Glycoside Donors Research Articles

Glycosylation of 2-(2-Propylsulfinyl)benzyl 1,2-Orthoester Glycosides Initiated by Sulfoxide Activation.

We have developed a highly effective glycosylation method that involves the activation of 2-(2-propylsulfinyl)benzyl 1,2-orthoester glycosides using triflic anhydride (Tf2O). Our research indicates that half of the glycosyl donor is activated through Tf2O via an interrupted Pummerer reaction mechanism, while the remaining portion is activated by triflic acid (TfOH) generated in situ. As a result, as little as 0.5 equiv of Tf2O is adequate for activating the orthoester glycoside donors. This glycosylation procedure offers several benefits, such as high efficiency, wide applicability, and the utilization of a recyclable leaving group.

Fe(III)-catalyzed stereoselective synthesis of deoxyglycosides using stable bifunctional deoxy-phenylpropiolate glycoside donors

Herein, we report a mild and economical route for the stereoselective synthesis of 2-deoxy and 2,6-dideoxyglycosides via FeCl3-catalyzed activation of bench stable deoxy-phenylpropiolate glycosyl donors (D-PPGs). Optimized reaction conditions work well under additive-free conditions to afford the corresponding 2-deoxy and 2,6-dideoxyglycosides in good yields with high α-anomeric selectivity by reacting with sugar and non-sugar-based acceptors. The optimized conditions were also extended for disarmed D-PPG donors. In addition, the developed strategy is amenable to high-scale-up synthesis.

α/β-Stereo- and diastereoselective glycosylation with n-pentenyl glycoside donors, promoted by N-iodosuccinimide and catalyzed by chiral Brønsted acid.

A method involving stereoselective glycosylation catalyzed by (+)-isomenthol ester of pentacarbomethoxycyclopentadiene as a chiral Brønsted acid, with n-pentenyl glycosides in the presence of N-iodosuccinimide as the promoter is described; this method offered a chiral recognition of racemic substrates.

Activation of Stable and Recyclable Phenylpropiolate Glycoside (PPG) Donors by Iron Catalysis

AbstractThe glycosylation reaction is one of the important aspects of carbohydrate chemistry, where two different units are frequently linked through C–O bonds. In the pursuit of advancing this field, the design and development of sustainable catalytic methods for O-glycosylation, which can provide an alternate and effective tool to traditional protocols involving stoichiometric promoters and classical donors, are considered as highly challenging, yet important facets of glycochemistry. Herein, we report a simple and efficient Fe(III)-catalyzed method for O-glycosylation through the activation of bifunctional phenylpropiolate glycoside (PPG) donors. This mild and effective method involves the use of the inexpensive and less toxic FeCl3 as catalyst and easily synthesizable, benchtop-stable glycosyl ester-based PPG donors, which react with various sugar as well as non-sugar-based acceptors to deliver the corresponding O-glycosides in good yields with moderate anomeric selectivity, along with regeneration of easily separable phenylpropiolic acid. Importantly, d-mannose and l-rhamnose-based PPG donors afforded the corresponding O-glycosides in high α-anomeric selectivity. The reaction conditions were further explored for the synthesis of trisaccharides.

Ortho-Methoxycarbonylethynylphenyl Thioglycosides (MCEPTs): Versatile Glycosyl Donors Enabled by Electron-Withdrawing Substituents and Catalyzed by Gold(I) or Cu(II) Complexes

With easily accessible and operator-friendly reagents, shelf-stable ortho-methoxycarbonylethynylphenyl thioglycosides were efficiently prepared. Based on these MCEPT glycoside donors, a novel glycosylation protocol featuring mild and catalytic promotion conditions with Au(I) or Cu(II) complexes, expanded substrate scope encompassing challenging donors and acceptors and clinically used pharmaceuticals, and versatility in various strategies for highly efficient synthesis of glycosides has been established. The practicality of the MCEPT glycosylation protocol was fully exhibited by highly efficient and scalable synthesis of surface polysaccharide subunits of Acinetobacter baumannii via latent-active, reagent-controlled divergent orthogonal one-pot and orthogonal one-pot strategies. The underlying reaction mechanism was investigated systematically through control reactions, leading to the isolation and characterization of the vital catalyst species in MCEPT glycosylation, the benzothiophen-3-yl-gold(I) complex. Based on the results obtained both from control reactions and from studies leading to the glycosylation protocol establishment, an operative mechanism was proposed and the effect of the vital catalyst species reactivity on the results of metal-catalyzed alkyne-containing donor-involved glycosylation was disclosed. Moreover, the mechanism for C-glycosylation side product formation from ortho-(substituted)ethynylphenyl thioglycoside donors with electron-donating substituents was also illuminated.

Cyclic Disaccharide Formation Enforced by a Ring Contraction: 2,3-Dideoxy Pyranoside Glycoside Donor to a Furanoside Macrocycle.

The synthesis of a disaccharide macrocycle through 2,3-dideoxy glucopyranosyl monosaccharide is reported. 2,3-Dideoxy-erythro-hexopyranosyl thioglycoside possessing a free hydroxy functionality at the C-4 carbon is prepared, and cycloglycosylation is conducted. In the event, the cycloglycosylation occurs with a ring contraction of the monosaccharide moiety and affords the cyclic furanoside disaccharide. Solution-phase and single-crystal X-ray diffraction structural characterizations permit the features of the macrocycle to be uncovered. The solubilization and encapsulation properties of the macrocycle are studied in aqueous solutions with 1-aminoadamantane.

Mimicry of the proton wire mechanism of enzymes inside a supramolecular capsule enables β-selective O-glycosylations

Enzymes achieve high substrate and product selectivities by orientating and activating the substrate(s) appropriately inside a confined and finely optimized binding pocket. Although some basic aspects of enzymes have already been mimicked successfully with man-made catalysts, substrate activation by proton wires inside enzyme pockets has not been recreated with man-made catalysts so far. A proton wire facilitates the dual activation of a nucleophile and an electrophile via a reciprocal proton transfer, enabling highly stereoselective reactions under mild conditions. Here we present evidence for such an activation mode inside the supramolecular resorcin[4]arene capsule and demonstrate that it enables catalytic and highly β-selective glycosylation reactions-still a major challenge in glycosylation chemistry. Extensive control experiments provide very strong evidence that the reactions take place inside the molecular container. We show that this activation strategy is compatible with a broad scope of glycoside donors and nucleophiles, and is only limited by the cavity size.

Cloning and expression analysis of UDP-rhamnose synthase from Citrus sinensis

Uridine diphosphate rhamnose(UDP-Rha), a glycoside donor synthesized with the catalysis of rhamnose synthase(RHM), is one of the important elements in the synthesis of rhamnosides. In this study, we cloned a RHM gene from Citrus sinensis(CsRHM) and analyzed its bioinformatic information and functions in vitro. The results showed the gene consisted of an open reading frame of 2 007 bp encoding 668 amino acid residues. The deduced protein had a presumed molecular weight of 75.27 kDa, a theoretical isoelectric point of 6.97, and the characteristic signal sequences(GxxxGxxG/A and YxxxK) of the RHM family. Multiple sequence alignments and the phylogenetic tree demonstrated that CsRHM shared homology with other RHMs. The results of enzymatic reactions in vitro showed that the recombinant protein CsRHM catalyzed the conversion of UDP-Glu to UDP-Rha, with the kinetic parameters V_(max), K_m, K_(cat), and K_(cat)/K_m of 0.373 7 μmol·L~(-1)·min~(-1), 21.29 μmol·L~(-1), 0.24 s~(-1), and 1.13×10~4 s~(-1)·L·mol~(-1), respectively. This study is the first report about CsRHM with validated catalytic function in vitro, which provides a foundation for further research on the biosynthesis of UDP-Rha.

Fisetin glycosides synthesized by cyclodextrin glycosyltransferase from Paenibacillus sp. RB01: characterization, molecular docking, and antioxidant activity.

Fisetin is a flavonoid that exhibits high antioxidant activity and is widely employed in the pharmacological industries. However, the application of fisetin is limited due to its low water solubility. In this study, glycoside derivatives of fisetin were synthesized by an enzymatic reaction using cyclodextrin glycosyltransferase (CGTase) from Paenibacillus sp. RB01 in order to improve the water solubility of fisetin. Under optimal conditions, CGTase was able to convert more than 400 mg/L of fisetin to its glycoside derivatives, which is significantly higher than the previous biosynthesis using engineered E. coli. Product characterization by HPLC and LC-MS/MS revealed that the transglycosylated products consisted of at least five fisetin glycoside derivatives, including fisetin mono-, di- and triglucosides, as well as their isomers. Enzymatic analysis by glucoamylase and α-glucosidase showed that these fisetin glycosides were formed by α-1,4-glycosidic linkages. Molecular docking demonstrated that there are two possible binding modes of fisetin in the enzyme active site containing CGTase-glysosyl intermediate, in which O7 and O4’ atoms of fisetin positioned close to the C1 of glycoside donor, corresponding to the isomers of the obtained fisetin monoglucosides. In addition, the water solubility and the antioxidant activity of the fisetin monoglucosides were tested. It was found that their water solubility was increased at least 800 times when compared to that of their parent molecule while still maintaining the antioxidant activity. This study revealed the potential application of CGTase to improve the solubility of flavonoids.

Synthesis of MUC1 Peptide Backbone Bearing Sialyl‐Tn Antigen for Structural and Functional Studies With Endogenous Lectins

ObjectiveTo synthesize the tumor associated analogs of MUC1 for use in structural and functional studies between sialyl‐Tn (sTn) antigen with macrophage galactose‐specific lectin (MGL).AbstractMucin 1 (MUC1) is a heavily glycosylated transmembrane protein, altered in both expression and glycosylation pattern in human carcinomas of the epithelium. The presence of incomplete or truncated glycan structures, often capped by sialic acid, commonly known as tumor‐associated carbohydrate antigens (TACAs), on the cell surface is a well‐known cancer biomarker. Accumulating evidence suggests that TACAs expression is associated with tumor escape from immune defenses through interaction with endogenous carbohydrate binding proteins (lectins). These interactions often lead to creation of a protumor microenvironment, favoring tumor initiation, progression, metastasis, and immune evasion. Macrophage galactose binding lectin (MGL) is a C‐type lectin receptor found on antigen‐presenting cells (APCs) which facilitates the uptake of carbohydrate antigens for antigen presentation, modulating the immune response homeostasis, autoimmunity, and cancer. Considering the crucial role of tumor‐associated forms of MUC1 with MGL in tumor immunology, a thorough understanding of this interaction is essential for it to be exploited for cancer vaccine strategies. The specific aim of this research is to synthesize structurally well‐defined chemical probes, mono and multiple glycosylated MUC1 glycopeptide models carrying the Tn or sTn, that allow for the control of the complexity of the chemical space of the multivalent ligands. For this purpose, a concise scheme was developed for the large synthesis of the sTn antigen building block. Thiophenyl glycoside donors, in the presence TfOH/NIS or TMSOTf/NIS as promoter system, were used for the galactosylation and sialylation steps of the amino acid building block synthesis, respectively. We explored the effect of activator, temperature, solvent, and excess equivalent of sialic acid thioglycoside donor on sialylation reaction. The most favorable reaction conditions improved the desired α‐stereoselectivity, decreased the competing 2,3‐elimination reaction on the sialyl donor during activation, leading to overall increase in the reaction yield. This building block was used in the stepwise solid‐phase peptide synthesis of MUC1 glycopeptide models. The secondary structure of synthesized peptides was determined by circular dichroism (CD) spectroscopy.

Investigations to mechanism and applications of the glycosylation protocol employing 8-methyltosylaminoethynyl-1-naphthyl (MTEAN) glycoside donors

Based on the capability of silyl ethers for efficient glycosylation, the new MTEAN glycosylation protocol utilizing 8-methyltosylaminoethynyl-1-naphthyl (MTEAN) glycosides as donors were extended to one-pot synthesis of nucleosides, late-stage modification of bioactive molecules, and stereoselective construction of 1,2-cis-glucosidic linkages using silyl ethers of the acceptors. Moreover, the reaction mechanism was systematically investigated by control reactions and side product characterizations, leading to the determination of a TfOH-catalyzed ynamide functionality-initiated process, which was further corroborated by identifications of the departure form of leaving group and the real catalyst.

Concise Synthesis of 1-Thioalkyl Glycoside Donors by Reaction of Per-O-acetylated Sugars with Sodium Alkanethiolates under Solvent-Free Conditions.

A relatively green method for synthesizing 1-thioalkyl glycosides has been developed, where sodium alkanethiolates were used to react with per-O-acetylated sugars instead of odorous alkyl mercaptans in the presence of BF3·Et2O without the use of solvents under mild conditions. Furthermore, we found that 1,2-trans-β-thioglycosides can be converted into corresponding 1,2-cis-α-thioglycosides in the presence of trifluoromethanesulfonic acid in nonpolar solvents under mild conditions. This provides a simple and efficient new approach for synthesizing challenging 1,2-cis-α-thioglycosides.

General Method for the Synthesis of α- or β-Deoxyaminoglycosides Bearing Basic Nitrogen.

The introduction of glycosides bearing basic nitrogen is challenging using conventional Lewis acid-promoted pathways owing to competitive coordination of the amine to the Lewis acid promoter. Additionally, because many aminoglycosides lack a C2 substituent, diastereomeric mixtures of O-glycosides are often produced. Herein, we present a method for the synthesis of α- or β- 2,3,6-trideoxy-3-amino- and 2,4,6-trideoxy-4-amino O-glycosides from a common precursor. Our strategy proceeds by the reductive lithiation of thiophenyl glycoside donors and trapping of the resulting anomeric anions with 2-methyltetrahydropyranyl peroxides. We apply this strategy to the synthesis of α- and β-forosamine, pyrrolosamine, acosamine, and ristosamine derivatives using primary and secondary peroxides as electrophiles. α-Linked products are obtained in 60-96% yield and with >50:1 selectivity. β-Linked products are obtained in 45-94% yield and with 1.7->50:1 stereoselectivity. Contrary to donors bearing an equatorial amine substituent, donors bearing an axial amine substituent favored β-products at low temperatures. This work establishes a general strategy to synthesize O-glycosides bearing a basic nitrogen.

Total Synthesis of Natural Disaccharide Sambubiose.

A practical and robust synthetic method to obtain the natural disaccharide sambubiose (2-O-β-D-xylopyranosyl-D-glucopyranose) is reported, exploring the key step in the synthesis, i.e., stereoselective O-glycosylation. Specifically, the best combinations of glycoside donors and acceptors were identified, stereospecific control of the reaction was achieved by screening several catalysts and protection/deprotection steps were evaluated and improved. The best result was obtained by coupling allyl 3,5,6-tri-O-benzyl-β-D-glucofuranoside with 2,3,4-tri-O-acetyl-D-xylopiranosyl-α-trichloro acetimidate in the presence of trimethylsilyl triflate as a catalyst giving the corresponding protected target compound as a correct single isomer. The latter was transformed accordingly into the desired final product by deprotection steps (deallylation, deacetylation, and debenzylation). Sambubiose was synthesized into a satisfactory and higher overall yield than previously reported and was also characterized.

Batch-feeding whole-cell catalytic synthesis of α-arbutin by amylosucrase from Xanthomonas campestris.

α-Arbutin is an effective skin-whitening cosmetic ingredient and can be synthesized through hydroquinone glycosylation. In this study, amylosucrase (Amy-1) from Xanthomonas campestris pv. campestris 8004 was newly identified as a sucrose-utilizing glycosylating hydroquinone enzyme. Its kinetic parameters showed a seven-time higher affinity to hydroquinone than maltose-utilizing α-glycosidase. The glycosylation of HQ can be quickly achieved with over 99% conversion when a high molar ratio of glycoside donor to acceptor (80:1) was used. A batch-feeding catalysis method was designed to eliminate HQ inhibition with high productivity (> 36.4mM h-1). Besides, to eliminate the serious inhibition caused by the accumulated hydroquinone oxidation products, the whole-cell catalysis was further proposed. 306mM of α-arbutin was finally achieved with 95% molar conversion rate within 15h. Hence, the batch-feeding whole-cell biocatalysis by Amy-1 is a promising technology for α-arbutin production with enhanced yield and molar conversion rate.

Regio/Stereoselective Glycosylation of Diol and Polyol Acceptors in Efficient Synthesis of Neu5Ac-α-2,3-LacNPhth Trisaccharide.

A concise approach to a Neu5Ac-α-2,3-LacNPhth trisaccharide derivative was developed. First, the regio/stereoselective glycosylation between glycoside donors and glucoNPhth diol acceptors was investigated. It was found that the regioselectivity depends not only on the steric hindrance of the C2-NPhth group and the C6-OH protecting group of the glucosamine acceptors, but also on the leaving group and protecting group of the glycoside donors. Under optimized conditions, LacNPhth derivatives were synthesized in up to 92 % yield through a regio/stereoselective glycosylation between peracetylated-α-galactopyranosyl trichloroacetimidate and p-methoxyphenyl 6-O-tert-butyldiphenylsilyl-2-deoxy-2-phthalimido-β-d-glucopyranoside, avoiding the formation of glycosylated orthoesters and anomeric aglycon transfer. Then, the LacNPhth derivative was deacylated and then protected on the primary position by TBDPS to form a LacNPhth polyol acceptor. Finally, the Neu5Ac-α-2,3-LacNPhth derivative was synthesized in 48 % yield through the regio/stereoselective glycosylation between the LacNPhth polyol acceptor and a sialyl phosphite donor. Starting from d-glucosamine hydrochloride, the target Neu5Ac-α-2,3-LacNPhth derivative was synthesized in a total yield of 18.5 % over only 10 steps.

Cooperative Brønsted Acid-Type Organocatalysis for the Stereoselective Synthesis of Deoxyglycosides.

A practical approach for the α-stereoselective synthesis of deoxyglycosides using cooperative Brønsted acid-type organocatalysis has been developed. The method is tolerant of a wide range of glycoside donors and acceptors, and its versatility is exemplified in the one-pot synthesis of a trisaccharide. Mechanistic studies suggest that thiourea-induced acid amplification of the chiral acid via H-bonding is key for the enhancement in reaction rate and yield, while stereocontrol is dependent on the chirality of the acid.

Correction: DNA-catalyzed glycosylation using aryl glycoside donors.

Correction for 'DNA-catalyzed glycosylation using aryl glycoside donors' by Anthony R. Hesser et al., Chem. Commun., 2016, 52, 9259-9262.

Broadening the scope of glycosyltransferase-catalyzed sugar nucleotide synthesis

We described the integration of the general reversibility of glycosyltransferase-catalyzed reactions, artificial glycosyl donors, and a high throughput colorimetric screen to enable the engineering of glycosyltransferases for combinatorial sugar nucleotide synthesis. The best engineered catalyst from this study, the OleD Loki variant, contained the mutations P67T/I112P/T113M/S132F/A242I compared with the OleD wild-type sequence. Evaluated against the parental sequence OleD TDP16 variant used for screening, the OleD Loki variant displayed maximum improvements in k(cat)/K(m) of >400-fold and >15-fold for formation of NDP-glucoses and UDP-sugars, respectively. This OleD Loki variant also demonstrated efficient turnover with five variant NDP acceptors and six variant 2-chloro-4-nitrophenyl glycoside donors to produce 30 distinct NDP-sugars. This study highlights a convenient strategy to rapidly optimize glycosyltransferase catalysts for the synthesis of complex sugar nucleotides and the practical synthesis of a unique set of sugar nucleotides.

Crystal Polymorphism of Methyl 2,3,4-tri-O-acetyl-1-O-(trichloroacetimidoyl)-α-d-glucopyranouronate

The polymorphism of the glycoside donor methyl 2,3,4-tri-O-acetyl-1-O-(trichloroacetimidoyl)-α-d-glucopyranouronate (1) has been investigated. Two polymorphic forms (labelled Forms I and II) have been elucidated and fully characterised by DSC, PXRD and single crystal analysis, both crystallizing in the space group P21. Form I was obtained by crystallization from a wide range of solvents, while Form II was obtained only from ethyl acetate or isopropanol on certain occasions. Unit cell dimensions for Form I are a 14.0292(12), b 8.9641(8), c 16.8580(14) A, β 94.285(2)°, and for Form II a 11.266(3), b 6.8889(17), c 13.921(4) A, β 101.161(6)°. Z’ is 2 for Form I and 1 for Form II. Form I displays two moderate intermolecular hydrogen bonds in the unit cell whereas Form II shows no moderate hydrogen-bonding motifs. All three molecules in the two polymorphs differ significantly in their conformations, especially with respect to the methyl carboxylate and trichloroacetimidoyl groups. Structures of two polymorphs of methyl 2,3,4-tri-O-acetyl-1-O-(trichloroacetimidoyl)-α-d-glucopyranouronate were determined.