2-(Hydroxycarbonyl)benzyl glycosides: a novel type of glycosyl donors for highly efficient beta-mannopyranosylation and oligosaccharide synthesis by latent-active glycosylation

Use of Novel Homochiral Thioureas Camphor Derived as Asymmetric Organocatalysts in the Stereoselective Formation of Glycosidic Bonds

We synthesized six new camphor-derived homochiral thioureas 1-6, from commercially available (1R)-(-)-camphorquinone. These new compounds 1-6 were evaluated as asymmetric organocatalysts in the stereoselective formation of glycosidic bonds, with 2,3,4,6-tetra-O-benzyl-D-glucopyranosyl and 2,3,4,6-tetra-O-benzyl-D-galactopyranosyl trichloroacetimidates as donors, and several alcohols as glycosyl acceptors, such as methanol, ethanol, 1-propanol, 1-butanol, 1-octanol, iso-propanol, tert-butanol, cyclohexanol, phenol, 1-naphtol, and 2-naphtol. Optimization of the asymmetric glycosylation reaction was achieved by modifying reaction conditions such as solvent, additive, loading of catalyst, temperature, and time of reaction. The best result was obtained with 2,3,4,6-tetra-O-benzyl-D-galactopyranosyl trichloroacetimidates, using 15 mol% of organocatalyst 1, in the presence of 2 equiv of MeOH in solvent-free conditions at room temperature for 1.5 h, affording the glycosidic compound in a 99% yield and 1:73 α:β stereoselectivity; under the same reaction conditions, without using a catalyst, the obtained stereoselectivity was 1:35 α:β. Computational calculations prior to the formation of the products were modeled, using density functional theory, M06-2X/6-31G(d,p) and M06-2X/6-311++G(2d,2p) methods. We observed that the preference for β glycoside formation, through a stereoselective inverted substitution, relies on steric effects and the formation of hydrogen bonds between thiourea 1 and methanol in the complex formed.

Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.

Conformationally restricted donors for stereoselective glycosylation

In nucleophilic reactions using sugars as electrophiles, i.e., glycosyl donors, their conformation affects the generation rate or stability of the glycosyl cation intermediates and determines at which side of the S_N2-S_N1 borderline and at what rate the reaction occurs. In addition, changes in the conformation create the steric or stereoelectronic effects of the substituents, which also change the reaction rate and stereoselectivity. Bulky silyl protecting groups, uronic acid esters, and transannular structures have been utilized to change the conformation. Consequently, reactions with unique reactivities and stereoselectivities have been developed. In this chapter, a discussion of the reaction mechanisms relating stereoselectivity to conformation is provided.

Synthesis and Glycosidation of Anomeric Halides: Evolution from Early Studies to Modern Methods of the 21st Century

Advances in synthetic carbohydrate chemistry have dramatically improved access to common glycans. However, many novel methods still fail to adequately address challenges associated with chemical glycosylation and glycan synthesis. Since a challenge of glycosylation has remained, scientists have been frequently returning to the traditional glycosyl donors. This review is dedicated to glycosyl halides that have played crucial roles in shaping the field of glycosciences and continue to pave the way toward our understanding of chemical glycosylation.

Small tools for sweet challenges: advances in microfluidic technologies for glycan synthesis

Glycans, including oligosaccharides and glycoconjugates, play an integral role in modulating the biological functions of macromolecules. Many physiological and pathological processes are mediated by interactions between glycans, which has led to the use of glycans as biosensors for pathogen and biomarker detection. Elucidating the relationship between glycan structure and biological function is critical for advancing our understanding of the impact glycans have on human health and disease and for expanding the repertoire of glycans available for bioanalysis, especially for diagnostics. Such efforts have been limited by the difficulty in obtaining sufficient quantities of homogenous glycan samples needed to resolve the exact relationships between glycan structure and their structural or modulatory functions on a given glycoconjugate. Synthetic strategies offer a viable route for overcoming these technical hurdles. In recent years, microfluidics have emerged as powerful tools for realizing high-throughput and reproducible syntheses of homogenous glycans for the potential use in functional studies. This critical review provides readers with an overview of the microfluidic technologies that have been developed for chemical and enzymatic glycan synthesis. The advantages and limitations associated with using microreactor platforms to improve the scalability, productivity, and selectivity of glycosylation reactions will be discussed, as well as suggested future work that can address certain pitfalls.

Anomeric alkylations and acylations of unprotected mono- and disaccharides mediated by pyridoneimine in aqueous solutions

A site-specific deprotonation followed by alkylations and acylations of sugar hemiacetals to the corresponding alkyl glycosides and acylated sugars in aqueous solutions is disclosed herein. Pyridoneimine as a new base is developed to mediate the deprotonation of readily available sugar hemiacetals and further reactions with alkylation and acylation agents.

Recent Advances in Stereocontrolled Mannosylation: Focus on Glycans Comprising Acidic and/or Amino Sugars

The main focus of this review is to describe accomplishments made in the stereoselective synthesis of β-linked mannosides functionalized with carboxyls or amines/amides. These ManNAc, ManA and ManNAcA residues found in many glycoconjugates, bacterial polysaccharides, and alginates have consistently captured interest of the glycoscience community both due to synthetic challenge and therapeutic potential.

Aglycon reactivity as a guiding principle in latent-active approach to chemical glycosylations

Chemical glycosylations critically depend on the activation of a glycosyl donor and the reaction of this activated donor intermediate with an acceptor alcohol. Whereas many strategies are developed for the activation of an anomeric aglycon substituent, the latent-active method of glycosylation is based specifically on tuning the reactivity of the aglycon substituent of a glycosyl donor. Several novel methods have emerged to install reactive aglycon moiety in a glycosyl donor and fine-tuning the reactivity of the moiety. Remote functionalizations of the aglycon plays a key role in the reactivity tuning. Activation of a remote functionality enables an otherwise latent aglycon to an active moiety, suitable as a glycosyl donor. The latent-active approach provides an advantage to avoid the conversion of the aglycon to another donor prior to a glycosylation, in addition to advancing the contemporary glycosylations with alternate insights. The review analyzes the methodologies that consolidate the latent-active approach to chemical glycosylations.

The study of the acid-catalyzed reaction between 2-methyl and 2-(2,2,2-trichloroethoxy) gluco-[2,1-d]-2-oxazolines. Synthesis of macrocyclic pseudo-tetrasaccharide derivative of d-glucosamine

The formation of macrocyclic pseudo-tetrasaccharide derivative of d-glucosamine as a result of the acid-catalyzed reaction between 2-methyl- and 2-(2,2,2-trichloroethoxy)-substituted oxazoline derivatives of sugars was discovered. The structure of the obtained product was determined using NMR spectroscopy and mass spectrometry. An explanation of the obtained results based on the mechanism of the reaction of electrophilic polymerization of 2-substituted glyco-[2,1-d]-2-oxazolines and the principle of hard and soft acids and bases (HSAB) was proposed.

Synthesis of Reverse Glycosyl Fluorides and Rare Glycosyl Fluorides Enabled by Radical Decarboxylative Fluorination of Uronic Acids

An efficient protocol for synthesizing reverse glycosyl fluorides is described, relying on silver-promoted decarboxylative fluorination of structurally diverse pentofuran- and hexopyranuronic acids under the mild reaction conditions. The potential applications of the reaction are further demonstrated by converting readily available d-uronic acid derivatives into uncommon d-/l-glycosyl fluorides through a C1-to-C5 switch strategy. The reaction mechanism is corroborated by 5-exo-trig radical cyclization of allyl α-d-C-glucopyranuronic acid triggered by decarboxylative fluorination.

Highly Selective β-Mannosylations and β-Rhamnosylations Catalyzed by Bis-thiourea

We report highly β-selective bis-thioureas-catalyzed 1,2-cis-O-pyranosylations employing easily accessible acetonide-protected donors. A wide variety of alcohol nucleophiles, including complex natural products, glycosides, and amino acids were β-mannosylated and β-rhamnosylated successfully using an operationally simple protocol under mild and neutral conditions. Less nucleophilic acceptors such as phenols were also glycosylated efficiently in excellent yields and with high β-selectivities.

One-pot oligosaccharide synthesis: latent-active method of glycosylations and radical halogenation activation of allyl glycosides

Chemical glycosylations occupy a central importance to synthesize tailor-made oligo- and polysaccharides of functional importance. Generation of the oxocarbenium ion or the glycosyl cation is the method of choice in order to form the glycosidic bond interconnecting a glycosyl moiety with a glycosyl/aglycosyl moiety. A number of elegant methods have been devised that allow the glycosyl cation formation in a fairly stream-lined manner to a large extent. The latent-active method provides a powerful approach in the protecting group controlled glycosylations. In this context, allyl glycosides have been developed to meet the requirement of latent-active reactivities under appropriate glycosylation conditions. Radical halogenation provides a newer route of activation of allyl glycosides to an activated allylic glycoside. Such an allylic halide activation subjects the glycoside reactive under acid catalysis, leading to the conversion to a glycosyl cation and subsequent glycosylation with a number of acceptors. The complete anomeric selectivity favoring the 1,2-trans-anomeric glycosides points to the possibility of a preferred conformation of the glycosyl cation. This article discusses about advancements in the selectivity of glycosylations, followed by delineating the allylic halogenation of allyl glycoside as a glycosylation method and demonstrates synthesis of a repertoire of di- and trisaccharides, including xylosides, with varied protecting groups.

Stereoselective β-Mannosylation with 2,6-Lactone-bridged Thiomannosyl Donor by Remote Acyl Group Participation

Stereoselective β-mannosylation has been recognized as one of the greatest challenges of carbohydrate chemistry. Herein, we described a practical method for stereoselective construction of β-mannosides by using a 2,6-lactone-bridged thiomannosyl donor through the remote acyl-group participation as well as the steric effect of O-4 substituent. The two effects are enabled through the conversion of a regular mannopyranosyl ⁴ C₁ conformation into a 2,6-lactone bridged conformation. The lactone donor could be readily prepared in three steps on a gram scale and the β-mannosylation proceeded smoothly with high stereoselectivity for primary, secondary and tertiary alcohol acceptors. In addition, this strategy was successfully applied to the synthesis of a naturally occurring trisaccharide.

Gold(i)-promoted α-selective sialylation of glycosylortho-hexynylbenzoates for the latent-active synthesis of oligosialic acids

A gold(i)-promoted α-selective glycosylation approach with sialylortho-hexynylbenzoates as donors is developed for the latent-active synthesis of α-(2 → 9)-linked oligosialic acids.

Acceptor reactivity in glycosylation reactions

The effect of the reactivity of the glycosyl acceptor on the outcome of glycosylation reactions is reviewed.

Automated Chemical Oligosaccharide Synthesis: Novel Approach to Traditional Challenges

Advances in carbohydrate chemistry have certainly made common oligosaccharides much more accessible. However, many current methods still rely heavily upon specialized knowledge of carbohydrate chemistry. The application of automated technologies to chemical and life science applications such as genomics and proteomics represents a vibrant field. These automated technologies also present opportunities for their application to organic synthesis, including that of the synthesis of oligosaccharides. However, application of automated methods to the synthesis of carbohydrates is an underdeveloped area as compared to other classes of biomolecules. The overarching goal of this review article is to present the advances that have been made at the interface of carbohydrate chemistry and automated technology.

Total Synthesis of Trisaccharide Repeating Unit of O-Specific Polysaccharide of Pseudomonas fluorescens BIM B-582

The first total synthesis of the trisaccharide repeating unit of the O-specific polysaccharide of Pseudomonas fluorescens BIM B-582 is reported. This efficient synthesis involves consecutive 1,2- cis glycosylations including β-l-rhamnosylation and α selective coupling of rare 4-deoxy-d- xylo-hexose as the key steps. The synthetic trisaccharide is equipped with an aminopropyl linker at the reducing end to allow for conjugation to proteins and microarrays for further immunological studies.

Practical synthesis of active O-2-(2-propylsulfinyl)benzyl (OPSB) glycosides via a catalytic and metal free oxidation of latent O-2-(2-propylthiol)benzyl (OPTB) glycosides

A catalytic and metal free sulfoxidation of O-2-(2-propylthiol)benzyl (OPTB) glycosides to O-2-(2-propylsulfinyl)benzyl (OPSB) glycosides has been developed by introducing NOBF₄ as catalyst, oxygen as terminal oxidant and TBAB as additive. Wide variety of OPTB glycosides were efficiently oxidized without observation of over oxidation. The allowance of large scale synthesis, easy operation and purification highlighted its practical application in construction of complex oligosaccharides and glycoconjugates employing interrupted Pummerer reaction mediated glycosylation strategy.

"One-Pot" Protection, Glycosylation, and Protection-Glycosylation Strategies of Carbohydrates

Carbohydrates, which are ubiquitously distributed throughout the three domains of life, play significant roles in a variety of vital biological processes. Access to unique and homogeneous carbohydrate materials is important to understand their physical properties, biological functions, and disease-related features. It is difficult to isolate carbohydrates in acceptable purity and amounts from natural sources. Therefore, complex saccharides with well-defined structures are often most conviently accessed through chemical syntheses. Two major hurdles, regioselective protection and stereoselective glycosylation, are faced by carbohydrate chemists in synthesizing these highly complicated molecules. Over the past few years, there has been a radical change in tackling these problems and speeding up the synthesis of oligosaccharides. This is largely due to the development of one-pot protection, one-pot glycosylation, and one-pot protection-glycosylation protocols and streamlined approaches to orthogonally protected building blocks, including those from rare sugars, that can be used in glycan coupling. In addition, new automated strategies for oligosaccharide syntheses have been reported not only for program-controlled assembly on solid support but also by the stepwise glycosylation in solution phase. As a result, various sugar molecules with highly complex, large structures could be successfully synthesized. To summarize these recent advances, this review describes the methodologies for one-pot protection and their one-pot glycosylation into the complex glycans and the chronological developments associated with automated syntheses of oligosaccharides.

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.

Radical halogenation-mediated latent–active glycosylations of allyl glycosides

Radical halogenation-mediated glycosylation using allyl glycosides as donors and as acceptors emerges to be an efficient and hither-to unknown glycosylation method, adhering to the concept of the latent–active methodology.

Novel 1,2-cis-stereoselective glycosylations utilizing organoboron reagents and their application to natural products and complex oligosaccharide synthesis

In this mini-review article, 1,2-cis-stereoselective glycosylation methods utilizing organoboron reagents and 1,2-anhydroglycosyl donors that were developed in our laboratory, including regio- and 1,2-cis-stereoselective glycosylations using glycosyl-acceptor-derived boronic ester catalysts and 1,2-cis-stereoselective glycosylations using glycosyl-acceptor-derived borinic ester catalysts, are recounted. Representative applications of these methods to the synthesis of biologically active natural products and a complex oligosaccharide are also described.

Preactivation-based chemoselective glycosylations: A powerful strategy for oligosaccharide assembly

Most glycosylation reactions are performed by mixing the glycosyl donor and acceptor together followed by the addition of a promoter. While many oligosaccharides have been synthesized successfully using this premixed strategy, extensive protective group manipulation and aglycon adjustment often need to be performed on oligosaccharide intermediates, which lower the overall synthetic efficiency. Preactivation-based glycosylation refers to strategies where the glycosyl donor is activated by a promoter in the absence of an acceptor. The subsequent acceptor addition then leads to the formation of the glycoside product. As donor activation and glycosylation are carried out in two distinct steps, unique chemoselectivities can be obtained. Successful glycosylation can be performed independent of anomeric reactivities of the building blocks. In addition, one-pot protocols have been developed that have enabled multiple-step glycosylations in the same reaction flask without the need for intermediate purification. Complex glycans containing both 1,2-cis and 1,2-trans linkages, branched oligosaccharides, uronic acids, sialic acids, modifications such as sulfate esters and deoxy glycosides have been successfully synthesized. The preactivation-based chemoselective glycosylation is a powerful strategy for oligosaccharide assembly complementing the more traditional premixed method.

A Natural Lipotrisaccharide and Its Derivatives Selectively Lyse Streptococcus pneumoniae via Interaction with Cell Membrane

A natural lipotrisaccharide (NP000778, 1a), a new triglycosidic tri-O-substituted glycolipid isolated from the Morinda citrifolia plant, and its chemical derivatives were identified to be active against major Gram-positive pathogens, particularly Streptococcus pneumoniae. Additional evidence indicated that 1a and its synthetic derivatives exerted their bactericidal activities against S. pneumoniae by selectively targeting the bacterial membrane, leading to the rapid lysis of the pneumococci. Efficient synthesis of 1a and its derivatives was performed using an application of the intramolecular aglycon delivery (IAD) reaction to establish its structure-activity relationships (SARs). SAR analysis indicated that trisaccharide glycolipid compounds showed good selectivity and high potency against S. pneumoniae. These compounds contain a linear chain with a chain length from C₃ to C₉ at the 2-position (R₁) and 4'-position (R₃), as well as a 2-methyl butyryl group at the 3'-position (R₂), without an aza substitution in the lipid chain. This is the first lipotrisaccharide identified with potent bactericidal activity via interaction with cell membrane. The results reported herein offer a valuable guideline for the design of glycolipid derivatives that selectively target pathogenic bacteria.

S-Benzimidazolyl (SBiz) Imidates as a Platform for Oligosaccharide Synthesis via Active-Latent, Armed-Disarmed, Selective, and Orthogonal Activations

This article describes the development of S-benzimidazolyl (SBiz) imidates as versatile building blocks for oligosaccharide synthesis. The SBiz imidates have been originally developed as a new platform for active-latent glycosylations. This article expands upon the utility of these compounds. The application to practically all common concepts for the expeditious oligosaccharide synthesis including selective, chemoselective, and orthogonal strategies is demonstrated. The strategy development was made possible thanks to our enhanced understanding of the reaction mechanism and the modes by which SBiz imidates interact with various promoters of glycosylation.

Regio- and stereoselective β-mannosylation using a boronic acid catalyst and its application in the synthesis of a tetrasaccharide repeating unit of lipopolysaccharide derived from E. coli O75

A novel regio- and stereoselective β-mannosylation using 1,2-anhydromannose and a diol sugar acceptor in the presence of a boronic acid catalyst and its application are reported.

Obstacles and solutions for chemical synthesis of syndecan-3 (53-62) glycopeptides with two heparan sulfate chains

Proteoglycans play critical roles in many biological events. Due to their structural complexities, strategies towards synthesis of this class of glycopeptides bearing well-defined glycan chains are urgently needed. In this work, we give the full account of the synthesis of syndecan-3 glycopeptide (53-62) containing two different heparan sulfate chains. For assembly of glycans, a convergent 3+2+3 approach was developed producing two different octasaccharide amino acid cassettes, which were utilized towards syndecan-3 glycopeptides. The glycopeptides presented many obstacles for post-glycosylation manipulation, peptide elongation, and deprotection. Following screening of multiple synthetic sequences, a successful strategy was finally established by constructing partially deprotected single glycan chain containing glycopeptides first, followed by coupling of the glycan-bearing fragments and cleavage of the acyl protecting groups.

Glycosyl-Acceptor-Derived Borinic Ester-Promoted Direct and β-Stereoselective Mannosylation with a 1,2-Anhydromannose Donor

β-Stereoselective mannosylations were conducted using a 1,2-anhydromannose donor and mono-ol acceptors in the presence of a glycosyl-acceptor-derived borinic ester. Reactions proceeded smoothly under mild conditions to provide the corresponding β-mannosides with high stereoselectivity in moderate to high yields. In addition, the present glycosylation method was applied successfully to the total synthesis of acremomannolipin A.

Glycosylation via remote activation of anomeric leaving groups: development of 2-(2-propylsulfinyl)benzyl glycosides as novel glycosyl donors

New glycosyl donors with recyclable and regenerable leaving groups, which could be activated via remote mode, were designed for latent-active glycosylation.

β-Mannosylation with 4,6-benzylidene protected mannosyl donors without preactivation

Mannosylations with benzylidene protected mannosyl donors were found to be β-selective even when no preactivation was performed. It was also found that the kinetic β-product in some cases anomerizes fast to the thermodynamically favored α-anomer under typical reaction conditions.

Cation Clock Reactions for the Determination of Relative Reaction Kinetics in Glycosylation Reactions: Applications to Gluco- and Mannopyranosyl Sulfoxide and Trichloroacetimidate Type Donors

The development of a cation clock method based on the intramolecular Sakurai reaction for probing the concentration dependence of the nucleophile in glycosylation reactions is described. The method is developed for the sulfoxide and trichloroacetimidate glycosylation protocols. The method reveals that O-glycosylation reactions have stronger concentration dependencies than C-glycosylation reactions consistent with a more associative, S(N)2-like character. For the 4,6-O-benzylidene-directed mannosylation reaction a significant difference in concentration dependence is found for the formation of the β- and α-anomers, suggesting a difference in mechanism and a rationale for the optimization of selectivity regardless of the type of donor employed. In the mannose series the cyclization reaction employed as clock results in the formation of cis and trans-fused oxabicyclo[4,4,0]decanes as products with the latter being strongly indicative of the involvement of a conformationally mobile transient glycosyl oxocarbenium ion. With identical protecting group arrays cyclization in the glucopyranose series is more rapid than in the mannopyranose manifold. The potential application of related clock reactions in other carbenium ion-based branches of organic synthesis is considered.