Highly diastereoselective alpha-mannopyranosylation in the absence of participating protecting groups

The effect of neighbouring group participation and possible long range remote group participation in O-glycosylation

Stereoselective glycosylations are one of the most challenging tasks of synthetic glycochemists. The protecting building blocks on the glycosides contribute significantly in attaining the required stereochemistry of the resulting glycosides. Strategic installation of suitable protecting groups in the C-2 position, vicinal to the anomeric carbon, renders neighbouring group participation, whereas protecting groups in the distal C-3, C-4, and C-6 positions are often claimed to exhibit remote group participation with the anomeric carbon. Neighbouring group participation and remote group participation are being widely studied to help the glycochemists design the synthetic protocols for multistep synthesis of complex oligosaccharides and in turn, standardise the process of the glycosylation towards a particular stereochemical output. While neighbouring group participation has been quite effective in achieving the required stereochemistry of the produced glycosides, remote participation exhibits comparatively less efficacy in achieving complete stereoselectivity in the glycosylation reactions. Remote participation is a still highly debated topic in the scientific community. However, implementing the participating role of the remote groups in glycosylation reactions is widely practised to achieve better stereocontrol and to facilitate the formation of synthetically challenging glycosidic linkages.

2,2-Dichloro-1,3-benzodioxole as a Hydroxyl Protection Reagent for Carbohydrate Compounds

The application of 2,2-dichloro-1,3-benzodioxole (DCBZ) as a versatile hydroxyl protection reagent in carbohydrate chemistry has been demonstrated. DCBZ exhibited high reactivity with diverse vicinal and remote dihydroxyl groups in furanoses and pyranoses to form benzodioxole (BZDO)-protected products. The BZDO group showed remarkable stability under basic, redox, and glycosylation conditions and can be efficiently removed under acid conditions in the presence of a diol, highlighting its utility in the orthogonal synthesis of complex glycans.

Anomeric Triflates versus Dioxanium Ions: Different Product-Forming Intermediates from 3-Acyl Benzylidene Mannosyl and Glucosyl Donors

Minimal structural differences in the structure of glycosyl donors can have a tremendous impact on their reactivity and the stereochemical outcome of their glycosylation reactions. Here, we used a combination of systematic glycosylation reactions, the characterization of potential reactive intermediates, and in-depth computational studies to study the disparate behavior of glycosylation systems involving benzylidene glucosyl and mannosyl donors. While these systems have been studied extensively, no satisfactory explanations are available for the differences observed between the 3-O-benzyl/benzoyl mannose and glucose donor systems. The potential energy surfaces of the different reaction pathways available for these donors provide an explanation for the contrasting behavior of seemingly very similar systems. Evidence has been provided for the intermediacy of benzylidene mannosyl 1,3-dioxanium ions, while the formation of the analogous 1,3-glucosyl dioxanium ions is thwarted by a prohibitively strong flagpole interaction of the C-2-O-benzyl group with the C-5 proton in moving toward the transition state, in which the glucose ring adopts a B2,5-conformation. This study provides an explanation for the intermediacy of 1,3-dioxanium ions in the mannosyl system and an answer to why these do not form from analogous glucosyl donors.

Detection and Characterization of Rapidly Equilibrating Glycosylation Reaction Intermediates Using Exchange NMR

The stereoselective introduction of glycosidic bonds (glycosylation) is one of the main challenges in the chemical synthesis of carbohydrates. Glycosylation reaction mechanisms are difficult to control because, in many cases, the exact reactive species driving product formation cannot be detected and the product outcome cannot be explained by the primary reaction intermediate observed. In these cases, reactions are expected to take place via other low-abundance reaction intermediates that are in rapid equilibrium with the primary reaction intermediate via a Curtin-Hammett scenario. Despite this principle being well-known in organic synthesis, mechanistic studies investigating this model in glycosylation reactions are complicated by the challenge of detecting the extremely short-lived reactive species responsible for product formation. Herein, we report the utilization of the chemical equilibrium between low-abundance reaction intermediates and the stable, readily observed α-glycosyl triflate intermediate in order to infer the structure of the former species by employing exchange NMR. Using this technique, we enabled the detection of reaction intermediates such as β-glycosyl triflates and glycosyl dioxanium ions. This demonstrates the power of exchange NMR to unravel reaction mechanisms as we aim to build a catalog of kinetic parameters, allowing for the understanding and eventual prediction of glycosylation reactions.

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 building blocks of D-altrosamine

Presented herein is a streamlined synthesis of building blocks of a rare sugar D-altrosamine. Also investigated was the glycosylation of different glycosyl acceptors with differentially protected altrosamine donors. High facial stereoselectivity was achieved with 3-O-picoloyl donors and reactive glycosyl acceptors via the H-bond-mediated aglycone delivery (HAD) pathway. In contrast, glycosidations of the altrosamine donor equipped with the 3-O-benzoyl group were poorly stereoselective.

Influence of 3-Thio Substituents on Benzylidene-Directed Mannosylation. Isolation of a Bridged Pyridinium Ion and Effects of 3-O-Picolyl and 3-S-Picolyl Esters

The influence on glycosyl selectivity of substituting oxygen for sulfur at the 3-position of 4,6-O-benzylidene-protected mannopyranosyl thioglycosides is reported and varies considerably according to the protecting group employed at the 3-position. The substitution of a thioether at the 3-position for the more usual 3-O-benzyl ether results in a significant loss of selectivity. The installation of a 3-S-picolinyl thioether results in a complex reaction mixture, from which a stable seven-membered bridged bicyclic pyridinium ion is isolated, while the corresponding 3-O-picolinyl ether affords a highly α-selective coupling reaction. A 3-O-picolyl ester provides excellent β-selectivity, while the analogous 3-S-picolyl thioester gives a highly α-selective reaction. The best β-selectivity is seen with a 3-deoxy-3-(2-pyridinyldisulfanyl) system. These observations are discussed in terms of the influence of the various substituents on the central glycosyl triflate - ion pair equilibrium.

Synthetic neoglycoconjugates of hepta- and nonamannoside ligands for eliciting oligomannose-specific HIV-1-neutralizing antibodies

Oligomannose-type glycans on the spike protein of HIV-1 constitute relevant epitopes to elicit broadly neutralizing antibodies (bnAbs). Herein we describe an improved synthesis of α- and β-linked hepta- and nonamannosyl ligands that, subsequently, were converted into BSA and CRM 197 neoglycoconjugates. We assembled the ligands from anomeric 3-azidopropyl spacer glycosides from select 3-O-protected thiocresyl mannoside donors. Chain extensions were achieved using 4+3 or 4+5 block synthesis of thiocresyl and trichloroacetimidate glycosyl donors. Subsequent global deprotection generated the 3-aminopropyl oligosaccharide ligands. ELISA binding data obtained with the β-anomeric hepta- and nonamannosyl conjugates with a selection of HIV-1 bnAbs showed comparable binding of both mannosyl ligands by Fab fragments yet lesser binding of the nonasaccharide conjugate by the corresponding IgG antibodies. These results support previous observations that a complete Man 9 structure might not be the preferred antigenic binding motif for some oligomannose-specific antibodies and have implications for glycoside designs to elicit oligomannose-targeted HIV-1-neutralizing antibodies.

Direct Synthesis of Glycans Containing Challenging ManNAcA Residues

A method for direct, highly stereoselective synthesis of glycans containing β-linked d-mannosaminuronic acid (ManNAcA) residues is reported herein, among which is the capsular polysaccharide of Staphylococcus aureus type 8. Previous chemical syntheses of this glycan relied on indirect methods comprising glucosylation followed by a multistep epimerization and oxidation sequence. The high β-stereocontrol with direct glycosidation of 3-O-picoloylated ManNAcA donors was achieved using the H-bond-mediated aglycone delivery (HAD) reaction. A method to achieve complete α-ManNAcA stereoselectivity with 3-O-benzoylated donors is also reported.

Direct Experimental Characterization of a Bridged Bicyclic Glycosyl Dioxacarbenium Ion by 1 H and 13 C NMR Spectroscopy: Importance of Conformation on Participation by Distal Esters

Low-temperature NMR studies with a 4-C-methyl-4-O-benzoyl galactopyranosyl donor enable the observation and characterization of a bridged bicyclic dioxacarbenium ion arising from participation by a distal ester. Variable-temperature NMR studies reveal this bridged ion to decompose at temperatures above ≈-30 °C. In the absence of the methyl group, the formation of a bicyclic ion is not observed. It is concluded that participation by typical secondary distal esters in glycosylation reactions is disfavored in the ground state conformation of the ester from which it is stereoelectronically impossible. Methylation converts the secondary ester to a conformationally more labile tertiary ester, removes this barrier, and renders participation more favorable. Nevertheless, the minor changes in selectivity in model glycosylation reactions on going from the secondary to the tertiary esters at both low and room temperature argue against distal group participation being a major stereodirecting factor even for the tertiary system.

2,3-Carbamate mannosamine glycosyl donors in glycosylation reactions of diacetone-d-glucose. An experimental and theoretical study

The role of the cyclic 2,3-N,O-carbamate protecting group in directing the selectivity of mannosylation reactions of diacetone-d-glucose, promoted by BSP/Tf2O via α-triflate intermediates, has been investigated through a combined computational and experimental approach. DFT calculations were used to locate the transition states leading to the α or β anomers. These data indicate the preferential formation of the β-adduct with mannosyl donors either equipped with the 4,6-O-benzylidene protection or without it. The synthetic results confirmed this preference, showing in both cases an α/β selectivity of 4:6. This highlights a role for the 2,3-N,O-carbamate in sharp contrast with what described in the case of 2,3-O-carbonate mannosyl donors.

A versatile approach to the synthesis of glycans containing mannuronic acid residues

Reported herein is a new method for a highly effective synthesis of β-glycosides from mannuronic acid donors equipped with the 3-O-picoloyl group. The stereocontrol of glycosylations was achieved by means of the H-bond-mediated aglycone delivery (HAD). The method was utilized for the synthesis of a tetrasaccharide linked viaβ-(1 → 3)-mannuronic linkages. We have also investigated 3,6-lactonized glycosyl donors that provided moderate to high β-manno stereoselectivity in glycosylations. A method to achieve complete α-manno stereoselectivity with mannuronic acid donors equipped with 3-O-benzoyl group is also reported.

En Route to the Transformation of Glycoscience: A Chemist's Perspective on Internal and External Crossroads in Glycochemistry

Carbohydrate chemistry is an essential component of the glycosciences and is fundamental to their progress. This Perspective takes the position that carbohydrate chemistry, or glycochemistry, has reached three crossroads on the path to the transformation of the glycosciences, and illustrates them with examples from the author's and other laboratories. The first of these potential inflexion points concerns the mechanism of the glycosylation reaction and the role of protecting groups. It is argued that the experimental evidence supports bimolecular SN2-like mechanisms for typical glycosylation reactions over unimolecular ones involving stereoselective attack on naked glycosyl oxocarbenium ions. Similarly, it is argued that the experimental evidence does not support long-range stereodirecting participation of remote esters through bridged bicyclic dioxacarbenium ions in organic solution in the presence of typical counterions. Rational design and improvement of glycosylation reactions must take into account the roles of the counterion and of concentration. A second crossroads is that between mainstream organic chemistry and glycan synthesis. The case is made that the only real difference between glycan and organic synthesis is the formation of C-O rather than C-C bonds, with diastereocontrol, strategy, tactics, and elegance being of critical importance in both areas: mainstream organic chemists should feel comfortable taking this fork in the road, just as carbohydrate chemists should traveling in the opposite direction. A third crossroads is that between carbohydrate chemistry and medicinal chemistry, where there are equally many opportunities for traffic in either direction. The glycosciences have advanced enormously in the past decade or so, but creativity, input, and ingenuity of scientists from all fields is needed to address the many sophisticated challenges that remain, not the least of which is the development of a broader and more general array of stereospecific glycosylation reactions.

Glycoside Hydrolases Restrict the Side Chain Conformation of Their Substrates To Gain Additional Transition State Stabilization

Carbohydrate side chain conformation confers a significant influence on reactivity during glycosylation and anomeric bond hydrolysis due to stabilization of the oxocarbenium-like transition state. By analysis of 513 pyranoside-bound glycoside hydrolase (GH) crystal structures, we determine that most glucosidases and β-mannosidases preferentially bind their substrates in the most reactive gauche,gauche (gg) conformation, thereby maximizing stabilization of the corresponding oxocarbenium ion-like transition state during hydrolysis. α-Galactoside hydrolases mostly show a preference for the second most activating gauche,trans (gt) conformation to avoid the energy penalty that would arise from imposing the gg conformation on galacto-configured ligands. These preferences stand in stark contrast to the side chain populations observed for these sugars both in free solution and bound to nonhydrolytic proteins, where for the most part a much greater diversity of side chain conformations is observed. Analysis of sequences of GH-ligand complexes reveals that side chain restriction begins with the enzyme-substrate complex and persists through the transition state until release of the hydrolysis product, despite changes in ring conformation along the reaction coordinate. This work will inform the design of new generations of glycosidase inhibitors with restricted side chains that confer higher selectivity and/or affinity.

A versatile approach to the synthesis of mannosamine glycosides

O-Picoloyl protecting groups at remote positions can affect the stereoselectivity of glycosylation by means of the H-bond-mediated aglycone delivery (HAD) pathway. A new practical method for the stereoselective synthesis of β-glycosides of mannosamine is reported. The presence of the O-picoloyl group at the C-3 position of a mannosamine donor can provide high or complete stereocontrol. The method was also utilized for the synthesis of a biologically relevant trisaccharide related to the capsular polysaccharide of Streptococcus pneumoniae serotype 4. Also reported herein is a method to achieve complete α-manno stereoselectivity with mannosamine donors equipped with 3-O-benzoyl group.

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.

Chiral auxiliaries: Usefullness in stereoselective glycosylation reactions and their synthetic applications

Oligosaccharides play a very important role in biological system and structure-activity relationships that is why it has a lot of application to medicinal chemistry and development of polysaccharide conjugate vaccines. The stereoselective introduction of a glycosidic linkage presents the principal challenge for biological importance oligosaccharide synthesis. The main aim of this review is to described the importance of chiral auxiliary and neibhouring group participation for the stereoselective 1,2-cis glycosidic bonds formation and their application in complex oligosaccharide synthesis.Numerous 1,2-cis-linked oligosaccharides and glyconjugates are naturally found in the compounds of blood group, human milk, antigens of bacterial lipopolysaccharide etc.that predominantly increased it's importance in this field.

4,6- O-Pyruvyl Ketal Modified N-Acetylmannosamine of the Secondary Cell Wall Polysaccharide of Bacillus anthracis Is the Anchoring Residue for Its Surface Layer Proteins

The secondary cell wall polysaccharide (SCWP) of Bacillus anthracis plays a key role in the organization of the cell envelope of vegetative cells and is intimately involved in host-guest interactions. Genetic studies have indicated that it anchors S-layer and S-layer-associated proteins, which are involved in multiple vital biological functions, to the cell surface of B. anthracis. Phenotypic observations indicate that specific functional groups of the terminal unit of SCWP, including 4,6- O-pyruvyl ketal and acetyl esters, are important for binding of these proteins. These observations are based on genetic manipulations and have not been corroborated by direct binding studies. To address this issue, a synthetic strategy was developed that could provide a range of pyruvylated oligosaccharides derived from B. anthracis SCWP bearing base-labile acetyl esters and free amino groups. The resulting oligosaccharides were used in binding studies with a panel of S-layer and S-layer-associated proteins, which identified structural features of SCWP important for binding. A single pyruvylated ManNAc monosaccharide exhibited strong binding to all proteins, making it a promising structure for S-layer protein manipulation. The acetyl esters and free amine of SCWP did not significantly impact binding, and this observation is contrary to a proposed model in which SCWP acetylation is a prerequisite for association of some but not all S-layer and S-layer-associated proteins.

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.

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.

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.

Stereoselectivity of Conformationally Restricted Glucosazide Donors

Glycosylations of 4,6-tethered glucosazide donors with a panel of model acceptors revealed the effect of acceptor nucleophilicity on the stereoselectivity of these donors. The differences in reactivity among the donors were evaluated in competitive glycosylation reactions, and their relative reactivities were found to be reflected in the stereoselectivity in glycosylations with a set of fluorinated alcohols as well as carbohydrate acceptors. We found that the 2-azido-2-deoxy moiety is more β-directing than its C-2-O-benzyl counterpart, as a consequence of increased destabilization of anomeric charge development by the electron-withdrawing azide. Additional disarming groups further decreased the α-selectivity of the studied donors, whereas substitution of the 4,6-benzylidene acetal with a 4,6-di-tert-butyl silylidene led to a slight increase in α-selectivity. The C-2-dinitropyridone group was also explored as an alternative for the nonparticipating azide group, but this protecting group significantly increased β-selectivity. All studied donors exhibited the same acceptor-dependent selectivity trend, and good α-selectivity could be obtained with the weakest acceptors and most reactive donors.

Determination of the Influence of Side-Chain Conformation on Glycosylation Selectivity using Conformationally Restricted Donors

The synthesis of a series of conformationally locked mannopyranosyl thioglycosides in which the C6-O6 bond adopts either the gauche,gauche, gauche,trans, or trans,gauche conformation is described, and their influence on glycosylation stereoselectivity investigated. Two 4,6-O-benzylidene-protected mannosyl thioglycosides carrying axial or equatorial methyl groups at the 6-position were also synthesized and the selectivity of their glycosylation reactions studied to enable a distinction to be made between steric and stereoelectronic effects. The presence of an axial methoxy group at C6 in the bicyclic donor results in a decreased preference for formation of the β-mannoside, whereas an axial methyl group has little effect on selectivity. The result is rationalized in terms of through-space stabilization of a transient intermediate oxocarbenium ion by the axial methoxy group resulting in a higher degree of SN 1-like character in the glycosylation reaction. Comparisons are made with literature examples and exceptions are discussed in terms of pervading steric effects layered on top of the basic stereoelectronic effect.

Recent Advances Toward Robust N-Protecting Groups for Glucosamine as Required for Glycosylation Strategies

2-Amino-2-deoxy-d-glucose (d-glucosamine) is among the most abundant monosaccharides found in natural products. This constituent, recognized for its ubiquity, is presented in most instances as its N-acetyl derivative 2-acetamido-2-deoxy-d-glucopyranose (N-acetylglucosamine, GlcNAc, NAG). It occurs as the β-linked pyranosyl group in polysaccharides and oligosaccharides, and sometimes as the monosaccharide itself, either in its native state or as a glycoconjugate. The compound's acylation profile and other aspects of its structure are important elements in determining the variety of reactivities and functions of the molecule as a whole. Methods elaborated to investigate these challenges have been intensively reviewed; however, a relatively more comprehensive reviewing of this subject is introduced here to cover some aspects that have not been sufficiently covered. This might enable those who are beginners in this field to be aware of the subject in a more comprehensive context. 2-Amino-2-deoxy-d-glucosylation strategies demand robust amino-protecting groups that survive under a variety of chemical conditions, yet provide groups that can be deprotected under relatively mild conditions. At the end of this review, a table that includes all the N-protecting groups that have been used for glucosamine is provided to introduce them at a glance to aid in constructing building blocks that will act as useful 2-amino-2-deoxy-d-glucosyl donors.

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.

Absence of Stereodirecting Participation by 2-O-Alkoxycarbonylmethyl Ethers in 4,6-O-Benzylidene-Directed Mannosylation

The preparation of a series of mannopyranosyl donors carrying 2-O-(2-oxoalkyl) ethers and their use in glycosylation reactions are described. The formation of cyclic products with the simple 2-O-phenacyl ether and with the 2-O-(t-butoxycarbonylmethyl) ether establishes the stereoelectronic feasibility of participation in such systems. The high β-selectivities observed with the bis-trifluoromethyl phenacyl ether indicate that participation can be suppressed through the introduction of electron-withdrawing substituents. The high β-selectivities and absence of cyclic products observed with the 2-O-(methoxycarbonylmethyl) ether exclude the effective participation of esters through six-membered cyclic intermediates in this series. The results are discussed in terms of the conformation of cyclic dioxenium ions (E,E-, E,Z-, or Z,Z-) and in the context of "neighboring group" participation by nonvicinal esters in glycosylation. Methods for the deprotection of the 2-O-phenacyl and 2-O-(methoxycarbonylmethyl) ethers are described.

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.

Stereocontrolled 1,2-cis glycosylation as the driving force of progress in synthetic carbohydrate chemistry

Recent developments in stereoselective 1,2-cis glycosylation that have emerged during the past decade are surveyed herein.

Recent developments in stereoselective 1,2-cis glycosylation that have emerged during the past decade are surveyed herein. For detailed coverage of the previous achievements in the field the reader is referred to our earlier reviews: A. V. Demchenko, Curr. Org. Chem. , 2003, 7 , 35–79 and Synlett , 2003, 1225–1240.

A propos of glycosyl cations and the mechanism of chemical glycosylation; the current state of the art

An overview of recent advances in glycosylation with particular emphasis on mechanism is presented. The mounting evidence for both the existence of glycosyl oxocarbenium ions as fleeting intermediates in some reactions, and the crucial role of the associated counterion in others is discussed. The extremes of the SN1 and SN2 manifolds for the glycosylation reaction are bridged by a continuum of mechanisms in which it appears likely that most examples are located.

Investigations of scope and mechanism of nickel-catalyzed transformations of glycosyl trichloroacetimidates to glycosyl trichloroacetamides and subsequent, atom-economical, one-step conversion to α-urea-glycosides

The development and mechanistic investigation of a highly stereoselective methodology for preparing α-linked-urea neo-glycoconjugates and pseudo-oligosaccharides is described. This two-step procedure begins with the selective nickel-catalyzed conversion of glycosyl trichloroacetimidates to the corresponding α-trichloroacetamides. The α-selective nature of the conversion is controlled with a cationic nickel(II) catalyst, [Ni(dppe)(OTf)2 ] (dppe=1,2-bis(diphenylphosphino)ethane, OTf=triflate). Mechanistic studies have identified the coordination of the nickel catalyst with the equatorial C2 -ether functionality of the α-glycosyl trichloroacetimidate to be paramount for achieving an α-stereoselective transformation. A cross-over experiment has indicated that the reaction does not proceed in an exclusively intramolecular fashion. The second step in this sequence is the direct conversion of α-glycosyl trichloroacetamide products into the corresponding α-urea glycosides by reacting them with a wide variety of amine nucleophiles in presence of cesium carbonate. Only α-urea-product formation is observed, as the reaction proceeds with complete retention of stereochemical integrity at the anomeric CN bond.

Stereoselective C-glycoside formation with 2-O-benzyl-4,6-O-benzylidene protected 3-deoxy gluco- and mannopyranoside donors: comparison with O-glycoside formation

Unlike alcohols, the reaction of C-nucleophiles with 2-O-benzyl-4,6-O-benzylidene-protected 3-deoxy-gluco- and mannopyranosyl thioglycosides is highly stereoselective providing the α-C-glycosides in the gluco-series and the β-C-glycosides in the manno-series. Conformational analysis of nucleophilic attack of putative intermediate glycosyl oxocarbenium ions suggests that the observed selectivities for C-glycoside formation can be explained by preferential attack on the opposite face of the oxocarbenium to the C2-H2 bond and that eclipsing interactions with this bond are the main stereodetermining factor. It is argued that the steric interactions in the attack of alcohols (sp(3)-hybridized O) and of typical carbon-based nucleophiles (sp(2) C) on oxocarbenium ions are very different, with the former being less severe, and thus that there is no a priori reason to expect O- and C-glycosylation to exhibit parallel stereoselectivities for attack on a given oxocarbenium ion.

Cation clock permits distinction between the mechanisms of α- and β-O- and β-C-glycosylation in the mannopyranose series: evidence for the existence of a mannopyranosyl oxocarbenium ion

The use of a cationic cyclization reaction as a probe of the glycosylation mechanism has been developed and applied to the 4,6-O-benzylidene-protected mannopyranoside system. Cyclization results in the formation of both cis- and trans-fused tricyclic systems, invoking an intermediate glycosyl oxocarbenium ion reacting through a boat conformation. Competition reactions with isopropanol and trimethyl(methallyl)silane are interpreted as indicating that β-O-mannosylation proceeds via an associative S(N)2-like mechanism, whereas α-O-mannosylation and β-C-mannosylation are dissociative and S(N)1-like. Relative rate constants for reactions going via a common intermediate can be estimated.

Influence of protecting groups on the anomeric equilibrium; case of the 4,6-O-benzylidene acetal in the mannopyranose series

It is reported that the replacement of the 4- and 6-O-benzyl ethers in 2,3,4,6-tetra-O-benzyl-α,β-mannopyranose by a 4,6-O-benzylidene acetal results in an increased population of the β-anomer at equilibrium in CDCl(3) solution. The phenomenon is considered to arise from the lower steric bulk of the benzylidene acetal that, through diminished buttressing interactions, reduces steric interactions normally present in the β-anomer.

Lewis acids as α-directing additives in glycosylations by using 2,3-O-carbonate-protected glucose and galactose thioglycoside donors based on preactivation protocol

Catalytic or stoichiometric amounts of Lewis acids were found to be very effective α-directing additives in the stereoselective glycosylations of diverse 2,3-O-carbonate-protected glucose and galactose thioglycoside donors by preactivation protocol. The poor stereoselectivities of 4,6-di-O-acetyl-2,3-O-carbonate protected thioglycoside donors in glycosyl coupling reactions were greatly improved, and excellent α-stereoselectivities were achieved by the addition of 0.2 equiv of BF(3)·OEt(2). On the other hand, the β-selectivities of 4,6-di-O-benzyl-2,3-O-carbonate-protected thioglucoside donor toward glycosylations were reversed completely to the α-selectivities by the use of 1 equiv of SnCl(4), making the stereoselectivity controllable. Furthermore, the poor stereoselectivities of 4,6-di-O-benzyl-2,3-O-carbonate-protected thiogalactoside donor in glycosylations were also improved by using SnCl(4) as additive.

Methodology development and physical organic chemistry: a powerful combination for the advancement of glycochemistry

This Perspective outlines work in the Crich group on the diastereoselective synthesis of the so-called difficult classes of glycosidic bond: the 2-deoxy-β-glycopyranosides, the β-mannopyranosides, the α-sialosides, the α-glucopyranosides, and the β-arabinofuranosides with an emphasis on the critical interplay between mechanism and methodology development.