Oleanolic acid synthetic oligoglycosides: a review on recent progress in biological activities

The natural product oleanolic acid has been widely used for treating hepatopathy in China, whereas its clinical application was confined by poor solubility in water. Inspired by remarkable bioactivities and physical properties of triterpenoid saponins, synthesis and biological evaluation of oleanolic acid oligoglycosides drew considerable attention. In the past several years, chemical efforts were made toward glycosylated modifications of oleanolic acid at C3-OH and C17-COOH, of the carbons at ring A/C, and of the functional groups of oleanolic acid lactone. To provide useful information for further study and applications of oleanolic acid derivatives, a total of 177 oleanolic acid synthetic oligoglycosides and their bioactivities (e.g., antiosteoporosis, antidiabetes, antibacterial, anticancer and hemolytic effects) were reviewed; structure-activity relationships and promising agents are indicated.

Total synthesis of the antitumor natural product polycarcin V and evaluation of its DNA binding profile

The convergent total synthesis of polycarcin V, a gilvocarcin-type natural product that shows significant cytotoxicity with selectivity for nonsmall-cell lung cancer, breast cancer, and melanoma cells, has been achieved in 13 steps from 7, 8, and 22; the sequence features a stereoselective α-C-glycosylation reaction for the union of protected carbohydrate 7 and naphthol 8. The association constant for the binding of polycarcin V to duplex DNA is similar to that previously reported for gilvocarcin V.

Synthesis of α-d-glucosyl substituted methyl glycosides of 3-deoxy-α-d-manno- and d-glycero-α-d-talo-oct-2-ulosonic acid (Kdo/Ko) corresponding to inner core fragments of Acinetobacter lipopolysaccharide

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Synthesis of Acinetobacter LPS fragments with orthogonal protecting pattern.

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First synthesis of Ko-glycosides substituted at position 5.

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α-Selective glucosylation using benzylidene trifluoroacetimidate donor.

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Regioselective phosphorylation at C-6 of a glucosyl substituent.

The α-d-glucopyranosyl-(1→5)-substituted methyl glycosides of 3-deoxy-α-d-manno-oct-2-ulosonic acid (Kdo), 3-deoxy-α-d-lyxo-hept-2-ulosonic acid (Kdh), and d-glycero-α-d-talo-oct-2-ulosonic acid (Ko) were prepared using orthogonally protected glycosyl acceptor derivatives via glycosylation with a torsionally disarmed 4,6-O-benzylidene protected trifluoroacetimidate glucosyl donor followed by global deprotection. The related 6-O-phosphoryl-α-d-glucopyranosyl-(1→5)-substituted Kdo and Kdh derivatives were derived from a benzylidene-protected glucosyl intermediate using phosphoramidite and phosphoryl chloride-based phosphorylation steps, respectively. The deprotected disaccharides serve as ligands to study lectin binding of Acinetobacter lipopolysaccharide core oligosaccharides.

Scope and limitations of 2-deoxy- and 2,6-dideoxyglycosyl bromides as donors for the synthesis of β-2-deoxy- and β-2,6-dideoxyglycosides

It is shown that 2-deoxy- and 2,6-dideoxyglycosyl bromides can be prepared in high yield (72-94%) and engaged in glycosylation reactions with β:α selectivities ≥6:1. Yields of product are 44-90%. Fully armed 2-deoxyglycoside donors are viable, while 2,6-dideoxyglycosides require one electron-withdrawing substituent for high efficiency and β-selectivity. Equatorial C-3 ester protecting groups decrease β-selectivity, and donors bearing an axial C-3 substituent are not suitable. The method is compatible with azide-containing donors and acid-sensitive functional groups.

Merremoside D: de novo synthesis of the purported structure, NMR analysis, and comparison of spectral data

The first synthesis of the purported structure of Merremoside D has been achieved in 22 longest linear steps. The de novo asymmetric synthesis relied on the use of asymmetric catalysis to selectively install all 21 stereocenters in the final compounds from commercially available achiral starting materials. Adiabatic gradient 2D NMR techniques (gHSQCAD, gHMBCAD, gH2BCAD, gHSQCTOXYAD, ROESYAD) were used to completely assign the structure of synthetic Merremoside D. Comparison of our assignments with the limited NMR data reported for natural Merremoside D allows for the tentative confirmation of its structure.

Component-based syntheses of trioxacarcin A, DC-45-A1 and structural analogues

The trioxacarcins are polyoxygenated, structurally complex natural products that potently inhibit the growth of cultured human cancer cells. Here we describe syntheses of trioxacarcin A, DC-45-A1 and structural analogues by late-stage stereoselective glycosylation reactions of fully functionalized, differentially protected aglycon substrates. Key issues addressed in this work include the identification of an appropriate means to activate and protect each of the two 2-deoxysugar components, trioxacarcinose A and trioxacarcinose B, as well as a viable sequencing of the glycosidic couplings. The convergent, component-based sequence we present allows for rapid construction of structurally diverse, synthetic analogues that would be inaccessible by any other means, in amounts required to support biological evaluation. Analogues that arise from the modification of four of five modular components are assembled in 11 steps or fewer. The majority of these are found to be active in antiproliferative assays using cultured human cancer cells.

Synthesis and antibacterial activity of doxycycline neoglycosides

A set of 37 doxycycline neoglycosides were prepared, mediated via a C-9 alkoxyamino-glycyl-based spacer reminiscent of that of tigecycline. Subsequent in vitro antibacterial assays against representative drug-resistant Gram negative and Gram positive strains revealed a sugar-dependent activity profile and one doxycycline neoglycoside, the 2'-amino-α-D-glucoside conjugate, to rival that of the parent pharmacophore. In contrast, the representative tetracycline-susceptible strain E. coli 25922 was found to be relatively responsive to a range of doxycycline neoglycosides. This study also extends the use of aminosugars in the context of neoglycosylation via a simple two-step strategy anticipated to be broadly applicable for neoglycorandomization.

Superarming of glycosyl donors by combined neighboring and conformational effects

A novel glycosyl donor that combines the concepts of both conformational and electronic superarming has been synthesized. The reactivity and selectivity of the donor have been tested in competition experiments.

Synthesis of stereochemically and skeletally diverse fused ring systems from functionalized C-glycosides

A diversity-oriented synthesis (DOS) strategy was developed for the synthesis of stereochemically diverse fused-ring systems containing a pyran moiety. Each scaffold contains an amine and methyl ester for further diversification via amine capping and amide coupling. Scaffold diversity was evaluated in comparison to previously prepared scaffolds by a shape-based principal moments of inertia (PMI) analysis.

Direct synthesis of β-N-glycosides by the reductive glycosylation of azides with protected and native carbohydrate donors

A simple and straightforward method for the stereocontrolled synthesis of β-linked N-glycosides uses alkyl and aryl azides as the nitrogen source. The N-glycosides are formed in high yields and with high β selectivities (typically >70% yield, >15:1 β:α selectivity). This approach is also amenable to the synthesis of N-glycosylated amino acids and peptides (see example, Fmoc=9-fluorenylmethoxycarbonyl).

Synthesis of biopterin and related pterin glycosides

Certain pterins having a hydroxyalkyl side chain at C-6 have been found as glycosidic forms in certain prokaryotes, such as 2'-O-(α-D-glucopyranosyl)biopterin from various kinds of cyanobacteria, and limipterin from a green sulfur photosynthetic bacterium. Synthetic studies on glycosides of biopterin and related pterins have been made in view of the structural proof as well as for closer examination of their biological activities and functions. The syntheses of these natural pterin glycosides have effectively been achieved, mostly through appropriately protected N(2) -(N,N-dimethylaminomethylene)-3-[2-(4-nitrophenyl)ethyl]pterin derivatives as glycosyl acceptors, and are reviewed here.

[Synthesis of p(1)-(11-phenoxyundecyl)-P(2)-(alpha-D-galactopyranosyl) diphosphate and p(1)-(11-phenoxyundecyl)-P(2)-(alpha-D-glucopyranosyl) diphosphate; assay of their acceptor properties for mannosyl residue in the enzmic reaction catalyzed by mannosyltransferase from Salmonella newport]

P(1)-(Phenoxyundecyl)-P(2)-(alpha-D-galactopyranosyl) diphosphate as well as P(1)-(11-phenoxyundecyl)-P(2)-(alpha-D-glucopyranosyl) diphosphate are newly synthesized and their ability to serve as substrate-acceptor of mannosyl residue in enzymic reaction catalyzed by mannosyltransferase from Salmonella newport membrane preparation is investigated. The possibility ofgalactosyl-containing derivative to serve as mannosyl acceptor from GDP-Man is established whereas glucosyl-containing compound is inactive in this process.

Dissecting the influence of oxazolidinones and cyclic carbonates in sialic acid chemistry

At a moment's notice: Thermal equilibration of 1 and mass spectral analysis of sialyl phosphates suggest that the 4O,5N-oxazolidinone and the 4,5-O-carbonate systems influence the anomeric effect and the mechanisms of sialidation by virtue of their dipole moment in the mean plane of the pyranose ring. The electron-withdrawing effect destabilizes 2 and promotes associative glycosylation mechanisms. TEMPO = 2,2,6,6-tetramethylpiperidine N-oxide.

Stereoselective C-glycoside formation with 2-O-benzyl-4,6-O-benzylidene protected 3-deoxy gluco- and mannopyranoside donors: comparison with O-glycoside formation

Unlike alcohols, the reaction of C-nucleophiles with 2-O-benzyl-4,6-O-benzylidene-protected 3-deoxy-gluco- and mannopyranosyl thioglycosides is highly stereoselective providing the α-C-glycosides in the gluco-series and the β-C-glycosides in the manno-series. Conformational analysis of nucleophilic attack of putative intermediate glycosyl oxocarbenium ions suggests that the observed selectivities for C-glycoside formation can be explained by preferential attack on the opposite face of the oxocarbenium to the C2-H2 bond and that eclipsing interactions with this bond are the main stereodetermining factor. It is argued that the steric interactions in the attack of alcohols (sp(3)-hybridized O) and of typical carbon-based nucleophiles (sp(2) C) on oxocarbenium ions are very different, with the former being less severe, and thus that there is no a priori reason to expect O- and C-glycosylation to exhibit parallel stereoselectivities for attack on a given oxocarbenium ion.

Chemoenzymatic synthesis of two new halogenated coumarin glycosides as potential antifungal agents

Two new potential antifungal coumarin glycosides, 6-chlorocoumarin 7-O-beta-D-glucopyranoside (1) and 7-hydroxy-4-trifluoromethyl-coumarin 5-O-beta-D-glucopyranoside (2), were synthesized via enzyme-mediated glycosylation of the respective aglycone, 6-chloro-7-hydroxycoumarin and 5,7-dihydroxy-4-trifluoromethylcoumarin, using transgenic hairy roots of Polygonum multiflorum. Instead of application of the isolated enzyme and exogenous sugar donors, hairy roots of P. multiflorum were successfully adapted as a whole-cell biocatalyst.

Synthesis and sensory evaluation of ent-kaurane diterpene glycosides

Catalytic hydrogenation of the three ent-kaurane diterpene glycosides isolated from Stevia rebaudiana, namely rubusoside, stevioside, and rebaudioside-A has been carried out using Pd(OH)₂ and their corresponding dihydro derivatives have been isolated as the products. Synthesis of reduced steviol glycosides was performed using straightforward chemistry and their structures were characterized on the basis of 1D and 2D NMR spectral data and chemical studies. Also, we report herewith the sensory evaluation of all the reduced compounds against their corresponding original steviol glycosides and sucrose for the sweetness property of these molecules.

Synthesis of flavonol 3-O-glycoside by UGT78D1

Glycosylation is an important method for the structural modification of various flavonols, resulting in the glycosides with increased solubility, stability and bioavailability compared with the corresponding aglycone. From the physiological point of view, glycosylation of plant flavonoids is of importance and interest. However, it is notoriously complicated that flavonols such as quercetin, kaempferol and myricetin, are glucosylated regioselectively at the specific position by chemical method. Compared to the chemical method, enzymatic synthesis present several advantages, such as mild reaction condition, high stereo or region selectivity, no protection/deprotection and high yield. UGT78D1 is a flavonol-specific glycosyltransferase, responsible for transferring rhamnose or glucose to the 3-OH position in vitro. In this study, the activity of UGT78D1 was tested against 28 flavonoids acceptors using UDP-glucose as donor nucleoside in vitro, and 5 acceptors, quercetin, myricetin, kaempferol, fisetin and isorhamnetin, were discovered to be glucosylated at 3-OH position. Herein, the small-scale 3-O-glucosylated quercetin, kaempferol and myricetin were synthesized by UGT78D1 and their chemical structures were confirmed by (1)H and (13)C nuclear magnetic resonance (NMR) and high resolution mass spectrometry (HRMS).

Semisynthesis of apigenin and acacetin-7-O-β-D-glycosides from naringin and their cytotoxic activities

Apigenin-7-O-β-D-glycosides 1-8 and acacetin-7-O-β-D-glycosides 9-16 were semisynthesized from 4'-O-benzyl apigenin 17 and acacetin 18 by glycosidation and deprotection with the corresponding α-acetylglycosyl bromide, respectively. Compounds 17 and 18 were prepared by iodination followed by base-induced elimination, 4'-O-benzylation, or 4'-O-methylation and acid hydrolysis using naringin as starting material which is readily available and cheap. Their cytotoxic potential against five human cancer cell lines (HL-60, SMMC-7721, A-549, MCF-7, and SW480) was evaluated by standard MTT method. The results show that compounds 2, 9, and 19 exhibit moderate cytotoxicity against HL-60, SMMC-7721, A-549, MCF-7, and SW480, while compound 3 exhibits potent cytotoxicity against MCF-7 selectively. Among the synthesized target compounds, 3, 4, 7, 11, 12, 15, and 16 were new compounds, the natural product 8 was the first synthesized and the synthesis of natural products 5, 6, 13, and 14 was efficiently improved by the new synthetic routes.

Diastereoselective additions of allylmetal reagents to free and protected syn-α,β-dihydroxyketones enable efficient synthetic routes to methyl trioxacarcinoside A

Two routes to the 2,6-dideoxysugar methyl trioxacarcinoside A are described. Each was enabled by an apparent α-chelation-controlled addition of an allylmetal reagent to a ketone substrate containing a free α-hydroxyl group and a β-hydroxyl substituent, either free or protected as the corresponding di-tert-butylmethyl silyl ether. Both routes provide practical access to gram quantities of trioxacarcinose A in a form suitable for glycosidic coupling reactions.

[Synthesis of heteroaromatic N-beta-glycosides of N-acetylghicosamine under phase transfer conditions. III. 1,2,4-triazoIin-3-ones glucosaminides]

The catalytic phase transfer reactions of 2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-a-D-glucopyranosyl chloride with thiosubstituted 4-phenyl-5-R-thio-Delta2-1,2,4-triazolin-3-ones in solid alkali-organic solvent and in aqueous alkali-organic solvent were studied. The main products ofglucosaminilation reaction are the appropriate N-beta-glucosaminides. The effect of reaction conditions on the yield and composition of products formed was elucidated on the example of the reaction alpha-D-glucosaminyl chloride with 5-methylthio-4-phenyl-Delta2-1,2,4-triazolin-3-one. The optimal conditions of phase transfer glycosylation were established. N-1,2-trans-grycosidic bond formation was proved by 1H NMR and IR data as well as a comparison with spectral data obtained previously for the glycosides of similar structure.

Physicochemical characterization of natural-like branched-chain glycosides toward formation of hexosomes and vesicles

Synthetic branched-chain glycolipids have become of great interest in biomimicking research, since they provide a suitable alternative for natural glycolipids, which are difficult to extract from natural resources. Therefore, branched-chain glycolipids obtained by direct syntheses are of utmost interest. In this work, two new branched-chain glycolipids are presented, namely, 2-hexyldecyl β(α)-D-glucoside (2-HDG) and 2-hexyldecyl β(α)-D-maltoside (2-HDM) based on glucose and maltose, respectively. The self-assembly properties of these glycolipids have been studied, observing the phase behavior under thermotropic and lyotropic conditions. Due to their amphiphilic characteristics, 2-HDG and 2-HDM possess rich phase behavior in dry form and in aqueous dispersions. In the thermotropic study, 2-HDG formed a columnar hexagonal liquid crystalline phase, whereas in a binary aqueous system, 2-HDG formed an inverted hexagonal liquid crystalline phase in equilibrium with excess aqueous solution. Furthermore, aqueous dispersions of the hexagonal liquid crystal could be obtained, dispersions known as hexosomes. On the other hand, 2-HDM formed a lamellar liquid crystalline phase (smectic A) in thermotropic conditions, whereas multilamellar vesicles have been observed in equilibrium with aqueous media. Surprisingly, 2-HDM mixed with sodium dodecyl sulfate or aerosol OT induced the formation of more stable unilamellar vesicles. Thus, the branched-chain glycolipids 2-HDG and 2-HDM not only provided alternative nonionic surfactants with rich phase behavior and versatile nanostructures, but also could be used as new drug carrier systems in the future.

Synthesis of solasodine glycoside derivatives and evaluation of their cytotoxic effects on human cancer cells

Solasodine glycosides, such as solamargine, have been proved to be very important anti-cancer agents. In order to discover more potent cytotoxic agents and explore the preliminary structure activity relationship, a new series of solasodine glycosides 2-9 were synthesized via a transglycosylation strategy, and their cytotoxic activity against a panel of human cancer cell lines (MCF-7, KB, K562, and PC3 cells) were evaluated by MTT assays. The results indicated that compounds 2, 8, and 9 with the substitute moiety of rhamnose, 2-hydroxyethoxymethyl, and 1,3-dihydroxypropan-2-yloxy-methyl, respectively, exhibited quite strong anticancer activity. The underlying mechanism tests demonstrated that these compounds could induce apoptosis detected by DAPI staining, and Annexin V and propidium iodide binding. Cell cycle analysis indicated that the cancer cells were predominantly arrested at the G2/M phase when exposure to these compounds was examined by flow cytometry. These compounds may serve as lead candidates in the development of novel chemotherapeutics for cancer treatment.

[Synthesis and antituberculosis activity of the derivatives of glycoside steviolbioside from the plant Stevia rebaudiana and diterpenoid isosteviol containing hydrazone, hydrazide and pyridinoyl moieties]

Conjugates of antitubercular drug Isoniazid (hydrazide of isonicotinic acid), nicotinic and alpha-picolinic acid hydrazides and glycoside steviolbioside from the plant Stevia rebaudiana as well as the product of its acid hydrolysis, diterpenoid isosteviol, were synthesized. Besides, isosteviol hydrazide and hydrazone derivatives as well as conjugates containing two isosteviol moieties connected by dihydrazide linker were also obtained. Both initial compounds and their synthetic derivatives inhibit the growth of Mycobacterium tuberculosis (H37Rv in vitro). The minimum concentration at which the growth of M. tuberculosis was inhibited by 100% (MIC) for stevioside and steviolbioside equals 7.5 and 3.8 microg/mL, respectively. MIC values for conjugates of the hydrazides of pyridine carbonic acids and steviolbioside as well as isosteviol are in the ranges 5-10 and 10-20 microg/mL, respectively. Maximum inhibitory effect against M. tuberculosis showed the conjugates of isosteviol and adipic acid dihydrazide (MIC values ranged from 1.7 to 3.1 microg/mL). Antitubercular activity of the compounds studied is higher than the activity of antitubercular drug Pyrizanamide (MIC = 12.5-20 microg/mL) but lower than the activity of antitubercular drug Isoniazid (MIC = 0.02-0.04 microg/mL).

Synthesis, thermal properties, and cytotoxicity evaluation of hydrocarbon and fluorocarbon alkyl β-D-xylopyranoside surfactants

Alkyl β-d-xylopyranosides are highly surface active, biodegradable surfactants that can be prepared from hemicelluloses and are of interest for use as pharmaceuticals, detergents, agrochemicals, and personal care products. To gain further insights into their structure-property and structure-activity relationships, the present study synthesized a series of hydrocarbon (-C(6)H(13) to -C(16)H(33)) and fluorocarbon (-(CH(2))(2)C(6)F(13)) alkyl β-d-xylopyranosides in four steps from d-xylose by acylation or benzoylation, bromination, Koenigs-Knorr reaction, and hydrolysis, with the benzoyl protecting group giving better yields compared to the acyl group in the Koenigs-Knorr reaction. All alkyl β-d-xylopyranosides formed thermotropic liquid crystals. The phase transition of the solid crystalline phase to a liquid crystalline phase increased linearly with the length of the hydrophobic tail. The clearing points were near constant for alkyl β-d-xylopyranosides with a hydrophobic tail ⩾8, but occurred at a significantly lower temperature for hexyl β-d-xylopyranoside. Short and long-chain alkyl β-d-xylopyranosides displayed no cytotoxicity at concentration below their aqueous solubility limit. Hydrocarbon and fluorocarbon alkyl β-d-xylopyranosides with intermediate chain length displayed some toxicity at millimolar concentrations due to apoptosis.

Synthesis and biological activity of novel organoselenium derivatives targeting multiple kinases and capable of inhibiting cancer progression to metastases

The present study reports synthesis and biological activity of novel benzoisoselenazolone compounds derived from ebselen and conjugated to a sugar molecule. Cell proliferation assay using cancer cells combined with in vitro biochemical assays revealed that benzoisoselenazolone 2d, 5a, and 6a exerted anti-proliferative activity, which correlated with selective in vitro inhibition of focal adhesion kinase, AKT-1, and protein kinase C-α. Active molecules were able to significantly inhibit cell migration and invasion in vitro compared to cells treated with the vehicle alone or ebselen. Moreover, in vivo anticancer activity focusing on lead compound 2d and using an invasive human breast cancer orthotopic mouse model revealed a potent anti-metastatic activity at well-tolerated doses. In summary, these novel benzoisoselenazolones we report herein target multiple kinases with established roles in cancer progression and possess anti-invasive and anti-metastatic activity in preclinical models supporting a potential for therapeutic application for human disease.