Activation of glycosyl methylpropiolates by TfOH.

Activation of glycosyl methylpropiolates by TfOH was investigated. Armed and superarmed glycosyl donors can be activated by use of 0.2 equivalent TfOH whereas 1.0 equivalent of TfOH was required for the activation of the disarmed glycosyl donors. All the glycosidations gave very good yields. The method is suitable for synthesis of glycosides and disaccharides and it may result in the hydrolysis of the interglycosidic bond if the sugar at the non-reducing end is armed or superarmed. These problems are not seen when gold-catalyzed activation procedures are invoked for the activation of glycosyl alkynoates.

Pd-catalyzed stereoselective synthesis of chromone C-glycosides.

Herein, we present an efficient Pd-catalysed method for stereoselective synthesis of chromone C-glycosides from various glycals. We successfully applied this method to various glycals with different protecting groups, yielding the corresponding glycosides in 41-78% yields. Additionally, we investigated the potential of this approach for the late-stage modification of natural products and pharmaceutical compounds linked to glycals, leading to the synthesis of their respective glycosides. Furthermore, we extended our research to gram-scale synthesis and demonstrated its applicability in producing various valuable products, including 2-deoxy-chromone C-glycosides. In summary, our work introduces a novel library of chromone glycosides, which holds promise for advancing drug discovery efforts.

A Strain-Promoted Divergent Chemical Steroidation Unveils Potent Anti-Inflammatory Pseudo-Steroidal Glycosides.

The development of novel agents with immunoregulatory effects is a keen way to combat the growing threat of inflammatory storms to global health. To synthesize pseudo-steroidal glycosides tethered by ether bonds with promising immunomodulatory potential, we develop herein a highly effective deoxygenative functionalization of a novel steroidal donor (steroidation) facilitated by strain-release, leveraging cost-effective and readily available Sc(OTf)3 catalysis. This transformation produces a transient steroid-3-yl carbocation which readily reacts with O-, C-, N-, S-, and P-nucleophiles to generate structurally diverse steroid derivatives. DFT calculations were performed to shed light on the mechanistic details of the regioselectivity, underlying an acceptor-dependent steroidation mode. This approach can be readily extended to the etherification of sugar alcohols to enable the achievement of a diversity-oriented, pipeline-like synthesis of pseudo-steroidal glycosides in good to excellent yields with complete stereo- and regiospecific control for anti-inflammatory agent discovery. Immunological studies have demonstrated that a meticulously designed cholesteryl disaccharide can significantly suppress interleukin-6 secretion in macrophages, exhibiting up to 99% inhibition rates compared to the negative control. These findings affirm the potential of pseudo-steroidal glycosides as a prospective category of lead agents for the development of novel anti-inflammatory drugs.

Direct Synthesis of Glycosyl Chlorides from Thioglycosides.

Reported herein is a new method for the direct synthesis of glycosyl chlorides from thioglycosides using sulfuryl chloride at rt. A variety of thioglycosides and thioimidates could be used as substrates. Both acid- and base-sensitive protecting groups were found compatible with these reaction conditions. Preliminary investigation of the reaction mechanism indicates chlorination of the leaving group at the anomeric sulfur as the key step of the reaction.

Syntheses of α(2,8) Sialosides Containing NeuAc and NeuGc by Using Double Carbonyl-Protected N-Acyl Sialyl Donors.

We report on the syntheses of NeuAc and NeuGc-containing glycosides via the use of double carbonyl-protected N-acetyl sialyl donors. The 7-O,9-O-carbonyl protection of an N-acyl-5-N,4-O-carbonyl-protected sialyl donor markedly increased the α-selectivity during glycosylation, particularly when glycosylating the C-8 hydroxyl group of sialic acids. The N-acyl carbamates were selectively opened with ethanethiol under basic conditions to provide N-acyl amines. It is noteworthy that N-glycolyl carbamate was more reactive to nucleophiles by comparison with the N-acetyl carbamate due to the electron-withdrawing oxygen in the N-acyl group and however, allowed selective opening of the carbamates without the loss of N-glycolyl groups. To demonstrate the utility of the approach, we began by synthesizing α(2,3) and α(2,6) sialyl galactosides. Glycosylation of the hydroxy groups of galactosides at the C-6 position with the NeuAc and NeuGc donors provided the corresponding sialyl galactoses in good yields with excellent α-selectivity. However, glycosylation of the 2,3-diol galactosyl acceptor selectively provided Siaα(2,2)Gal. Next, we prepared a series of α(2,8) disialosides composed of NeuAc and NeuGc. Glycosylation of NeuGc and NeuAc acceptors at the C-8 hydroxyl group with NeuGc and NeuAc sialyl donors provided the corresponding α(2,8) disialosides, and no significant differences were detected in the reactivities of these acceptors.

A "biphasic glycosyltransferase high-throughput screen" identifies novel anthraquinone glycosides in the diversification of phenolic natural products.

The sugar moieties of many glycosylated small molecule natural products are essential for their biological activity. Glycosyltransferases (GTs) are enzymes responsible for installing these sugar moieties on a variety of biomolecules. Many GTs active on natural products are inherently substrate promiscuous and thus serve as useful tools in manipulating natural product glycosylation to generate new combinations of sugar units (glycones) and scaffold molecules (aglycones) in a process called glycodiversification. It is important to have an effective screening tool to detect the activity of promiscuous enzymes and their resulting glycoside products. Toward this aim, we developed a strategy for screening natural product GTs in a high-throughput fashion enabled by rapid isolation and detection of chromophoric or fluorescent glycosylated natural products. This involves a solvent extraction step to isolate the resulting polar glycoside product from the unreacted aglycone acceptor substrate and the detection of the formed glycoside by the innate absorbance or fluorescence of the aglycone moiety. Using our approach, we screened a collection of natural product GTs against a panel of precursors to therapeutically important molecules. Three GTs showed previously unreported promiscuity toward anthraquinones resulting in novel ε-rhodomycinone glycosides. Considering the pharmaceutical value of clinically used anthraquinone glycosides that are biosynthesized from an ε-rhodomycinone precursor, and the significance that the sugar moiety has on the biological activity of these drugs, our results are of particular importance toward the glycodiversification of therapeutics in this class. The GTs identified and the novel compounds they produce show promise toward new biocatalytic tools and therapeutics.

Deoxy sugars. General methods for carbohydrate deoxygenation and glycosidation.

Deoxy sugars represent an important class of carbohydrates, present in a large number of biomolecules involved in multiple biological processes. In various antibiotics, antimicrobials, and therapeutic agents the presence of deoxygenated units has been recognized as responsible for biological roles, such as adhesion or great affinity to receptors, or improved efficacy. The characterization of glycosidases and glycosyltranferases requires substrates, inhibitors and analogous compounds. Deoxygenated sugars are useful for carrying out specific studies for these enzymes. Deoxy sugars, analogs of natural substrates, may behave as substrates or inhibitors, or may not interact with the enzyme. They are also important for glycodiversification studies of bioactive natural products and glycobiological processes, which could contribute to discovering new therapeutic agents with greater efficacy by modification or replacement of sugar units. Deoxygenation of carbohydrates is, thus, of great interest and numerous efforts have been dedicated to the development of methods for the reduction of sugar hydroxyl groups. Given that carbohydrates are the most important renewable chemicals and are more oxidized than fossil raw materials, it is also important to have methods to selectively remove oxygen from certain atoms of these renewable raw materials. The different methods for removal of OH groups of carbohydrates and representative or recent applications of them are presented in this chapter. Glycosidic bonds in general, and 2-deoxy glycosidic linkages, are included. It is not the scope of this survey to cover all reports for each specific technique.

Synthesis and Preliminary Anticancer Activity Assessment of N-Glycosides of 2-Amino-1,3,4-thiadiazoles.

The addition of 2-amino-1,3,4-thiadiazole derivatives with parallel iodination of differently protected glycals has been achieved using a double molar excess of molecular iodine under mild conditions. The corresponding thiadiazole derivatives of N-glycosides were obtained in good yields and anomeric selectivity. The usage of iodine as a catalyst makes this method easy, inexpensive, and successfully useable in reactions with sugars. Thiadiazole derivatives were tested in a panel of three tumor cell lines, MCF-7, HCT116, and HeLa. These compounds initiated biological response in investigated tumor models in a different rate. The MCF-7 is resistant to the tested compounds, and the cytometry assay indicated low increase in cell numbers in the sub- G1 phase. The most sensitive are HCT-116 and HeLa cells. The thiadiazole derivatives have a pro-apoptotic effect on HCT-116 cells. In the case of the HeLa cells, an increase in the number of cells in the sub-G1- phase and the induction of apoptosis was observed.

Synthesis, biological evaluation, and molecular docking studies of deoxygenated C-glycosides as LpxC inhibitors.

The bacterial deacetylase LpxC is a promising target for the development of novel antibiotics being selectively active against Gram-negative bacteria. In chiral pool syntheses starting from d- and l-ribose, a series regio- and stereoisomeric monohydroxytetrahydrofuran derivatives was synthesized and tested for LpxC inhibitory and antibacterial activities. Molecular docking studies were performed to rationalize the obtained structure-activity relationships. The (2S,3R,5R)-configured 3-hydroxytetrahydrofuran derivative ent-8 ((2S,3R,5R)-N,3-Dihydroxy-5-(4-{[4-(morpholinomethyl)phenyl]ethynyl}phenyl)tetrahydrofuran-2-carboxamide) was found to be the most potent LpxC inhibitor (Ki = 3.5 µM) of the synthesized series of monohydroxytetrahydrofuran derivatives and to exhibit the highest antibacterial activity against E. coli BL21(DE3) and the D22 strain.

Synthesis, anticancer activity and molecular docking of new triazolo[4,5-d]pyrimidines based thienopyrimidine system and their derived N-glycosides and thioglycosides.

A series of new substituted triazolo[4,5-d]pyrimidine derivatives linked to thienopyrimidine ring system were prepared as a hybrid heterocyclic systems, as possible nucleobases analogs, starting from the key carboxamide derivative 2 and its azide precursor via heterocyclization reactions and their structures were characterized. Glycosylation of the prepared triazolopyrimidine derivatives was performed and afforded, regioselctively, the corresponding thienopyrimidine-triazolopyrimidine hybrid N1-glycosides and their thioglycoside analogues in good yields. The synthesized glycosyl heterocycles were studied for their cytotoxic activity against HepG-2 and MCF-7 human cancer cells and significant results were obtained. Compounds 7a, 8 b, 9 b, 9a and 7 b demonstrated promising activities comparable to the activity of the doxorubicin for (HepG-2) cell line. Furthermore, a number of the afforded triazolopyrimidine glycosides were found potent against cancer cells (MCF-7). Furthermore, docking simulation the promising thienopyrimidine analogues 7-13 was done against EGFR kinase to provide a binding model that could serve in discovery of further anticancer agents.

Leloir glycosyltransferases enabled to flow synthesis: Continuous production of the natural C-glycoside nothofagin.

C-glycosyltransferase (CGT) and sucrose synthase (SuSy), each fused to the cationic binding module Zbasic2 , were co-immobilized on anionic carrier (ReliSorb SP400) and assessed for continuous production of the natural C-glycoside nothofagin. The overall reaction was 3'-C-ß-glycosylation of the polyphenol phloretin from uridine 5'-diphosphate (UDP)-glucose that was released in situ from sucrose and UDP. Using solid catalyst optimized for total (∼28 mg/g) as well as relative protein loading (CGT/SuSy = ∼1) and assembled into a packed bed (1 ml), we demonstrate flow synthesis of nothofagin (up to 52 mg/ml; 120 mM) from phloretin (≥95% conversion) solubilized by inclusion complexation in hydroxypropyl ß-cyclodextrin. About 1.8 g nothofagin (90 ml; 12-26 mg/ml) were produced continuously over 90 reactor cycles (2.3 h/cycle) with a space-time yield of approximately 11 mg/(ml h) and a total enzyme turnover number of up to 2.9 × 103 mg/mg (=3.8 × 105 mol/mol). The co-immobilized enzymes exhibited useful effectiveness (∼40% of the enzymes in solution), with limitations on the conversion rate arising partly from external liquid-solid mass transfer of UDP under packed-bed flow conditions. The operational half-life of the catalyst (∼200 h; 30°C) was governed by the binding stability of the glycosyltransferases (≤35% loss of activity) on the solid carrier. Collectively, the current study shows integrated process technology for flow synthesis with co-immobilized sugar nucleotide-dependent glycosyltransferases, using efficient glycosylation from sucrose via the internally recycled UDP-glucose. This provides a basis from engineering science to promote glycosyltransferase applications for natural product glycosides and oligosaccharides.

Total Syntheses of Aturanosides A and B.

Total syntheses of aturanosides A and B, two antiangiogenic anthraquinone glycosides, have been achieved in an expeditious manner, highlighting anthraquinone synthesis, phenol glycosylation, α-d-glucosaminoside installation, and judicious use of protecting groups.

Δ8(14)-Ergostenol Glycoside Derivatives Inhibit the Expression of Inflammatory Mediators and Matrix Metalloproteinase.

Arthritis is a chronic inflammatory disease accompanied by pathological reactions such as swelling, redness, fever, and pain in various joint areas. The drugs currently available to treat arthritis are associated with diverse side-effects. Therefore, there is a need for safer and more effective treatments to alleviate the inflammation of arthritis with fewer side-effects. In this study, a new sterol, Δ8(14)-ergostenol, was discovered, and its glycosides were synthesized and found to be more efficient in terms of synthesis or anti-inflammatory activity than either spinasterol or 5,6-dihydroergosterol is. Among these synthetic glycosides, galactosyl ergostenol inhibited the expression of inflammatory mediators in TNF-α-stimulated FLS and TNF-α-induced MMPs and collagen type II A1 degradation in human chondrocytes. These results suggest the new galactosyl ergostenol as a treatment candidate for arthritis.

Facile access to C-glycosyl amino acids and peptides via Ni-catalyzed reductive hydroglycosylation of alkynes.

C-Glycosyl peptides/proteins are metabolically stable mimics of the native glycopeptides/proteins bearing O/N-glycosidic linkages, and are thus of great therapeutical potential. Herein, we disclose a protocol for the syntheses of vinyl C-glycosyl amino acids and peptides, employing a nickel-catalyzed reductive hydroglycosylation reaction of alkyne derivatives of amino acids and peptides with common glycosyl bromides. It accommodates a wide scope of the coupling partners, including complex oligosaccharide and peptide substrates. The resultant vinyl C-glycosyl amino acids and peptides, which bear common O/N-protecting groups, are amenable to further transformations, including elongation of the peptide and saccharide chains.

A "Traceless" Directing Group Enables Catalytic SN2 Glycosylation toward 1,2-cis-Glycopyranosides.

Generally applicable and stereoselective formation of 1,2-cis-glycopyranosidic linkage remains a long sought after yet unmet goal in carbohydrate chemistry. This work advances a strategy to this challenge via stereoinversion at the anomeric position of 1,2-trans glycosyl ester donors. This SN2 glycosylation is enabled under gold catalysis by an oxazole-based directing group optimally tethered to a leaving group and achieved under mild catalytic conditions, in mostly excellent yields, and with good to outstanding selectivities. The strategy is also applied to the synthesis of oligosaccharides.

Stereoselective Synthesis of ß-C-Glycosides of 3-Deoxy-d-manno-oct-2-ulosonic Acid (Kdo) via SmI2-Mediated Reformatsky Reactions.

An efficient and simple approach for stereoselective synthesis of ß-Kdo C-glycosides was described, which relies on easily available peracetylated anomeric acetate or anomeric 2-pyridyl sulfide to couple with carbonyl compounds via SmI2-mediated Reformatsky reactions. The utility of this methodology is exemplified by the streamlined synthesis of a practical ß-Kdo C-glycoside with an anomeric aminopropyl linker to conjugate with other biomolecules for further biological studies.

Nickel-Catalyzed Radical Migratory Coupling Enables C-2 Arylation of Carbohydrates.

Nickel catalysis offers exciting opportunities to address unmet challenges in organic synthesis. Herein we report the first nickel-catalyzed radical migratory cross-coupling reaction for the direct preparation of 2-aryl-2-deoxyglycosides from readily available 1-bromosugars and arylboronic acids. The reaction features a broad substrate scope and tolerates a wide range of functional groups and complex molecular architectures. Preliminary experimental and computational studies suggest a concerted 1,2-acyloxy rearrangement via a cyclic five-membered-ring transition state followed by nickel-catalyzed carbon-carbon bond formation. The novel reactivity provides an efficient route to valuable C-2-arylated carbohydrate mimics and building blocks, allows for new strategic bond disconnections, and expands the reactivity profile of nickel catalysis.

A One-Pot Approach to Novel Pyridazine C-Nucleosides.

The synthesis of glycosides and modified nucleosides represents a wide research field in organic chemistry. The classical methodology is based on coupling reactions between a glycosyl donor and an acceptor. An alternative strategy for new C-nucleosides is used in this approach, which consists of modifying a pre-existent furyl aglycone. This approach is applied to obtain novel pyridazine C-nucleosides starting with 2- and 3-(ribofuranosyl)furans. It is based on singlet oxygen [4+2] cycloaddition followed by reduction and hydrazine cyclization under neutral conditions. The mild three-step one-pot procedure leads stereoselectively to novel pyridazine C-nucleosides of pharmacological interest. The use of acetyls as protecting groups provides an elegant direct route to a deprotected new pyridazine C-nucleoside.

1,2-trans-Stereoselective Synthesis of C-Glycosides of 2-Deoxy-2-amino-sugars Involving Glycosyl Radicals.

We report for the first time that the imidate radical can be efficiently added to glycals to generate glycosyl radicals, based on which a general, toxic-reagent-free synthesis of C-glycosides of 2-deoxy-2-amino sugars has been developed. Complementary to previous strategies, the reaction is 1,2-trans-stereoselective and could use aryl alkenes as substrates. The late-stage functionalization and density functional theory calculations are reported.

Synthesis of novel oligomeric anionic alkyl glycosides using laccase/TEMPO oxidation and cyclodextrin glucanotransferase (CGTase)-catalyzed transglycosylation.

Modification of alkyl glycosides, to alter their properties and widen the scope of potential applications, is of considerable interest. Here, we report the synthesis of new anionic alkyl glycosides with long carbohydrate chains, using two different approaches: laccase/2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidation of a long-carbohydrate-chain alkyl glycoside and cyclodextrin glucanotransferase (CGTase)-catalyzed elongation of anionic alkyl glycosides. The laccase/TEMPO oxidation of dodecyl ß- d-maltooctaoside proceeded efficiently with the formation of aldehyde and acid products. However, depolymerization occurred to a large extent, limiting the product yield and purity. On the other hand, CGTase-catalyzed coupling/disproportionation reactions with α-cyclodextrin and dodecyl ß- d-maltoside diuronic acid (DDM-2COOH) or octyl ß- d-glucuronic acid (OG-COOH) as substrates gave high conversions, especially when the CGTase Toruzyme was used. It was found that pH had a strong influence on both the enzyme activity and the acceptor specificity. With non-ionic substrates (dodecyl ß- d-maltoside and octyl ß- d-glucoside), Toruzyme exhibited high catalytic activity at pH 5-6, but for the acidic substrates (DDM-2COOH and OG-COOH) the activity was highest at pH 4. This is most likely due to the enzyme favoring the protonated forms of DDM-2COOH and OG-COOH, which exist at lower pH (pKa about 3).