Investigation of α-Thioglycoside Donors: Reactivity Studies toward Configuration-Controlled Orthogonal Activation in One-Pot Systems

Synthesis of 2-Deoxyglycosides with Exclusive β-Configuration Using 2-SAc Glycosyl Bromide Donors

In this study, we developed an indirect method for the synthesis of 2-deoxyglycosides with an exclusive β-configuration using glucosyl and galactosyl bromide donors with 2-thioacetyl (SAc) groups. The 2-SAc glucosyl and galactosyl bromide donors were easily obtained through the treatment of 1-OAc, 2-SAc glucose and galactose with HBr-CH3COOH solution, respectively. The glycosylation of such donors with acceptors under an improved Koenigs-Knorr condition resulted in glycosylation products with an exclusive β-configuration in excellent yields. The synthetic approach of 2-SAc glycosyl donors using glycals as the starting materials was also investigated. Based on these studies, the synthetic method of using 2-deoxyglycosides with an exclusive β-configuration through desulfurization will have more practical applications.

Mild and Effective Method for the Nickel-Catalyzed Arylation of Glycosyl Thiols in Aqueous Surfactant Solution

Aryl thioglycosides have broad applicability as both glycosyl donors and glycomimetic compounds. Their synthesis via the cross-coupling of glycosyl thiols with aryl halides has become a popular method for their construction because it allows better selectivity for anomeric configuration as well as a wider functional group tolerance compared to traditional methods. Herein, we report a nickel-catalyzed method for the synthesis of aryl thioglycosides which utilizes an aqueous micellar environment as the reaction medium. This alternative method allows for mild conditions while circumventing expensive palladium, leading to the successful synthesis of over 30 aryl thioglycosides, including challenging 1,2-cis thioglycoside products.

Catalytic Thioglycoside Activation with Diazo-Derived Copper Carbenes

Traditional glycosylation methods using thioglycosides often require harsh conditions or expensive metal catalysts. This study presents a more sustainable alternative by employing copper, an earth-abundant catalyst. We developed diazo-based thioglycoside donors that, through copper catalysis, undergo intramolecular activation to form glycosyl sulfonium ions, leading to the generation of oxocarbenium ions. This versatile approach efficiently accommodates a variety of O-nucleophiles, including primary, secondary, and tertiary, as well as complex bioactive molecules. It is compatible with various glycosyl donors and protecting groups, including superarmed, armed, and disarmed systems. Notably, the methodology operates orthogonally to traditional thioglycoside and alkyne donors and has been successfully applied to the orthogonal iterative synthesis of trisaccharides. Mechanistic insights were gained by studying the electronic effects of electron-donating (OMe) and electron-withdrawing (NO2) groups on the donors, offering a valuable understanding of the intramolecular reaction pathway.

Benzoylation of Tetrols: A Comparison of Regioselectivity Patterns for O- and S-Glycosides of d-Galactose

Efficient protecting group strategies are important for glycan synthesis and represent a unique synthetic challenge in differentiating sugar ring hydroxyl groups. Direct methods to enable regioselective protecting group installation are thus desirable. Herein, we explore a one-step regioselective benzoylation to deliver 2,3,6-protected d-galactose building blocks from tetrols across a variety of α- and β-, O- and S-glycoside substrates. We focus on benzoyl chloride as the esterifying reagent and a reaction temperature of -40 °C to screen the regioselectivity outcome for twenty-two different glycosides, based on isolated yields. Using this methodology, we demonstrate the capability for α-linked aryl and alkyl glycosides (O- and S- d-galactosides, d-galactosamines, and l-fucose), delivering consistent isolated yields (>65%) for 2,3,6-benzoylated products. We extend to explore β-linked systems, where the observed regioselectivity is not paralleled. We posit that both steric and electronic factors from the anomeric substituent contribute to modulating the reactivity competition between 2-OH and 4-OH, enabling the formation of regioisomeric mixtures. However, a certain balance of these factors within the aglycon can deliver 2,3,6-regioselectivity, notably for β-O-Et and β-O-CH2CF3 glycosides. The methodology contributes toward understanding the peculiarities of regioselective carbohydrate-protecting group installation, exploring the importance of the anomeric substituent upon ring hydroxyl group reactivity.

Progress and challenges in the synthesis of sequence controlled polysaccharides

The sequence, length and substitution of a polysaccharide influence its physical and biological properties. Thus, sequence controlled polysaccharides are important targets to establish structure-properties correlations. Polymerization techniques and enzymatic methods have been optimized to obtain samples with well-defined substitution patterns and narrow molecular weight distribution. Chemical synthesis has granted access to polysaccharides with full control over the length. Here, we review the progress towards the synthesis of well-defined polysaccharides. For each class of polysaccharides, we discuss the available synthetic approaches and their current limitations.

(C6F5)3B·(HF)n-catalyzed glycosylation of disarmed glycosyl fluorides and reverse glycosyl fluorides

(C₆F₅)₃B·(HF)_n-catalyzed glycosylation of disarmed glycosyl fluorides and reverse glycosyl fluorides with structurally diverse nucleophiles has been achieved.

Chemoselective activation of ethyl vs. phenyl thioglycosides: one-pot synthesis of oligosaccharides

Ethyl and phenyl thioglycosides are the two most common types of thioglycoside donors in carbohydrate chemistry. However, the chemoselective activation of ethyl vs. phenyl thioglycosides is very rare in the literature. In this work, ethyl thioglycosides could be readily activated with an N-trifluoromethylthiosaccharin/TMSOTf system in the presence of phenyl thioglycosides carrying the same or even more armed protecting group pattern. Both armed and disarmed thioglycosides exhibited high chemoselectivity towards the promoter system. Chemoselective glycosylation was subsequently applied to one-pot synthesis, thus providing an efficient means to oligosaccharides.

Mechanisms of Stereodirecting Participation and Ester Migration from Near and Far in Glycosylation and Related Reactions

This review is the counterpart of a 2018 Chemical Reviews article (Adero, P. O.; Amarasekara, H.; Wen, P.; Bohé, L.; Crich, D. Chem. Rev. 2018, 118, 8242-8284) that examined the mechanisms of chemical glycosylation in the absence of stereodirecting participation. Attention is now turned to a critical review of the evidence in support of stereodirecting participation in glycosylation reactions by esters from either the vicinal or more remote positions. As participation by esters is often accompanied by ester migration, the mechanism(s) of migration are also reviewed. Esters are central to the entire review, which accordingly opens with an overview of their structure and their influence on the conformations of six-membered rings. Next the structure and relative energetics of dioxacarbeniun ions are covered with emphasis on the influence of ring size. The existing kinetic evidence for participation is then presented followed by an overview of the various intermediates either isolated or characterized spectroscopically. The evidence supporting participation from remote or distal positions is critically examined, and alternative hypotheses for the stereodirecting effect of such esters are presented. The mechanisms of ester migration are first examined from the perspective of glycosylation reactions and then more broadly in the context of partially acylated polyols.

The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1-SN2 Interface

A critical review of the state-of-the-art evidence in support of the mechanisms of glycosylation reactions is provided. Factors affecting the stability of putative oxocarbenium ions as intermediates at the SN1 end of the mechanistic continuum are first surveyed before the evidence, spectroscopic and indirect, for the existence of such species on the time scale of glycosylation reactions is presented. Current models for diastereoselectivity in nucleophilic attack on oxocarbenium ions are then described. Evidence in support of the intermediacy of activated covalent glycosyl donors is reviewed, before the influences of the structure of the nucleophile, of the solvent, of temperature, and of donor-acceptor hydrogen bonding on the mechanism of glycosylation reactions are surveyed. Studies on the kinetics of glycosylation reactions and the use of kinetic isotope effects for the determination of transition-state structure are presented, before computational models are finally surveyed. The review concludes with a critical appraisal of the state of the art.

Stereoselective Epimerizations of Glycosyl Thiols

Glycosyl thiols are widely used in stereoselective S-glycoside synthesis. Their epimerization from 1,2-trans to 1,2-cis thiols (e.g., equatorial to axial epimerization in thioglucopyranose) was attained using TiCl₄, while SnCl₄ promoted their axial-to-equatorial epimerization. The method included application for stereoselective β-d-manno- and β-l-rhamnopyranosyl thiol formation. Complex formation explains the equatorial preference when using SnCl₄, whereas TiCl₄ can shift the equilibrium toward the 1,2-cis thiol via 1,3-oxathiolane formation.

"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.

Dissection of the effects that govern thioglucoside and thiomannoside reactivity

Neighboring group effects were investigated in gluco- and manno-configured thioglycosides under NIS/TfOH activation. Donors possessing a 2-O-benzoyl group that are capable (1,2-trans) and incapable (1,2-cis) of exerting nucleophilic push were compared with donors possessing a participatory neutral 2-O-benzyl group. By using competition experiments between sets of glycosyl donors the direct effect of neighboring group participation and the electron withdrawing effect of the 2-O-benzoyl group could be separated. The study brings insight into how the stereochemistry of the 1 and 2 position and how the nature of the aglycon (Ph or Et) have a pronounced effect on glycosyl donor reactivity.

2- O- N-Benzylcarbamoyl as a Protecting Group To Promote β-Selective Glycosylation and Its Applications in the Stereoselective Synthesis of Oligosaccharides

This study examines the utility of the N-benzylcarbamoyl (BnCar) protecting group in glycosylation reactions of the parent O-2 protected carbohydrate donor. It was found that the BnCar group imparted exclusively β-selectivity with primary and secondary alcohols. A mechanistic study revealed the activated intermediate to be the glycosyl triflate in a skew conformation, which results in β-selective glycosylation via an S_N2-like pathway. The BnCar group can be readily cleaved using tetrabutylammonium nitrite, without affecting ester and ether protecting groups. Taken together, these results show BnCar to be useful for the synthesis of complex oligosaccharides, an undertaking that requires delicate chemical differentiation of various protecting groups.