccines. However, subunit vaccines are inherently less immunogenic; thus, coadministration of an adjuvant to increase the immunogenicity of the antigen is often necessary to elicit a potent immune response. QS-21, an immunostimulatory saponin natural product, has been used as an adjuvant in conjunction with various vaccines in numerous clinical trials, but suffers from several inherent liabilities, including scarcity, chemical instability, and dose-limiting toxicity. Moreover, little is known about its mechanism of action. Over a decade-long effort, beginning at the University of Illinois at Urbana-Champaign and continuing at the Memorial Sloan Kettering Cancer Center (MSKCC), the group of Prof. David Y. Gin accomplished the total synthesis of QS-21 and developed a practical semisynthetic approach to novel variants that overcome the liabilities of the natural product. First, semisynthetic QS-21 variants were designed with stable amide linkages in the acyl chain domain that exhibited comparable in vivo adjuvant activity and lower toxicity than the natural product. Further modifications in the acyl chain domain and truncation of the linear tetrasaccharide domain led to identification of a trisaccharide variant with a simple carboxylic acid side chain that retained potent adjuvant activity, albeit with reemergence of toxicity. Conversely, an acyl chain analogue terminating in a free amine was inactive but enabled chemoselective functionalization with radiolabeled and fluorescent tags, yielding adjuvant-active saponin probes that, unlike inactive congeners, accumulated in the lymph nodes in vaccinated mice and internalized into dendritic cells. Subtle variations in length, stereochemistry, and conformational flexibility around the central glycosidic linkage provided QS-21 variants with adjuvant activities that correlated with specific conformations found in molecular dynamics simulations. Notably, deletion of the entire branched trisaccharide domain afforded potent, truncated saponin variants with negligible toxicity and improved synthetic access, facilitating subsequent investigation of the triterpene core. The triterpene C4-aldehyde substituent, previously proposed to be important for QS-21 adjuvant activity, proved to be dispensable in these truncated saponin variants, while the presence of the C16 hydroxyl group enhanced activity. Novel adjuvant conjugates incorporating the small-molecule immunopotentiator tucaresol at the acyl chain terminus afforded adjuvant-active variants but without significant synergistic enhancement of activity. Finally, a new divergent synthetic approach was developed to provide versatile and streamlined access to additional linear oligosaccharide domain variants with modified sugars and regiochemistries, opening the door to the rapid generation of diverse, synthetically accessible analogues. In this Account, we summarize these multidisciplinary studies at the interface of chemistry, immunology, and medicine, which have provided critical information on the structure-activity relationships (SAR) of this Quillaja saponin class; access to novel, potent, nontoxic adjuvants for use in subunit vaccines; and a powerful platform for investigations into the mechanisms of saponin immunopotentiation.

Nitric oxide-donating derivatives of hederacolchiside A1: Synthesis and biological evaluation in vitro and in vivo as potential anticancer agents.

A series of nitric oxide (NO) donating derivatives of hederacolchiside A1 bearing triterpenoid saponin motif were designed, synthesized and evaluated for their anticancer activity. All of the tested furoxan-based NO releasing compounds showed significant proliferation inhibitory activities. Especially compound 6a exhibited strong cytotoxicity (IC50=1.6-6.5µM) against four human tumor cell lines (SMMC-7721, NCI-H460, U251, HCT-116) in vitro and the highest level of NO releasing. Furthermore, compound 6a was revealed low acute toxicity to mice and weak haemolytic activity with potent tumor growth inhibition against mice H22 hepatocellular cells in vivo (51.5%).

Syntheses and mucosal adjuvant activity of simplified oleanolic acid saponins possessing cinnamoyl ester.

A series of new simplified oleanolic acid saponins with a glycosyl ester moiety at C28, were efficiently prepared. Furthermore, the effect of nasal administration of the synthetic oleanolic acid saponins on the nasal anti-influenza virus antibody titer against secondary nasal inoculation of the influenza split vaccine was examined. The result revealed cinnamoyl saponin as a suitable candidate vaccine adjuvant.

Synthesis and cytotoxic activity of two steroids: icogenin aglycone analogs.

During the process of icogenin analog research, we obtained two cytotoxic steroids: compound 4 and compound 6 casually. Their in vitro antitumor activities were tested by the standard MTT assay. The results disclosed that compound 4 (IC50 = 3.65-6.90 µM) showed potential antitumor activities against HELA, KB cell lines and compound 6 (IC50 = 2.40-9.05 µM) showed potential antitumor activities against HELA, BGC-823, KB, A549, HCT-8 cell lines.

Synthesis and biological evaluation of Hederacolchiside A1 derivatives as anticancer agents.

Modification of Hederacolchiside A1 (HA1) on 28-COOH gave a series of novel triterpenoid saponin compounds containing ester or amide group. Comparing with natural product HA1, several derivatives showed decreased toxicity in the mice acute toxicity trial and increased the anticancer activity in vitro. Especially compound 1 exhibited the strongest antiproliferative activities against human cancer cell lines tested (IC50=1.1-4.6µM) and potent tumor inhibition rate in vivo (46.8%).

Synthesis of furostanol glycosides: discovery of a potent α-glucosidase inhibitor.

A convenient approach to the synthesis of furostanol glycosides has been developed with the features of both highly efficient incorporation of a 26-O-ß-d-glucopyranosyl unit and ready formation of hemiketal ring E. The total syntheses of seven furostanol saponins including funlioside B, lilioglycoside, protobioside I, protodioscin, pallidifloside I, coreajaponins A and parisaponin I are efficiently achieved using an easily available 16ß-acetoxy-22-oxo-26-hydroxy-cholestanic derivative as a powerful building block. The α-glucosidase inhibitory activity of the synthesized saponins is also evaluated, which reveals that funlioside B is a highly potential lead for developing α-glucosidase inhibitors.

Synthesis of 28a-homoselenolupanes and 28a-homoselenolupane saponins.

A practical synthesis of 28a-homo-28a-selenolupane triterpenes and the corresponding selenosaponins containing d-mannose, l-arabinose, l-rhamnose, and d-idose moieties is described. Selenium containing triterpenes were obtained from the readily available 3-O-allyl-homobetulin mesylate by nucleophilic substitution with the selenocyanate ion which upon reduction of the -SeCN group afforded the free selenol. Glycosylation using classical Schmidt donors gave 1,2-trans selenosaponins as the main product as well as minute amounts of 1,2-cis isomers. This is one of the very few examples of the synthesis of selenoglycosides by direct glycosylation of free selenols. The studied selenol showed high resistance to air oxidation resulting in good stability during the synthesis of selenolupane derivatives. Cytotoxic activities of new homoselenolupane derivatives were also evaluated in vitro and revealed that some triterpenes exhibited an interesting profile against human cancer cell lines.