Stereoselective Glycosylation Research Articles

Boron-Catalyzed Regio- and Stereoselective Glycosylation via Fluoride Migration

Boron-Catalyzed Regio- and Stereoselective Glycosylation via Fluoride Migration

Selective Reduction Leading to 3,5-cis-3-Aminosugars: Synthesis and Stereoselective Glycosylation.

Synthesis of 3,5-cis-3-amino glycals with a cis-fused cyclic sulfamidate group has been achieved by selective reduction of sulfamidate ketimine groups. The efficient access to the structurally unique glycals allowed the subsequent divergent synthesis of various naturally occurring 3-amino-2,3,6-trideoxysugars. In addition, Lewis acid-promoted glycosylation of the glycals provided a simple solution for the stereoselective installation of O- and C-linked aglycons on the amino sugar scaffolds.

Protein Engineering of Glucosylglycerol Phosphorylase Facilitating Efficient and Highly Regio- and Stereoselective Glycosylation of Polyols in a Synthetic System

Protein Engineering of Glucosylglycerol Phosphorylase Facilitating Efficient and Highly Regio- and Stereoselective Glycosylation of Polyols in a Synthetic System

Convergent synthesis of the hexasaccharide repeating unit of the capsular polysaccharide of Klebsiella serotype K-34

A convergent [4 + 2] block synthetic strategy has been developed for the synthesis of the hexasaccharide repeating unit of the capsular polysaccharide of Klebsiella serotype K-34 in very good yield. A tetrasaccharide acceptor was synthesized by sequential stereoselective glycosylations of four suitably protected d-glucose and l-rhamnose units and a disaccharide thioglycoside was prepared by orthogonal glycosylation of d-galactosyl thioglycoside acceptor and l-rhamnosyl trichloroacetimidate donor. A combination of N-iodosuccinimide (NIS) and perchloric acid supported over silica (HClO4–SiO2) has been used as thiophilic glycosylation activator in the stereoselective glycosylations. A late stage oxidation of primary hydroxyl group to a carboxylic group in the hexasaccharide was achieved in excellent yield using a combination of TEMPO and bis(acetoxy)iodobenzene (BAIB). All glycosylation reactions were highly stereoselective.

Total Synthesis of Cordyceps militaris Glycans via Stereoselective Orthogonal One-Pot Glycosylation and α-Glycosylation Strategies.

Cordyceps militaris glycans with potent antioxidant activities have been efficiently and collectively synthesized. The synthetic approach features the following: (1) a stereoselective orthogonal one-pot glycosylation strategy for both 1,2-trans- and 1,2-cis-glycosidic linkages; (2) a merged reagent modulation and remote anchimeric assistance (RMRAA) α-glucosylation strategy; (3) DMF-modulated α-glucosylation; (4) convergent [3 + 4 + 6] orthogonal one-pot assembly of the target tridecasaccharide.

Chemical Synthesis of a Colanic Acid Hexasaccharide.

Chemical synthesis of a hexasaccharide relevant to colanic acid, which bears a high prevalence of polyanionic and acetyl decoration, has been achieved, highlighting stereoselective glycosylation and effective installation of O-acetyl and pyruvate residues.

Competing C-4 and C-5-Acyl Stabilization of Uronic Acid Glycosyl Cations.

Uronic acids are carbohydrates carrying a terminal carboxylic acid and have a unique reactivity in stereoselective glycosylation reactions. Herein, the competing intramolecular stabilization of uronic acid cations by the C-5 carboxylic acid or the C-4 acetyl group was studied with infrared ion spectroscopy (IRIS). IRIS reveals that a mixture of bridged ions is formed, in which the mixture is driven towards the C-1,C-5 dioxolanium ion when the C-5,C-2-relationship is cis, and towards the formation of the C-1,C-4 dioxepanium ion when this relation is trans. Isomer-population analysis and interconversion barrier computations show that the two bridged structures are not in dynamic equilibrium and that their ratio parallels the density functional theory computed stability of the structures. These studies reveal how the intrinsic interplay of the different functional groups influences the formation of the different regioisomeric products.

Syntheses of Clofarabine and Related C2′-β-fluorinated Nucleoside Analogues

A multistep synthesis of 2-chloro-9-(2′-deoxy-2′-fluoro-β-D-arabinofuranosyl) adenine (clofarabine) is described from methyl β-D-ribofuranoside. A new improved method for preparation of 1,2-diacetyl D-ribofuranose derivative was developed via acetolysis of tri-O-pivaloylated D-ribofuranoside and plausible mechanism of the reaction was proposed. Synthesis of 3′,5′-di-O-pivaloyl-2,6-dichloropurine β-D-riboside along with isomeric 2′,5′-di-O-pivaloyl nucleoside was carried out by stereoselective glycosylation reaction of 2,6-dichloropurine with peracylated D-ribofuranose followed by regioselective 2´-O-deacetylation of protected β-ribonucleoside with different bases. Mild C2′-β-fluorination of the purine 3′,5′-di-O-pivaloyl ribonucleoside with an excess of diethylaminosulfur trifluoride afforded protected 2,6-dichloropurine 2′-fluoro β-D-arabinoside as the key intermediate. Efficient route to clofarabine was also investigated using anion glycosylation of 2-chloroadenine potassium salt with the 1-α-bromide and potassium tert-butoxide in binary solvent mixture, chromatography for separation of a mixture of anomeric nucleosides (a β/α ratio of 3.0:1) and deacylation of benzoylated 2′-fluoro β-nucleoside. Novel N6-isopentyl clofarabine analogue was synthesized by a direct alkylation of the parent nucleoside

Recent advances in stereoselective 1,2-cis-O-glycosylations.

For the stereoselective assembly of bioactive glycans with various functions, 1,2-cis-O-glycosylation is one of the most essential issues in synthetic carbohydrate chemistry. The cis-configured O-glycosidic linkages to the substituents at two positions of the non-reducing side residue of the glycosides such as α-glucopyranoside, α-galactopyranoside, β-mannopyranoside, β-arabinofuranoside, and other rather rare glycosides are found in natural glycans, including glycoconjugate (glycoproteins, glycolipids, proteoglycans, and microbial polysaccharides) and glycoside natural products. The way to 1,2-trans isomers is well sophisticated by using the effect of neighboring group participation from the most effective and kinetically favored C-2 substituent such as an acyl group, although high stereoselective synthesis of 1,2-cis glycosides without formation of 1,2-trans isomers is far less straightforward. Although the key factors that control the stereoselectivity of glycosylation are largely understood since chemical glycosylation was considered to be one of the useful methods to obtain glycosidic linkages as the alternative way of isolation from natural sources, strictly controlled formation of these 1,2-cis glycosides is generally difficult. This minireview introduces some of the recent advances in the development of 1,2-cis selective glycosylations, including the quite recent developments in glycosyl donor modification, reaction conditions, and methods for activation of intermolecular glycosylation, including the bimodal glycosylation strategy for 1,2-cis and 1,2-trans glycosides, as well as intramolecular glycosylations, including recent applications of NAP-ether-mediated intramolecular aglycon delivery.

Synthesis of the pentasaccharide repeating unit corresponding to the capsular polysaccharide of Escherichia coli O20:K83:H26

A concise synthetic strategy has been developed for the synthesis of the pentasaccharide repeating unit of the capsular polysaccharide of Escherichia coli (E. coli) strain applying block [2 + 3] stereoselective glycosylation. A functionalized disaccharide thioglycoside donor prepared using “armed-disarmed” glycosylation concept, was coupled with suitably protected trisaccharide glycosyl acceptor in the presence of a combination of N-iodosuccinimide (NIS) and perchloric acid supported over silica (HClO4–SiO2) to give the target pentasaccharide derivative in 65% yield. A late-stage regioselective oxidation of the primary hydroxyl group using a combination of TEMPO and Bis(acetoxy)iodobenzene (BAIB) furnished d-galacturonic acid moiety in the functionalized pentasaccharide derivative.

Recent Chemical and Chemoenzymatic Strategies to Complex-Type N-Glycans.

Glycosylation is one of the major forms of protein post-translational modification. N-glycans attached to proteins by covalent bonds play an indispensable role in intercellular interaction and immune function. In human bodies, most of the cell surface glycoproteins and secreted glycopeptides are modified with complex-type N-glycans. Thus, for analytical or medicinal purposes, efficient and universal methods to provide homogeneous complex-type N-glycans have been an urgent need. Despite the extremely complicated structures, tremendous progress in the synthesis of N-glycans has been achieved. On one hand, chemical strategies are shown to be effective to prepare core oligosaccharides of N-glycans by focusing on stereoselective glycosylations such as β-mannosylation and α-sialylation, as well as the methodology of the N-glycan assembly. On the other hand, chemoenzymatic strategies have also become increasingly powerful in recent years. This review attempts to highlight the very recent advancements in chemical and chemoenzymatic strategies for eukaryotic complex-type N-glycans.

Chemical synthesis of the pentasaccharide related to the anti-inflammatory oleanane type saponins isolated from medicinal plant Aster tataricus L. f.

Total synthesis of the pentasaccharide related to the saponin isolated from the medicinal plant Aster tataricus L. f. is reported in the form of its allyl glycoside. The synthesis is accomplished by following a [3 + 2] block synthesis strategy where the trisaccharide acceptor and the disaccharide donor are rationally designed and obtained from semi-protected monosaccharides through stereoselective glycosylations either by activation of the thioglycoside or glycosyl trichloroacetimidate. The apiose moiety has been synthesized by literature procedure with little modification. The target pentasaccharide in the form of its allyl glycoside will be beneficial for further biological evaluation of the said structure and possible glycoconjugate formation as per need.

Halogen-bond-assisted radical activation of glycosyl donors enables mild and stereoconvergent 1,2-cis-glycosylation.

The chemistry of carbohydrates has a history of over 100 years, but simple, stereoselective and efficient glycosylation methods remain highly needed to facilitate the studies of sugars in various disciplines. Here we report a strategy for 1,2-cis-glycosylation without using metals, strong (Lewis) acids, elaborate catalysts or labile substrates. Our method operates by a unique mechanism: it activates glycosyl donors through a radical cascade rather than the conventional acid-promoted, ionic process. As elucidated by computational and experimental studies, the allyl glycosyl sulfones (as donors) form halogen bond complexes with perfluoroalkyl iodides, which-merely by visible light irradiation-fragment via radical intermediates to give the electrophilic glycosyl iodides. In situ trapping by various nucleophiles affords, in a stereoconvergent manner, the challenging 1,2-cis-glycosides. This metal- and acid-free reaction shows remarkable tolerance to functional groups. The high stereoselectivity holds for a broad array of donors. This study suggests that the simple C2-alkoxy group can serve as an effective directing group for building 1,2-cis-glycosidic bonds.

Bridge between Total Synthesis of Bioactive Natural Products and Development of Drug Leads

Although natural products are rich sources for drug discovery, only a small percentage of natural products themselves have been approved for clinical use, thus it is necessary to modulate various properties, such as efficacy, toxicity, and metabolic stability. A question in natural product drug discovery is how to logically design natural product derivatives with desired biological properties. This review describes our recent studies regarding the medicinal chemistry of tunicamycin. Tunicamycin inhibits bacterial phospho-N-acetylmuramic acid (MurNAc)-pentapeptide translocase (MraY), which is an essential enzyme in bacteria and a good target for antibacterial drug discovery. The usefulness of tunicamycin as antibacterial agents is limited by off-target inhibition of human UDP-N-acetylglucosamine (GlcNAc): polyprenol phosphate translocase (GPT). We positioned the total synthesis of tunicamycin as a starting point for the research and have accomplished the synthesis of tunicamycin V by using the Achmatowicz reaction, [3,3] sigmatropic rearrangement of allyl cyanate, and stereoselective glycosylation as key reactions. Next, the minimum structural requirements for tunicamycin V for MraY inhibition were established by systematic structure-activity relationship studies with truncated analogs of tunicamycin V. Our collaborative study elucidated a crystal structure of human GPT in complex with tunicamycin. This structural information was then exploited to rationally design an MraY-specific inhibitor of tunicamycin V in which the GlcNAc moiety was modified to a MurNAc amide. The analog was identified as a highly selective MraYAA inhibitor.

Cover Feature: Black Swan in Phase Transfer Catalysis: Influence of Mixing Mode on the Stereoselectivity of Glycosylation (Eur. J. Org. Chem. 14/2022)

The Cover Feature allegorizes differently arranged long-lived supramolecular structures (supramers), formed from reagent molecules in the reaction solution, as different groups of dancers holding hands. The supramer hypothesis has been invoked to rationally discuss the experimentally observed change in reaction mechanism induced by the change in mixing mode. Cover design by Maria Panova. More information can be found in the Research Article by L. O. Kononov et al.

Black Swan in Phase Transfer Catalysis: Influence of Mixing Mode on the Stereoselectivity of Glycosylation

AbstractPhase transfer catalysis (PTC) is widely used for the preparation of anomerically pure aryl glycosides by glycosylation of phenols with glycosyl halides. Here, we attempted to synthesize 4‐(3‐chloropropoxy)phenyl (CPP) sialoside – a useful Janus glycoside for neoglycoconjugate synthesis. A batch glycosylation reaction under typical PTC conditions (N‐acetylsialyl chloride, 4‐(3‐chloropropoxy)phenol, Bu4NHSO4, AcOEt, 10 % aq Na2CO3) resulted in formation of CPP sialoside as a single anomer, albeit in low yield. Conversely, the PTC glycosylation in flow using Comet X‐01 micromixer doubled the yield of CPP sialoside, which was obtained as a mixture of anomers suggesting the change in the reaction mechanism apparently induced by the change in mixing mode. An increase in flow rate led to further loss of stereoselectivity while the use of T‐shaped mixer increased stereoselectivity. Similar results were obtained for N,N‐diacetylsialyl chloride and two other para‐alkoxy‐substituted phenols suggesting generality of the discovered phenomenon.

Photolabile 2-(2-Nitrophenyl)-propyloxycarbonyl (NPPOC) for Stereoselective Glycosylation and Its Application in Consecutive Assembly of Oligosaccharides.

A photolabile protecting group (PPG) 2-(2-nitrophenyl)-propyloxycarbonyl (NPPOC) was explored in glycosylation and applied in the consecutive synthesis of oligosaccharides. NPPOC displays a strong neighboring group participation (NGP) effect to facilitate the construction of 1,2-trans glycosides in excellent yield. Notably, NPPOC could be efficiently removed by photolysis, and the deprotection conditions are friendly to typical protecting groups. A branched and asymmetric oligomannose Man6 was rapidly prepared, and the consecutive assembly of oligosaccharides without intermediate purification was further investigated owing to the compatibility conditions between NPPPOC's photolysis and glycosylation.

4,5-Dioxo-imidazolinium Cation-Promoted α-Selective Dehydrative Glycosylation of 2-Deoxy- and 2,6-Dideoxy Sugars

Herein, we report the great potential of the 4,5-dioxo-imidazolinium cation activation strategy for dehydrative glycosylation reactions employing the readily available and economical geminal dichloroimidazolidinediones (DCIDs) that promotes the glycosylation between 2-deoxy- and 2,6-dideoxy-sugar hemiacetals with various acceptors in good yields and high α-selectivity. This research not only provides a mild and efficient alternative approach for stereoselective dehydrative glycosylation but also extends the dichloroimidazolidinedione as a novel promoter in the field of glycoscience.

Synthetic Modifications of the Linkage Region of Proteoglycans and Impact on CSGalNAcT‐1

AbstractWe herein report for the first time a multi‐step synthesis of a library of disaccharides and trisaccharides of the linkage region of proteoglycans (PGs) having a d‐galactose modified at position C‐2, C‐4 or C‐6 by replacing the hydroxyl group by methoxy or deoxy moieties. Efficient radical dehalogenation, Barton‐McCombie reaction, O‐methylation and stereoselective glycosylation are described. We also report preliminary results of the effect of these modifications upon the activity of recombinant chondroitin sulfate N‐acetylgalactosaminyltransferase‐1 (CSGalNAcT‐1), an enzyme involved at the bifurcation step of the biosynthesis of PGs.

Bismuth-Catalyzed Stereoselective 2-Deoxyglycosylation of Disarmed/Armed Glycal Donors

Bi(OTf)3 promoted direct and highly stereoselective glycosylation of "disarmed" and "armed" glycals to synthesize 2-deoxyglycosides has been reported. The tunable and solvent-controlled chemoselective activation of deactivated glycal donors distinguishing the competitive Ferrier and 1,2-addition pathways was discovered to improve substrate scope. The practical versatility of the method has been amply demonstrated with the oligosaccharide syntheses and 2-deoxyglycosylation of high-value natural products and drugs.