Deoxy Sugars: Occurrence and Synthesis

Influence of C-4 Axial/Equatorial Configuration and Neighboring Group/Remote Group Participation (NGP/RGP) Driven Conformational Evidence in Chemoselective Activation of Glycals

We herein reveal the possibility of the C-4 neighboring group/remote group participation (NGP/RGP) facilitating the stabilization of the anomeric center via dioxolenium intermediates in the chemoselective activation of glycal donors. We further realized that the axial/equatorial configuration of the C-4 group in the galacto- and gluco-glycal series enables diverse pathways to give direct 1,2-addition or Ferrier rearrangement, respectively. A proof-of-principle for stereoselective glycosylation was amply illustrated by employing carbohydrates, amino acids, natural products, and bioactive molecules to develop 2-deoxy-glycan analogs.

Synthesis of Salmonella enteritidis Antigenic Tetrasaccharide Repeating Unit by Employing Cationic Gold(I)-Catalyzed Glycosylation Involving Glycosyl N-1,1-Dimethylpropargyl Carbamate Donors

Synthesis of an antigenic tetrasaccharide repeating unit of the O-polysaccharide of Salmonella enteritidis lipopolysaccharide has been accomplished. Those four monosaccharides were assembled stereoselectively by employing our recently developed cationic gold(I)-catalyzed glycosylation methodology involving various glycosyl N-1,1-dimethylpropargyl carbamate donors. The newly formed α-anomeric stereochemical configuration was controlled by the axial C2-OBz of the glycosyl donors via anchimeric assistance.

Total Synthesis of a Conjugation-Ready Tetrasaccharide Repeating Unit of Vibrio cholerae O:3 O-antigen Polysaccharide

Herein, we report the first total synthesis of the tetrasaccharide repeating unit of Vibrio cholerae O:3 O-antigen polysaccharide. The highly complex tetrasaccharide contains rare amino sugars such as d-bacillosamine and l-fucosamine, highly labile sugar ascarylose, and higher carbon sugar d-d-heptose. Stereoselective glycosylation of the notoriously reactive ascarylose with d-d-heptose, poor nucleophilicity of the axial C4-OH of l-fucosamine, and amide coupling are the key challenges encountered in the total synthesis, which was completed via a longest linear sequence of 23 steps in 4.2% overall yield.

Site-Selective C-O Bond Editing of Unprotected Saccharides

Glucose and its polyhydroxy saccharide analogs are complex molecules that serve as essential structural components in biomacromolecules, natural products, medicines, and agrochemicals. Within the expansive realm of saccharides, a significant area of research revolves around chemically transforming naturally abundant saccharide units to intricate or uncommon molecules such as oligosaccharides or rare sugars. However, partly due to the presence of multiple hydroxyl groups with similar reactivities and the structural complexities arising from stereochemistry, the transformation of unprotected sugars to the desired target molecules remains challenging. One such formidable challenge lies in the efficient and selective activation and modification of the C-O bonds in saccharides. In this study, we disclose a modular 2-fold "tagging-editing" strategy that allows for direct and selective editing of C-O bonds of saccharides, enabling rapid preparation of valuable molecules such as rare sugars and drug derivatives. The first step, referred to as "tagging", involves catalytic site-selective installation of a photoredox active carboxylic ester group to a specific hydroxyl unit of an unprotected sugar. The second step, namely, "editing", features a C-O bond cleavage to form a carbon radical intermediate that undergoes further transformations such as C-H and C-C bond formations. Our strategy constitutes the most effective and shortest route in direct transformation and modification of medicines and other molecules bearing unprotected sugars.

Reagent-controlled chemo/stereoselective glycosylation of ʟ-fucal to access rare deoxysugars

2,6-Dideoxy sugars constitute an important class of anticancer antibiotics natural products and serve as essential medicinal tools for carbohydrate-based drug discovery and vaccine development. In particular, 2-deoxy ʟ-fucose or ʟ-oliose is a rare sugar and vital structural motif of several potent antifungal and immunosuppressive bioactive molecules. Herein, we devised a reagent-controlled stereo and chemoselective activation of ʟ-fucal, enabling the distinctive glycosylation pathways to access the rare ʟ-oliose and 2,3-unsaturated ʟ-fucoside. The milder oxo-philic Bi(OTf)3 catalyst induced the direct 1,2-addition predominantly, whereas B(C6F5)3 promoted the allylic Ferrier-rearrangement of the enol-ether moiety in ʟ-fucal glycal donor, distinguishing the competitive mechanisms. The reagent-tunable modular approach is highly advantageous, employing greener catalysts and atom-economical transformations, expensive ligand/additive-free, and probed for a diverse range of substrates comprising monosaccharides, amino-acids, bioactive natural products, and drug scaffolds embedded with susceptible or labile functionalities.

Mild method for the synthesis of α-glycosyl chlorides: A convenient protocol for quick one-pot glycosylation

A simple and efficient protocol for the preparation of α-glycosyl chlorides within 15-30 min is described which employs a stable, cheap, and commercially available Trichloroisocyanuric acid (TCCA) as non-toxic chlorinating agent along with PPh3. This process involved a wide range of substrate scope and is well-suited with labile hydroxyl protecting groups such as benzyl, acetyl, benzoyl, isopropylidene, benzylidene, and TBDPS (tert-butyldiphenylsilyl) groups. This process is operationally simple, mild conditions and obtained good yields with excellent α selectivity. Moreover, a multi-catalyst one-pot glycosylation can be carried out to transform the glycosyl hemiacetals directly to a various O-glycosides in high overall yields without the need for separation or purification of the α-glycosyl chloride donors.

All-in-One Synthesis of 3,6-Dideoxysugars: An Olefin Metathesis-Isomerization Approach

A collective synthesis of 3,6-dideoxysugars, including seven naturally known congeners, has been reported using commercially available methyl lactates in five steps. The essential tandem process involving the olefin cross-metathesis and isomerization steps was enabled by the dual function of Grubbs-II catalyst, affording the products in good yields and providing concise and practical access to a class of biologically important deoxysugars.

Chemical Synthesis of a Keto Sugar Nucleotide

Keto sugar nucleotides (KSNs) are common and versatile precursors to various deoxy sugar nucleotides, which are substrates for the corresponding glycosyltransferases involved in the biosynthesis of glycoproteins, glycolipids, and natural products. However, there has been no KSN synthesized chemically due to the inherent instability. Herein, the first chemical synthesis of the archetypal KSN TDP-4-keto-6-deoxy-d-glucose (1) is achieved by an efficient and optimized route, providing feasible access to other KSNs and analogues, thereby opening a new avenue for new applications.

From a Carbohydrate Raw Material to an Important Building Block: Cost-Efficient Conversion of d-Fructose into 2-Deoxy-l-ribose

A straightforward method for the conversion of a low-cost carbohydrate (d-fructose) into an important carbohydrate building block (2-deoxy-l-ribose) is reported. This methodology involves a novel radical cyclization followed by a fragmentation reaction, selective enzymatic hydrolysis using a lipase, and oxidative cleavage of the vicinal diol. This method uses the cheapest starting material and employs the shortest synthetic route (7 steps) for converting a d-sugar into 2-deoxy-l-ribose.

A De Novo Route to 3,6-Dideoxy Sugars

We report the de novo asymmetric synthesis of the 3,6-dideoxy sugars abequose, paratose, and tyvelose from 2-acetylfuran. Conversion of this readily available ketone to a pyranone derivative was followed by transformation to either an α- or β-glycoside via diasteroselective acylation. Michael addition at C2 controlled primarily by the C1 configuration in the glycoside produced 3,6-dideoxy-4-keto sugars, which could be reduced and converted to either fully deprotected monosaccharides or to immediate precursors of glycosyl donors.

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.

Catalytic Stereoselective Synthesis of 2-Deoxy α-glycosides Using Glycosyl Ortho-[1-(p-MeOPhenyl)Vinyl]Benzoates (PMPVB) Donors

2-Deoxy Glycosyl Ortho-[1-(p-MeOPhenyl)Vinyl]Benzoates (PMPVB) donors have been presented as stable, reactive glycosyl donors for the synthesis of 2-deoxy α-glycosides. The donors react under Brønsted acid conditions to provide the 2-deoxy-α-glycosides...

Thiocarbonyl as a Switchable Relay-Auxiliary Group in Carbohydrate Synthesis

A multifunctional O-phenyl thiocarbonyl (O(C═S)OPh) group was introduced in glycosylation reactions. This auxiliary group exhibits three features (1) C6-long-range participation effect, (2) relay activation, and (3) switchable promoter-controlled carbonylation, which enables the facile synthesis of both 6-deoxy glucoside and 6-alcohol glucoside. In addition, we successfully quantified the extent of the C6-acyl participation effect and developed its application toward the α-trisaccharide motif.

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.

Excited-State Palladium-Catalyzed 1,2-Spin-Center Shift Enables Selective C-2 Reduction, Deuteration, and Iodination of Carbohydrates

Excited-state catalysis, a process that involves one or more excited catalytic species, has emerged as a powerful tool in organic synthesis because it allows access to the excited-state reaction landscape for the discovery of novel chemical reactivity. Herein, we report the first excited-state palladium-catalyzed 1,2-spin-center shift reaction that enables site-selective functionalization of carbohydrates. The strategy features mild reaction conditions with high levels of regio- and stereoselectivity that tolerate a wide range of functional groups and complex molecular architectures. Mechanistic studies suggest a radical mechanism involving the formation of hybrid palladium species that undergoes a 1,2-spin-center shift followed by the reduction, deuteration, and iodination to afford functionalized 2-deoxy sugars. The new reactivity will provide a general approach for the rapid generation of natural and unnatural carbohydrates.

Iridium-promoted deoxyglycoside synthesis: stereoselectivity and mechanistic insight

Herein, we devised a method for stereoselective O-glycosylation using an Ir(i)-catalyst which enables both hydroalkoxylation and nucleophilic substitution of glycals with varying substituents at the C3 position. In this transformation, 2-deoxy-α-O-glycosides were acquired when glycals equipped with a notoriously poor leaving group at C3 were used; in contrast 2,3-unsaturated-α-O-glycosides were produced from glycals that bear a good leaving group at C3. Mechanistic studies indicate that both reactions proceed via the directing mechanism, through which the acceptor coordinates to the Ir(i) metal in the α-face-coordinated Ir(i)-glycal π-complex and then attacks the glycal that contains the O-glycosidic bond in a syn-addition manner. This protocol exhibits good functional group tolerance and is exemplified with the preparation of a library of oligosaccharides in moderate to high yields and with excellent stereoselectivities.

Ir(i)-catalyzed α-selective O-glycosylation of glycals provided an access to both 2-deoxyglycosides and 2,3-unsaturated glycosides with a broad substrate scope. The underlying rationale of α-selectivity has been illustrated by the DFT study.

Copper(ii)-catalyzed stereoselective 1,2-addition vs. Ferrier glycosylation of "armed" and "disarmed" glycal donors

Selective activation of "armed' and ''disarmed" glycal donors enabling the stereo-controlled glycosylations by employing Cu(ii)-catalyst as the promoter has been realized. The distinctive stereochemical outcome in the process is mainly influenced by the presence of diverse protecting groups on the donor and the solvent system employed. The protocol is compatible with a variety of aglycones including carbohydrates, amino acids, and natural products to access deoxy-glycosides and glycoconjugates with high α-anomeric selectivity. Notably, the synthetic practicality of the method is amply verified for the stereoselective assembling of trisaccharides comprising 2-deoxy components. Mechanistic studies involving deuterated experiments validate the syn-diastereoselective 1,2-addition of acceptors on the double bond of armed donors.

Stereoselective Electro-2-deoxyglycosylation from Glycals

We report a novel and highly stereoselective electro-2-deoxyglycosylation from glycals. This method features excellent stereoselectivity, scope, and functional-group tolerance. This process can also be applied to the modification of a wide range of natural products and drugs. Furthermore, a scalable synthesis of glycosylated podophyllotoxin and a one-pot trisaccharide synthesis through iterative electroglycosylations were achieved.

Sugar-Pirating as an Enabling Platform for the Synthesis of 4,6-Dideoxyhexoses

An efficient divergent synthetic strategy that leverages the natural product spectinomycin to access uniquely functionalized monosaccharides is described. Stereoselective 2'- and 3'-reduction of key spectinomycin-derived intermediates enabled facile access to all eight possible 2,3-stereoisomers of 4,6-dideoxyhexoses as well as representative 3,4,6-trideoxysugars and 3,4,6-trideoxy-3-aminohexoses. In addition, the method was applied to the synthesis of two functionalized sugars commonly associated with macrolide antibiotics-the 3-O-alkyl-4,6-dideoxysugar d-chalcose and the 3-N-alkyl-3,4,6-trideoxysugar d-desosamine.

Epimerization of an Ascaroside-Type Glycolipid Downstream of the Canonical β-Oxidation Cycle in the Nematode Caenorhabditis nigoni

A species-specific ascaroside-type glycolipid was identified in the nematode Caenorhabditis nigoni using HPLC-ESI-(-)-MS/MS precursor ion scanning, HR-MS/MS, and NMR techniques. Its structure containing an l-3,6-dideoxy-lyxo-hexose unit was established by total synthesis. The identification of this novel 4-epi-ascaroside (caenorhabdoside) in C. nigoni along with the previous identification of 2-epi-ascarosides (paratosides) in Pristionchus pacificus indicate that nematodes can generate highly specific signaling molecules by epimerization of the ascarylose building block downstream of the canonical β-oxidation cycle.

Tuning the Chemoselectivity of Silyl Protected Rhamnals by Temperature and Brønsted Acidity: Kinetically Controlled 1,2-Addition vs Thermodynamically Controlled Ferrier Rearrangement

An acidity- and temperature-dependent chemoselective glycosylation of silyl-protected rhamnals with alcohols has been revealed. The reaction undergoes a 1,2-addition pathway with (±)-CSA as the catalyst at rt, affording kinetically controlled 2-deoxyl rhamnosides. In contrast, only thermodynamically controlled 2,3-unsaturated rhamnosides are formed via Ferrier rearrangement when elevating reaction temperature to 85 °C or using CF₃SO₃H instead. This tunable glycosylation allows facile and practical access to both 2-deoxyl and 2,3-unsaturated rhamnosides with excellent yields and high α-stereoselectivity.

Synthesis and antitumor activities of aquayamycin and analogues of derhodinosylurdamycin A

An analogue without the sugar moiety of natural derhodinosylurdamycin A is better.

Thio-click reaction of 2-deoxy-exo-glycals towards new glycomimetics: stereoselective synthesis of C-2-deoxy-d-glycopyranosyl compounds

Photoinitiated addition of thiols to 2-deoxy-exo-glycals obtained from endo-glycals of d-arabino, d-lyxo, d-erythro and d-threo configurations resulted in highly regio- and stereoselective formation of glycosylmethyl sulfide type glycomimetics.

Synthesis of l-Deoxyribonucleosides from d-Ribose

The preparation of 2-deoxy-l-ribose derivatives or mirror image deoxyribonucleosides (l-deoxyribonucleosides) from d-ribose is reported. Starting from inexpensive d-ribose, an acyclic d-form carbohydrate precursor was synthesized to study a unique carbonyl translocation process. In this novel radical reaction, not only was the configuration of the sugar transformed from the d-form to the l-form, but also deoxygenation at the C(2) position of the sugar was successfully achieved. This is one of the most practical methods for converting a d-sugar to a 2-deoxy-l-sugar in a one-step reaction. To further identify the reaction product, radical reactions followed by treatment with 1,3-propanedithiol and then benzoylation were performed to afford a dithioacetal derivative. The stereochemistry and configuration of the 2-deoxy-l-ribose dithioacetal derivative were confirmed by its X-ray crystal structure. To further apply this methodology, a diethyl thioacetal derivative was formed, followed by selective benzoyl protection, and an NIS-initiated cyclization reaction to give the desired ethyl S-l-2-deoxyriboside, which can be used as a 2-deoxy-l-ribosyl synthon in the formal total synthesis of various l-deoxyribonucleosides, such as l-dT.

Highly reactive 2-deoxy-2-iodo-d-allo and d-gulo pyranosyl sulfoxide donors ensure β-stereoselective glycosylations with steroidal aglycones

The preparation of well-defined d-xylo and d-ribo glycosides represents a synthetic challenge due to the limited configurational availability of starting materials and the laborious synthesis of homogeneous 2-deoxy-β-glycosidic linkages, in particular that of the sugar-steroid motif, which represents the “stereoselective determining step” of the overall synthesis. Herein we describe the use of 2-deoxy-2-iodo-glycopyranosyl sulfoxides accessible from widely available d-xylose and d-ribose monosaccharides as privileged glycosyl donors that permit activation at very low temperature. This ensures a precise kinetic control for a complete 1,2-trans stereoselective glycosylation of particularly challenging steroidal aglycones.

Highly stereoselective synthesis of challenging steroidal 2-deoxy-β-glycosides with d-xylo and d-ribo configurations enabled by low temperature activation of 2-deoxy-2-iodoglycopyranosyl sulfoxides.

Efficient glycosylation of natural Danshensu and its enantiomer by sugar and 2-deoxy sugar donors

An efficient methodology of the glycosylation process of a secondary plant metabolite (R-Danshensu) and its enantiomer by sugar and 2-deoxy sugar donors was developed. The overall synthesis of the new sugar derivatives involved two steps, starting from the previously synthesized protected R and S Danshensu (1 and 2). The deoxy sugar derivatives of R and S Danshensu were obtained from available tri-O-acetyl-2-deoxy-D-glucal and di-O-acetyl-2-deoxy-D-ramnal. The direct glycosylation of 1 and 2 using glycal activation by an acid catalyst in all cases led to the α-anomers of deoxy sugar derivatives with good yields. As a result, a novel group of sugar and deoxy sugar conjugates with optically pure polyphenolic acids was successfully synthesized and their cytotoxic profile against two cancer cell lines was tested. An advantageous ADME profile and antiproliferative data classified this new group of compounds as a promising scaffold for further modification of more potent and selective anticancer agents.

Pre-activation Based Stereoselective Glycosylations

Due to the wide presence of carbohydrates in nature and their crucial roles in numerous important biological processes, oligosaccharides have attracted a lot of attention in synthetic organic chemistry community. Many innovative synthetic methods have been developed for oligosaccharide synthesis, among which the pre-activation based glycosylation is particularly noteworthy. Traditionally, glycosylation reactions are carried out when the glycosyl donor and the acceptor are both present when the promoter is added. In comparison, the pre-activation based glycosylation is unique, where the glycosyl donor is activated by the promoter in the absence of the acceptor. Upon complete donor activation, the acceptor is added to the reaction mixture enabling glycosylation. The key step in any oligosaccharide synthesis is the stereoselective formation of the glycosidic bond. As donor activation and acceptor glycosylation are temporally separated, pre-activation based glycosylation can bestow unique stereochemical control. This review systematically discusses factors impacting the stereochemical outcome of a pre-activation based glycosylation reaction including substituents on the glycosyl donor, reaction solvent, and additives. Applications of pre-activation based stereoselective glycosylation in assembly of complex oligosaccharides are also discussed.

Chemical Access to d-Sarmentose Units Enables the Total Synthesis of Cardenolide Monoglycoside N-1 from Nerium oleander

Herein we present a chemical approach for the ready preparation of d-sarmentosyl donors enabling the first total synthesis and structure validation of cardenolide N-1, a challenging 2,6-dideoxy-3-O-methyl-β-d-xylo-hexopyranoside extracted from Nerium oleander twigs that displays anti-inflammatory properties and cell growth inhibitory activity against tumor cells. The strategy highlights the synthetic value of the sequential methodology developed in our group for the synthesis of 2-deoxyglycosides. Key steps include Wittig-Horner olefination of a d-xylofuranose precursor, [I⁺]-induced 6-endo cyclization, and 1,2-trans stereoselective glycosylation.

Chemical O-Glycosylations: An Overview

The development of glycobiology relies on the sources of particular oligosaccharides in their purest forms. As the isolation of the oligosaccharide structures from natural sources is not a reliable option for providing samples with homogeneity, chemical means become pertinent. The growing demand for diverse oligosaccharide structures has prompted the advancement of chemical strategies to stitch sugar molecules with precise stereo- and regioselectivity through the formation of glycosidic bonds. This Review will focus on the key developments towards chemical O-glycosylations in the current century. Synthesis of novel glycosyl donors and acceptors and their unique activation for successful glycosylation are discussed. This Review concludes with a summary of recent developments and comments on future prospects.

Fluorine-directed 1,2-trans glycosylation of rare sugars

To reconcile the urgent need to access well defined β-configured 2,6-di-deoxypyranose analogues for chemical biology, with the intrinsic α-selectivity of the native system, the directing role of fluorine at C2 has been explored. Localised partial charge inversion (C-H(δ+)→ C-F(δ-)) elicits a reversal of the substrate-based α-stereoselectivity, irrespective of the protecting group electronics.

C6 picoloyl protection: a remote stereodirecting group for 2-deoxy-β-glycoside formation

We reported a remote control glycosylation method using the picoloyl protecting group for 2-deoxy-β-glycosidic bond formation. The method is applicable to various 2-deoxythioglycosyl donors and the utility is illustrated by the synthesis of a deoxytrisaccharide component of landomycins.