Glycosylation Strategy Research Articles

Highly Reactive Glycosylation with 1-O-(Methylthio)thiocarbonyl Glycosyl Donors under Acidic to Neutral Reaction Conditions.

In this study, we have successfully developed a glycosylation method using 1-O-(methylthio)thiocarbonyl-glycoses as donors. Such xanthate donors are easily accessible and shelf-stable. The glycosylation reaction could be promoted by cations (acidic to neutral conditions) under mild conditions, exhibiting a reactivity intermediate between that with glycosyl trichloroacetimidate as the donor and that with thioglycoside as the donor. This methodology tolerates both "armed" and "disarmed" glycosyl donors, as well as various sugar acceptors, and affords the corresponding glycosides in good to excellent yields. Based on the relative higher reactivity of such xanthate donors than thioglycoside donors under the same glycosylation conditions, a trisaccharide was further synthesized in a one-pot glycosylation strategy.

Synthesis of the pentasaccharide repeating unit with a conjugation-ready linker corresponding to the O-antigenic polysaccharide of Acinetobacter junii strain 65

A straightforward synthesis of the pentasaccharide with a readily available linker arm corresponding to the O-antigenic polysaccharide of Acinetobacter junii strain 65 has been achieved in good yield. The synthesis has been carried out using thioglycosides as glycosyl donor in the presence of a combination of N-iodosuccinimide (NIS) and trifluoromethanesulfonic acid (TfOH) as thiophilic activator. The yields of the glycosylation steps were very good with satisfactory stereochemistry at the glycosidic linkages. The pentasaccharide derivative has also been obtained using a one-pot iterative glycosylation strategy.

Total synthesis of a structurally complex zwitterionic hexasaccharide repeating unit of polysaccharide B from Bacteroides fragilis via one-pot glycosylation

Zwitterionic polysaccharides (ZPSs) present on the surface of a common gut commensal Bacteroides fragilis are endowed with unique immunological properties as they can directly bind to T-cells in the absence of protein conjugation. ZPSs are therefore considered to be potential antigens for the development of totally carbohydrate-based vaccines. Herein, we disclose the first total synthesis of a highly branched phosphorylated zwitterionic capsular polysaccharide repeating unit of Bacteroides fragilis. The hexasaccharide repeating unit bearing six different monosaccharides comprises three 1,2-cis-glycosidic linkages, a challenging 1,2-trans linkage in D-QuipNAc-β-(1→4)-D-Gal motif, and a 2-aminoethyl phosphonate appendage. The synthesis of target ZPS was accomplished utilizing an expeditious, highly stereoselective and convergent (1 + 2 + 2 + 1) one-pot glycosylation strategy. The striking features include efficient synthesis of rare deoxy amino sugars D- and L-quinovosamine, stereoselective installation of three 1,2-cis glycosidic linkages, glycosylation of D-quinovosamine donor with a sterically crowded, poorly reactive 4-OH galactose moiety, as well as late stage phosphorylation.

Total Synthesis of Nona-decasaccharide Motif from Ganoderma sinense Polysaccharide Enabled by Modular and One-Pot Stereoselective Glycosylation Strategy.

Polysaccharides from a medicinal fungus Ganoderma sinense represent important and adjunctive therapeutic agents for treating various diseases, including leucopenia and hematopoietic injury. However, the synthetic accessibility to long, branched, and complicated carbohydrates chains from Ganoderma sinense polysaccharides remains a challenging task in chemical synthesis. Here, we report the modular chemical synthesis of nona-decasaccharide motif from Ganoderma sinense polysaccharide GSPB70-S with diverse biological activities for the first time through one-pot stereoselective glycosylation strategy on the basis of glycosyl ortho-(1-phenyvinyl)benzoates, which not only sped up carbohydrates synthesis but also reduced chemical waste and avoided aglycones transfer issues inherent to one-pot glycosylation on the basis of thioglycosides. The synthetic route also highlights the following key steps: (1) preactivation-based one-pot glycosylation for highly stereoselective constructions of several 1,2-cis-glycosidic linkages, including three α-d-GlcN-(1 → 4) linkages and one α-d-Gal-(1 → 4) bond via the reagent N-methyl-N-phenylformamide modulation; (2) orthogonal one-pot assembly of 1,2-trans-glycosidic linkages in various linear and branched glycans fragments by strategic combinations of glycosyl N-phenyltrifluoroacetimidates, glycosyl ortho-alkynylbenzoates, and glycosyl ortho-(1-phenyvinyl)benzoates; and (3) the final [1 × 4 + 15] Yu glycosylation for efficient assembly of nona-decasaccharide target. Additionally, shorter sequences of 4-mer, 5-mer, and 6-mer are also prepared for structure-activity relationship biological studies. The present work shows that this one-pot stereoselective glycosylation strategy can offer a reliable and effective means to streamline chemical synthesis of long, branched, and complex carbohydrates with many 1,2-cis-glycosidic bonds.

Convergent synthesis of the pentasaccharide repeating unit corresponding to the cell wall O-polysaccharide of Salmonella milwaukee (group U) O:43 strain

Synthesis of the pentasaccharide repeating unit of the cell O-polysaccharide produced by Salmonella milwaukee O:43 strain (group U) has been achieved in very good yield adopting a convergent stereoselective [3 + 2] block glycosylation strategy. Thioglycosides and glycosyl trichloroacetimidate derivative were used as glycosyl donors in the presence of a combination of N-iodosuccinimide (NIS) and trimethylsilyl trifluoromethanesulfonate (TMSOTf) as thiophilic activator and TMSOTf as trichloroacetimidate activator respectively. The stereochemical outcome of all glycosylation reactions was excellent.

Efficient glycosylation and in vitro neuroprotective evaluation of abundant prenylflavonoid in hops

Structurally unique isopentenyl flavonoids are extremely low in natural plant species, and their cumbersome chemical pathways, scarcity of donors, and poor stability have severely hindered their application scenarios. Xanthohumol (XN), an isopentenyl chalcone, is a unique and abundant herbal efficacy substance in hops (Humulus upulus L.), which is an herb with a variety of clinical benefits. However, 90% of XN is left behind in the production of hop residue, resulting in a waste of natural resources. The poor water solubility and low bioavailability of XN also limit its wide application. Combined biotransformation strategies can greatly enhance the added value of resource materials and become one of the important means for the sustainable utilization of traditional Chinese medicine resources. In this study, an extreme microbial screening strategy for efficient bioconversion of phrenylflavonids is established to achieve Bacillus zanthoxyli GQ8 obtained from brewer's dregs and a non-aqueous biosynthesis system of xanthohumol glycosides (XNG, substrate concentration of 0.5 g/L, conversion rate of 73.2%) is constructed by GQ8. With HT22 cell model of H2O2-induced oxidative damage and BV2 cell model of LPS-induced neuroinflammation, XNG showed little cytotoxicity and exhibited better anti-inflammatory and antioxidant effects compared with the original compound. Our experimental results firstly provide a high-efficiency biotransformation strategy for glycosylation of XN with bacteria and provide a support that prenylflavonoid glycoside derivatives have remarkable biological activity and show better development potential.

Antigenicity evaluation of synthetic α-(1,3)-linked D, D-heptoglycan of Helicobacter pylori serotype O6 lipopolysaccharide

Helicobacter pylori, a gram‐negative bacterium, is known to be associated with various gastric pathologies including chronic gastritis, peptic ulcers and gastric carcinoma. Despite the emerging issue of bacterial resistance to antibiotic-based combination therapy, there is currently no vaccine available for H. pylori infection in the market. Here, we report the synthesis of α-(1,3)-D,D-heptoglycan with different chain length from the lipopolysaccharide of H. pylori serogroup O6. The [n + 1] iterative glycosylation strategy was used for heptoglycan chain elongation. The trifluoroacetimidate heptoside donor exhibited much higher efficiency than thioglycoside donor during glycosylation. An antigenicity evaluation using glycan microarrays indicated that α-(1,3)-D,D-hepto-disaccharide, pentasaccharide and hexasaccharide showed stronger binding affinity to IgG antibodies of H. pylori O6 LPS immunized rabbit serum or serum of H. pylori infected patients. These findings provide significant structure − activity relationship information for developing carbohydrate-based vaccines against H. pylori containing α-(1,3)-D,D-heptoglycan.

Bimodal Glycosyl Donors as an Emerging Approach Towards a General Glycosylation Strategy.

Organic synthesis provides an accessible route to preparative scale biological glycans, although schemes to access these complex structures are often complicated by preparation of multiple monosaccharide building blocks. Bimodal glycosyl donors capable of forming both α- and β-anomers selectively, are an emerging tactic to reduce the required number of individual synthetic components in glycan construction. This review discusses examples of bimodal donors in the literature, and how they achieve their stereocontrol for both anomers. Notable examples include a bespoke O-2 benzyl protecting group, a strained glycal for reaction using organometallic catalysis, and a simple perbenzylated donor optimised for stereoselective glycosylation through extensive reaction tuning.

Donor Preactivation-Based Glycan Assembly: from Manual to Automated Synthesis.

Carbohydrates are called the third chain of life. Carbohydrates participate in many important biochemical functions in living species, and the biological information carried by them is several orders of magnitude larger than that of nucleic acids and proteins. However, due to the intrinsic complexity and heterogeneity of carbohydrate structures, furnishing pure and structurally well-defined glycans for functional studies is a formidable task, especially for homogeneous large-size glycans. To address this issue, we have developed a donor preactivation-based one-pot glycosylation strategy enabling multiple sequential glycosylations in a single reaction vessel.The donor preactivation-based one-pot glycosylation refers to the strategy in which the glycosyl donor is activated in the absence of a glycosyl acceptor to generate a reactive intermediate. Subsequently, the glycosyl acceptor with the same anomeric leaving group is added, leading to a glycosyl coupling reaction, which is then iterated to rapidly achieve the desired glycan in the same reactor. The advantages of this strategy include the following: (1) unique chemoselectivity is obtained after preactivation; (2) it is independent of the reactivity of glycosyl donors; (3) multiple-step glycosylations are enabled without the need for intermediate purification; (4) only stoichiometric building blocks are required without complex protecting group manipulations. Using this protocol, a range of glycans including tumor-associated carbohydrate antigens, various glycosaminoglycans, complex N-glycans, and diverse bacterial glycans have been synthesized manually. Gratifyingly, the synthesis of mycobacterial arabinogalactan containing 92 monosaccharide units has been achieved, which created a precedent in the field of polysaccharide synthesis. Recently, the synthesis of a highly branched arabinogalactan from traditional Chinese medicine featuring 140 monosaccharide units has been also accomplished to evaluate its anti-pancreatic-cancer activity. In the spirit of green and sustainable chemistry, this strategy can also be applied to light-driven glycosylation reactions, where either UV or visible light can be used for the activation of glycosyl donors.Automated synthesis is an advanced approach to the construction of complex glycans. Based on the two preactivation modes (general promoter activation mode and light-induced activation mode), a universal and highly efficient automated solution-phase synthesizer was further developed to drive glycan assembly from manual to automated synthesis. Using this synthesizer, a library of oligosaccharides covering various glycoforms and glycosidic linkages was assembled rapidly, either in a general promoter-activation mode or in a light-induced-activation mode. The automated synthesis of a fully protected fondaparinux pentasaccharide was realized on a gram scale. Furthermore, the automated synthesis of large-size polysaccharides was performed, allowing the assembly of arabinans up to an astonishing 1080-mer using the automated multiplicative synthesis strategy, taking glycan synthesis to a new height far beyond the synthesis of nucleic acids (up to 200-mer) and proteins (up to 472-mer).

Synthesis of the Pentasaccharide of the K14 Capsular Polysaccharide of Antibiotic Resistant ST25 Acinetobacter baumannii isolate, D46 and O‐Polysaccharide of Acinetobacter baumannii O11

AbstractSynthesis of the pentasaccharide corresponding to the K14 capsular polysaccharide from ST25 Acinetobacter baumannii isolate, D46 and O‐polysaccharide of Acinetobacter baumannii O11 has been achieved in very good yield using a convergent stereoselective [3+2] block glycosylation strategy. The synthetic scheme includes several regio‐ and stereose lective glycosylations as well as orthogonal glycosylation with high yield. Thioglycosides have been used as glycosyl donors using a combination of copper(II) bromide and tetra‐n‐butylammonium bromide (TBAB) as thiophilic activator for 1,2‐cis glycosylation and a combination of N‐iodosuccinimide (NIS) and trimethylsilyl trifluoromethanesulfonate (TMSOTf) as thiophilic activator for 1,2‐trans glycosylation. Glycosyl trichloroacetimidate derivative was used as glycosyl donor using perchloric acid supported on silica (HClO4–SiO2) as activator. A catalytic transfer hydrogenation reaction has been carried out for the removal of benzyloxy and benzylidene functional groups using triethylsilane as hydrogen source.

Chemical Synthesis of a Key Precursor Relevant to the Tetrasaccharide Repeating Unit from Treponema mediumATCC 700293

Comprehensive SummaryTreponema is a Gram‐negative anaerobic bacterium, among which the pathogenic Treponema can cause various diseases, such as venereal syphilis (Treponema pallidum), yaws (Treponema carateum), and oral diseases (Treponema denticola and Treponema medium). Although different from conventional lipopolysaccharides, the extracellular glycoconjugate of Treponema may still be a potential antigen and provide a candidate for vaccine development. Hence, we completed the first chemical synthesis of Treponema medium ATCC 700293 tetrasaccharide precursor containing L‐ornithine (L‐Orn) and D‐aspartic acid (D‐Asp) derivatives. The efficiency of non‐reducing end disaccharide formation has been improved by optimizing the assembly of the protecting groups in the donors and acceptors. Our [3+1] glycosylation strategy attempted to reduce the length of the acceptor to increase the nucleophilicity of the hydroxyl group, thereby improving the efficiency of synthesizing the target tetrasaccharide. The L‐Orn derivative was introduced at the final stage due to its influence on the glycosylation stereospecificity and efficiency. Therefore, the successful introduction of two amino acid derivatives and the synthesis of a tetrasaccharide precursor with complex functional‐group modifications have provided valuable insights for synthesizing other complex bacterial glycans.

Anti-Tumor Potential of Post-Translational Modifications of PD-1.

Programmed cell death protein-1 (PD-1) is a vital immune checkpoint molecule. The location, stability, and protein-protein interaction of PD-1 are significantly influenced by post-translational modification (PTM) of proteins. The biological information of PD-1, including its gene and protein structures and the PD-1/PD-L1 signaling pathway, was briefly reviewed in this review. Additionally, recent research on PD-1 post-translational modification, including the study of ubiquitination, glycosylation, phosphorylation, and palmitoylation, was summarized, and research strategies for PD-1 PTM drugs were concluded. At present, only a part of PD-1/PD-L1 treated patients (35-45%) are benefited from immunotherapies, and novel strategies targeting PTM of PD-1/PD-L1 may be important for anti-PD-1/PD-L1 non-responders (poor responders).

Convenient Synthesis of Hexa- and Pentasaccharide Repeating Units Corresponding to the O-Polysaccharides of Acinetobacter baumannii O7 and Acinetobacter baumannii O10 Strains

AbstractThe synthesis of the hexa- and pentasaccharide repeating units of the cell wall O-polysaccharides of Acinetobacter baumannii O7 and O10 strains, respectively, has been achieved in very good yield using [4+2] and [4+1] block glycosylation strategies. The p-methoxybenzyl (PMB) group was used as an in situ removable protecting group, which was removed after glycosylation in the same pot by tuning the reaction condition. A challenging β-l-rhamnopyranosyl linkage was constructed using ‘armed-disarmed glycosylation’ conditions by the influence of a remotely located, H-bond mediating, picolinoyl group in the glycosyl donor. A d-mannosamine intermediate was prepared from d-glucose using minimum reaction steps. The hexa- and pentasaccharide were achieved as their p-methoxyphenyl (PMP) glycosides.

Exploring the Dual Functions of Distal Acyl Group Direction in Various Nucleophilic Environments.

A universal glycosylation strategy could significantly simplify glycoside synthesis. One approach to achieving this goal is through acyl group direction for the corresponding 1,2-, 1,3-, 1,4-, or 1,6-trans glycosylation; however, this approach has been challenging for glycosidic bonds that require distal equatorial-acyl group direction. We developed an approach in weakly nucleophilic environments for selective 1,4-trans glycosylation directed by the equatorial-4-O-acyl group. Here, we explored this condition in other distal acyl groups and found that, besides 1,n-trans direction, acyl groups also mediated hydrogen bonding between acyl groups and alcohols. The latter showed a diverse effect and classified the acyl group direction into axial and equatorial categories. Corresponding glycosylation conditions were distinguished as guidance for acyl group direction from either category. Hence, acyl group direction may serve as a general glycosylation strategy.

One-Pot Assembly of Mannose-Capped Lipoarabinomannan Motifs up to 101-Mer from the Mycobacterium tuberculosis Cell Wall.

Lipoarabinomannan (LAM) from the Mycobacterium tuberculosis cell envelope represents important targets for the development of new therapeutic agents against tuberculosis, which is a deadly disease that has plagued mankind for a long time. However, the accessibility of long, branched, and complex lipoarabinomannan over 100-mer remains a long-standing challenge. Herein, we report the modular synthesis of mannose-capped lipoarabinomannan 101-mer from the M. tuberculosis cell wall using a one-pot assembly strategy on the basis of glycosyl ortho-(1-phenylvinyl)benzoates (PVB), which not only accelerates the modular synthesis but also precludes the potential problems associated with one-pot glycosylation with thioglycosides. Shorter sequences including 18-mer, 19-mer, and 27-mer are also synthesized for in-depth structure-activity relationship biological studies. Current synthetic routes also highlight the following features: (1) streamlined synthesis of various linear and branched glycans using one-pot orthogonal glycosylation on the combination of glycosyl N-phenyltrifluoroacetimidates, glycosyl ortho-alkynylbenzoates, and glycosyl PVB; (2) highly stereoselective construction of 10 1,2-cis-arabinofuranosyl linkages using 5-O-(2-quinolinecarbonyl)-directing 1,2-cis-arabinofuranosylation via a hydrogen-bond-mediated aglycone delivery strategy; and (3) convergent [(18 + 19) × 2 + 27] one-pot synthesis of the 101-mer LAM polysaccharide. The present work demonstrates that this orthogonal one-pot glycosylation strategy can highly streamline the chemical synthesis of long, branched, and complex polysaccharides.

Design, Synthesis, and Anti-Osteoporotic Characterization of Arginine N-Glycosylated Teriparatide Analogs via the Silver-catalyzed Solid-Phase Glycosylation Strategy.

In spite of effective antiosteoporosis potency, teriparatide, a bone-building agent approved by the FDA (Food and Drug Administration), was proven to exhibit various side effects. In our previous work, we developed a universal strategy for synthesizing arginine N-glycosylated peptides termed silver-promoted solid-phase glycosylation (SSG) strategy. However, it is unknown whether the SSG strategy can be applied in the peptide drug design. Herein, we first reported the optimization of teriparatide via SSG strategy. Using Arg20 and/or Arg25 as the modifying positions, three series of arginine N-glycosylated teriparatide analogs were successfully synthesized, of which the introduced sugar groups included glucose, galactose, mannose, rhamnose, ribose, 2-acetamino-2-deoxy-glucose, xylose, lactose, and maltose. Among the 27 arginine N-glycosylated derivatives, Arg20-xylose and Arg25-maltose teriparatide analogs, termed PTH-1g and PTH-2i, respectively, indicated enhanced serum stability and significantly improved antiosteoporotic activities in vitro and in vivo compared with the native counterpart. They may serve as effective therapeutic candidates for treating osteoporosis.

Zinc(ii)-mediated stereoselective construction of 1,2-cis 2-azido-2-deoxy glycosidic linkage: assembly of Acinetobacter baumannii K48 capsular pentasaccharide derivative.

The capsular polysaccharide (CPS) is a major virulence factor of the pathogenic Acinetobacter baumannii and a promising target for vaccine development. However, the synthesis of the 1,2-cis-2-amino-2-deoxyglycoside core of CPS remains challenging to date. Here we develop a highly α-selective ZnI2-mediated 1,2-cis 2-azido-2-deoxy chemical glycosylation strategy using 2-azido-2-deoxy glucosyl donors equipped with various 4,6-O-tethered groups. Among them the tetraisopropyldisiloxane (TIPDS)-protected 2-azido-2-deoxy-d-glucosyl donor afforded predominantly α-glycoside (α : β = >20 : 1) in maximum yield. This novel approach applies to a wide acceptor substrate scope, including various aliphatic alcohols, sugar alcohols, and natural products. We demonstrated the versatility and effectiveness of this strategy by the synthesis of A. baumannii K48 capsular pentasaccharide repeating fragments, employing the developed reaction as the key step for constructing the 1,2-cis 2-azido-2-deoxy glycosidic linkage. The reaction mechanism was explored with combined experimental variable-temperature NMR (VT-NMR) studies and mass spectroscopy (MS) analysis, and theoretical density functional theory calculations, which suggested the formation of covalent α-C1GlcN-iodide intermediate in equilibrium with separated oxocarbenium-counter ion pair, followed by an SN1-like α-nucleophilic attack most likely from separated ion pairs by the ZnI2-activated acceptor complex under the influence of the 2-azido gauche effect.

Chemical Synthesis of the Trisaccharide Repeating Unit of the O-Antigen of Fusobacterium nucleatum ATCC 51191.

Herein, the trisaccharide repeating unit of Fusobacterium nucleatum ssp. animalis ATCC 51191, which is used to develop oncomicrobial vaccines, was efficiently synthesized for the first time. The synthetic approach featured the following: (i) construction of the 1,2-cis-glycosidic linkage using the large steric hindrance of a phthalimide group at C4 of fucosamine; (ii) synthesis of the trisaccharide via a linear [2 + 1] glycosylation strategy; and (iii) installation of l-alanine using hexafluorophosphate azabenzotriazole tetramethyl uronium as a promoter.

Convenient Synthesis of the Branched Hexasaccharide Repeating Unit of the Cell Wall O-Antigen of Escherichia coli O80 Strain

AbstractChemical synthesis of a branched hexasaccharide corresponding to the cell wall O-antigen of Escherichia coli (E. coli) O80 strain has been achieved in very good yield with satisfactory stereochemical outcome around the glycosidic linkages by applying a regio- and stereoselective [4+2] block glycosylation strategy. The tetrasaccharide diol derivative was synthesized from suitably functionalized monosaccharide intermediates using sequential stereoselective glycosylations and the disaccharide thioglycoside donor was obtained applying a regio- and stereoselective orthogonal armed-disarmed glycosylation strategy using protected thioglycoside derivatives. A late-stage TEMPO and bis(acetoxy)iodobenzene (BAIB) mediated regioselective oxidation of the primary hydroxyl group of the protected hexasaccharide derivative to the carboxylic acid was achieved leaving secondary hydroxyl groups unaffected.

Synthesis of the Hexasaccharide Repeating Unit Corresponding to the Cell Wall O-Antigen of Providencia alcalifaciens O9:H8 Strain

AbstractA straightforward synthesis of the hexasaccharide repeating unit of the O-specific polysaccharide of Providencia alcalifaciens O9:H8 strain was achieved in very good yield by applying a stereo- and regioselective [4+2] block glycosylation strategy. The tetrasaccharide acceptor and disaccharide donor were synthesized by sequential stereo- and regioselective glycosylations. Thioglycoside and glycosyl trichloroacetimidate derivatives were used as glycosyl donors in the synthetic strategy. The glycosylation steps were high-yielding and gave satisfactory stereochemical outcomes.