Is donor-acceptor hydrogen bonding necessary for 4,6-O-benzylidene-directed beta-mannopyranosylation? Stereoselective synthesis of beta-C-mannopyranosides and alpha-C-glucopyranosides

Neighboring-Group Participation by C-2 Acyloxy Groups: Influence of the Nucleophile and Acyl Group on the Stereochemical Outcome of Acetal Substitution Reactions

A single acyloxy group at C-2 can control the outcome of nucleophilic substitution reactions of pyran-derived acetals, but the extent of the neighboring-group participation depends on a number of factors. We show here that neighboring-group participation does not necessarily control the stereochemical outcome of acetal substitution reactions with weak nucleophiles. The 1,2-trans selectivity increased with increasing reactivity of the incoming nucleophile. This trend suggests the intermediacy of both cis-fused dioxolenium ions and oxocarbenium ions in the stereochemistry-determining step. In addition, as the electron-donating ability of the neighboring group decreased, the preference for the 1,2-trans products increased. Computational studies show how the barriers for the ring-opening reaction on the dioxolenium ions and the transition states to provide the oxocarbenium ions change with the electron-donating capacity of the C-2-acyloxy group and the reactivity of the nucleophile.

Computational evidence of glycosyl cations

Glycosyl cations are key intermediates in the glycosylation reactions taking place through a S_N1-type mechanism. To obtain a reliable description of the glycosylation reaction mechanism a combination of computational studies and experimental data such as kinetic isotopic effects is needed. Computational studies have elucidated S_N2-type glycosylation reaction mechanisms, but elucidation of mechanisms in which ion pairs can be formed presents some difficulties because of the recombination of the ions. Recent topological and dynamic studies open the door to the ultimate confirmation of the presence of glycosyl cations in the form of intimate ion pairs during certain glycosylation reactions. This review covers the state-of-the-art tools and applications of computational chemistry mainly developed during the last ten years to understand glycosylation reactions in which an oxocarbenium ion could be involved.

Mild Cu(OTf)2-Mediated C-Glycosylation with Chelation-Assisted Picolinate as a Leaving Group

C-Glycosylation reactions of glycosyl picolinates with allyltrimethylsilane or silyl enol ethers were developed. Picolinate as a chelation-assisted leaving group could be activated by Cu(OTf)2 and avoided the use of harsh Lewis acids. The glycosylations were operated under mild neutral conditions and gave the corresponding C-glycosides in up to 95% yield with moderate to excellent stereoselectivities.

Chemical Synthesis Elucidates the Key Antigenic Epitope of the Autism-Related Bacterium Clostridium bolteae Capsular Octadecasaccharide

The gut pathogen Clostridium bolteae has been associated with the onset of autism spectrum disorder (ASD). To create vaccines against C. bolteae, it is important to identify exact protective epitopes of the immunologically active capsular polysaccharide (CPS). Here, a series of C. bolteae CPS glycans, up to an octadecasaccharide, was prepared. Key to achieving the total syntheses is a [2+2] coupling strategy based on a β-d-Rhap-(1→3)-α-d-Manp repeating unit that in turn was accessed by a stereoselective β-d-rhamnosylation. The 4,6-O-benzylidene-induced conformational locking is a powerful strategy for forming a β-d-mannose-type glycoside. An indirect strategy based on C2 epimerization of β-d-quinovoside was efficiently achieved by Swern oxidation and borohydride reduction. Sequential glycosylation, and regioselective and global deprotection produced the disaccharide and tetrasaccharide, up to the octadecasaccharide. Glycan microarray analysis of sera from rabbits immunized with inactivated C. bolteae bacteria revealed a humoral immune response to the di- and tetrasaccharide, but none of the longer sequences. The tetrasaccharide may be a key motif for designing glycoconjugate vaccines against C. bolteae.

Photoinitiated Thiol-ene Reactions of Enoses: A Powerful Tool for Stereoselective Synthesis of Glycomimetics with Challenging Glycosidic Linkages

Thioglycosides and C-glycosides represent pharmacologically useful classes of glycomimetics that possess a high degree of biological stability. One emerging tool for the stereoselective synthesis of thioglycosides is the photoinitiated addition of thiols to unsaturated sugars. Moreover, thiyl radical-mediated reactions of exo-glycals and 1-substituted endo-glycals offer facile routes to β-C-glycosidic structures. This Concept article summarizes the thiol-ene coupling strategies developed recently by our group and Somsák's group for the synthesis of several kinds of glycomimetics which are difficult to synthesize by conventional methods. One unusual characteristic of the thiol-ene reactions of endo-glycals is that heating inhibits, whereas cooling promotes the reaction. This unique temperature dependence as well as the effects of the enose structures and thiol configurations on the efficacy and stereoselectivity of the reactions are also discussed.

C-Glycosylation enabled by N-(glycosyloxy)acetamides

The C-glycosylation of C-nucleophiles including allyltrimethylsilane, silyl enol ethers and phenols with N-(glycosyloxy)acetamides as glycosyl donors has been realized. This protocol provides a convenient and practical route for the synthesis of alkyl C-glycosides and aryl 2-deoxy-β-C-glycosides under mild reaction conditions.

Manual and Automated Syntheses of the N-Linked Glycoprotein Core Glycans

Presented herein are two complementary approaches to the synthesis of the core N-glycan pentasaccharide. The first, a traditional manual approach in solution, makes use of the H-bond-mediated aglycone delivery method for the highly diastereoselective introduction of the β-mannosidic linkage at room temperature. The synthesis of the core pentasaccharide was also accomplished using an high-performance liquid chromatography-assisted automated approach. The overall assembly was swift (8 h) and efficient (31%).

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.

Venturing beyond Donor-Controlled Glycosylation: New Perspectives toward Anomeric Selectivity

Glycans are complex compounds consisting of sugars linked glycosidically, existing either as pure polysaccharides or as part of glycoconjugates. They are prevalent in nature and possess important functions in regulating biological pathways. However, their diversity coupled with physiochemical similarities makes it challenging to isolate them in large quantities for biochemical studies, hence hampering progress in glycobiology and glycomedicine. Glycochemistry presents an alternative strategy to obtain pure glycan compounds through artificial synthetic methods. Efforts in glycochemistry have been centered on glycosylation, the key reaction in glycochemistry, especially with regards to anomeric stereoselectivity in polysaccharides and glycoconjugates. In particular, the stereoelectronic and steric properties of glycosyl donors are commonly used to direct the stereoselectivity in glycosylation reactions. Classic glycosylation strategies typically involve saturated glycosyl donors, proceeding either directly using hydrogen bonds and conformational constraints or indirectly by installing moieties covalently through neighboring group participation and intramolecular aglycon delivery. Over the past years, new glycosylation strategies, tapping on the foundations of transition metal catalysis, have emerged. To leverage the power of coordination chemistry, unsaturated glycosyl donors were introduced. Not only are the number of protection/deprotection steps reduced, the resultant unsaturated glycoside provides opportunities for downstream functionalizations, allowing quick access to a variety of sugars, including rare sugars. Alongside the glycosyl donor, an equally important but neglected aspect for targeting stereoselective glycosylation is the glycosyl acceptor. In the case of dual-directing donors, glycosyl acceptors have proved themselves capable of becoming the dominating factor for stereocontrol. Interestingly, rational manipulation or selection of glycosyl acceptors with particular nucleophilicity and p K_a values can lead to different stereoselectivities, thereby proving the tunability of such acceptors to favor the formation of one anomer over the other stereoselectively. By further venturing beyond substrate controlled stereoselectivity, we are presented with the opportunity to effect stereoselective glycosylation through glycosylating reagents. Of the key reagents, stereoselective catalyst stands out as a greener and efficient alternative to direct stereoselective control with stoichiometric substrates. Recently, investigations into this approach of stereocontrol presented an intriguing range of stereoselectivities, achieved by merely varying the nature of catalysts used. Another crucial effort in glycochemistry is enhancing the efficiencies of glycosylations, by reducing the number of preparative steps before or during glycosylation. Through using transient masking groups or one-pot synthetic strategies, these streamlined approaches provide enormous convenience and practicability for oligosaccharide syntheses. This Account presents mainly our advancements beyond the conventional donor-controlled strategies over the past decade, with emphasis placed on mechanistic explanations of anomeric selectivities, thereby providing perspectives to inspire further progress toward a generalized unified strategy for preparing every type of glycan.

Synthesis of Conformationally-Locked cis- and trans-Bicyclo[4.4.0] Mono-, Di-, and Trioxadecane Modifications of Galacto- and Glucopyranose; Experimental Limiting 3JH,H Coupling Constants for the Estimation of Carbohydrate Side Chain Populations and Beyond

Hexopyranose side chains populate three staggered conformations, whose proportions can be determined from the three sets of ideal limiting 3JH5,H6R and 3JH5,H6S coupling constants in combination with the time-averaged experimental coupling constants. Literature values for the limiting coupling constants, obtained by the study of model compounds, the use of the Haasnoot-Altona and related equations, or quantum mechanical computations, can result in computed negative populations of one of the three ideal conformations. Such values arise from errors in the limiting coupling constants and/or from the population of nonideal conformers. We describe the synthesis and analysis of a series of cis- and trans-fused mono-, di-, and trioxabicyclo[4.4.0]octane-like compounds. Correction factors for the application of data from internal models (-CH(OR)-CH(OR)-) to terminal systems (-CH(OR)-CH2(OR)) are deduced from comparison of further models, and applied where necessary. Limiting coupling constants so-derived are applied to the side chain conformations of three model hexopyranosides, resulting in calculated conformer populations without negative values. Although, developed primarily for hexopyranose side chains, the limiting coupling constants are suitable, with the correction factors presented, for application to the side chains of higher carbon sugars and to conformation analysis of acyclic diols and their derivatives in a more general sense.

The influence of acceptor nucleophilicity on the glycosylation reaction mechanism

A set of model nucleophiles of gradually changing nucleophilicity is used to probe the glycosylation reaction mechanism. Glycosylations of ethanol-based acceptors, bearing varying amounts of fluorine atoms, report on the dependency of the stereochemistry in condensation reactions on the nucleophilicity of the acceptor. Three different glycosylation systems were scrutinized, that differ in the reaction mechanism, that - putatively - prevails during the coupling reaction. It is revealed that the stereoselectivity in glycosylations of benzylidene protected glucose donors are very susceptible to acceptor nucleophilicity whereas condensations of benzylidene mannose and mannuronic acid donors represent more robust glycosylation systems in terms of diastereoselectivity. The change in stereoselectivity with decreasing acceptor nucleophilicity is related to a change in reaction mechanism shifting from the SN2 side to the SN1 side of the reactivity spectrum. Carbohydrate acceptors are examined and the reactivity-selectivity profile of these nucleophiles mirrored those of the model acceptors studied. The set of model ethanol acceptors thus provides a simple and effective "toolbox" to investigate glycosylation reaction mechanisms and report on the robustness of glycosylation protocols.

Theoretical Investigation of Solvent Effects on Glycosylation Reactions: Stereoselectivity Controlled by Preferential Conformations of the Intermediate Oxacarbenium-Counterion Complex

The mechanism of solvent effects on the stereoselectivity of glycosylation reactions is investigated using quantum-mechanical (QM) calculations and molecular dynamics (MD) simulations, considering a methyl-protected glucopyranoside triflate as a glycosyl donor equivalent and the solvents acetonitrile, ether, dioxane, or toluene, as well as gas-phase conditions (vacuum). The QM calculations on oxacarbenium-solvent complexes do not provide support to the usual solvent-coordination hypothesis, suggesting that an experimentally observed β-selectivity (α-selectivity) is caused by the preferential coordination of a solvent molecule to the reactive cation on the α-side (β-side) of the anomeric carbon. Instead, explicit-solvent MD simulations of the oxacarbenium-counterion (triflate ion) complex (along with corresponding QM calculations) are compatible with an alternative mechanism, termed here the conformer and counterion distribution hypothesis. This new hypothesis suggests that the stereoselectivity is dictated by two interrelated conformational properties of the reactive complex, namely, (1) the conformational preferences of the oxacarbenium pyranose ring, modulating the steric crowding and exposure of the anomeric carbon toward the α or β face, and (2) the preferential coordination of the counterion to the oxacarbenium cation on one side of the anomeric carbon, hindering a nucleophilic attack from this side. For example, in acetonitrile, the calculations suggest a dominant B2,5 ring conformation of the cation with preferential coordination of the counterion on the α side, both factors leading to the experimentally observed β selectivity. Conversely, in dioxane, they suggest a dominant (4)H3 ring conformation with preferential counterion coordination on the β side, both factors leading to the experimentally observed α selectivity.

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.

In a glycosylation reaction how does a hydroxylic nucleophile find the activated anomeric carbon?

The mechanism by which nucleophilic hydroxyls are attracted to activated glycopyranosyl donors is not known. Besides the intrinsic attraction of oxygen centred negative dipoles towards the developing electron deficiency at the anomeric carbon only a few suggestions have been given in the literature. By studying the effect on Density Functional Theory (DFT) modelled glycosylation reactions on the presence of polar additives as tested with acetonitrile two possible effects have been identified. One was noted in a previous publication (Carbohydr. Res.2012, 356, 180-190) and two further examples discovered here that suggest that a lone pair of a nucleophile approaching a donor with a β-leaving group from the α-face can act as the antiperiplanar lone pair that assists leaving group departure. This interaction starts at just under a nucleophile C-1 separation of 3Å and has an incipient bond angle of O-5-C-1-Nuc(O or N) of very close to 90° which can be at C-1 with the p-type orbital at C-1-O-5 of the incipient oxacarbenium ion, that is, the LUMO of the activated donor. The 2nd interaction is less well studied and is suggested to be a similar bonding interaction which moves β-face nucleophiles to O-Nuc-C-1-leaving groups angles close to 180°.

Hydrogen-bond-mediated aglycone delivery: focus on β-mannosylation

O-Picoloyl groups at remote positions can mediate the course of glycosylation reactions by providing high facial selectivity for the H-bond-mediated attack of the glycosyl acceptor. A new practical method for the stereoselective synthesis of β-mannosides at ambient temperature is presented.

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.

Dissecting the mechanisms of a class of chemical glycosylation using primary ¹³C kinetic isotope effects

Although arguably the most important reaction in glycoscience, chemical glycosylations are among the least well understood of organic chemical reactions, resulting in an unnecessarily high degree of empiricism and a brake on rational development in this critical area. To address this problem, primary (13)C kinetic isotope effects have now been determined for the formation of β- and α-manno- and glucopyranosides using a natural abundance NMR method. In contrast to the common current assumption, for three of the four cases studied the experimental and computed values are indicative of associative displacement of the intermediate covalent glycosyl trifluoromethanesulfonates. For the formation of the α-mannopyranosides, the experimentally determined KIE differs significantly from that computed for an associative displacement, which is strongly suggestive of a dissociative mechanism that approaches the intermediacy of a glycosyl oxocarbenium ion. The application of analogous experiments to other glycosylation systems should shed further light on their mechanisms and thus assist in the design of better reactions conditions with improved stereoselectivity.

Do glycosyl sulfonium ions engage in neighbouring-group participation? A study of oxathiane glycosyl donors and the basis for their stereoselectivity

Neighbouring-group participation has long been used to control the synthesis of 1,2-trans-glycosides. More recently there has been a growing interest in the development of similar strategies for the synthesis of 1,2-cis-glycosides, in particular the use of auxiliary groups that generate sulfonium ion intermediates. However, there has been some debate over the role of sulfonium ion intermediates in these reactions: do sulfonium ions actually engage in neighbouring-group participation, or are they a resting state of the system prior to reaction through an oxacarbenium ion intermediate? Herein, we describe the reactivities and stereoselectivities of a family of bicyclic thioglycosides in which an oxathiane ring is fused to the sugar to form a trans-decalin-like structure. A methyl sulfonium ion derived from one such glycosyl donor is so stable that it can be crystallised from ethanol, yet it reacts with complete stereoselectivity at high temperature. The importance of a ketal group in the oxathiane ring for maintaining this high stereoselectivity is investigated using a combination of experiment and ab initio calculations. The data are discussed in terms of S(N)1 and S(N)2 type mechanisms. Trends in stereoselectivity across a series of compounds are more consistent with selective addition to oxacarbenium ions rather than a shift between S(N)1 and S(N)2 mechanisms.

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.

Influence of the O3 protecting group on stereoselectivity in the preparation of C-mannopyranosides with 4,6-O-benzylidene protected donors

α-C-Glucopyranosides and mannopyranosides are obtained in 65-85% yields from 4,6-O-benzylidene-protected glucosyl and mannosyl thioglycosides bearing ester functionality at the 3-O-position by a coupling reaction with C-nucleophiles on activation with diphenyl sulfoxide, 2,4,6-tri-tert-butylpyrimidine, and trifluoromethanesulfonic anhydride.

Mechanism of a chemical glycosylation reaction

Glycosylation is arguably the most important reaction in the field of glycochemistry, yet it involves one of the most empirically interpreted mechanisms in the science of organic chemistry. The beta-mannopyranosides, long considered one of the more difficult classes of glycosidic bond to prepare, were no exception to this rule. A number of logical but circuitous routes for their preparation were described in the literature, but they were accompanied by an even greater number of mostly ineffective recipes with which to access them directly. This situation changed in 1996 with the discovery of the 4,6-O-benzylidene acetal as a control element permitting direct entry into the beta-mannopyranosides, typically with high yield and selectivity. The unexpected nature of this phenomenon demanded study of the mechanism, leading first to the demonstration of the alpha-mannopyranosyl triflates as reaction intermediates and then to the development of alpha-deuterium kinetic isotope effect methods to probe their transformation into the product glycosides. In this Account, we assemble our observations into a comprehensive assessment consistent with a single mechanistic scheme. The realization that in the glucopyranose series the 4,6-O-benzylidene acetal is alpha- rather than beta-directing led to further investigations of substituent effects on the stereoselectivity of these glycosylation reactions, culminating in their explanation in terms of the covalent alpha-glycosyl triflates acting as a reservoir for a series of transient contact and solvent-separated ion pairs. The function of the benzylidene acetal, as explained by Bols and co-workers, is to lock the C6-O6 bond antiperiplanar to the C5-O5 bond, thereby maximizing its electron-withdrawing effect, destabilizing the glycosyl oxocarbenium ion, and shifting the equilibria as far as possible toward the covalent triflate. beta-Selective reactions result from attack of the nucleophile on the transient contact ion pair in which the alpha-face of the oxocarbenium ion is shielded by the triflate counterion. The alpha-products arise from attack either on the solvent-separated ion pair or on a free oxocarbenium ion, according to the dictates of the anomeric effect. Changes in selectivity from varying stereochemistry (glucose versus mannose) or from using different protecting groups can be explained by the shifting position of the key equilibria and, in particular, by the energy differences between the covalent triflate and the ion pairs. Of particular note is the importance of substitutents at the 3-position of the donor; an explanation is proposed that invokes their evolving torsional interaction with the substituent at C2 as the chair form of the covalent triflate moves toward the half-chair of the oxocarbenium ion.

Erosion of stereochemical control with increasing nucleophilicity: O-glycosylation at the diffusion limit

Nucleophilic substitution reactions of 2-deoxyglycosyl donors indicated that the reactivity of the oxygen nucleophile has a significant impact on stereoselectivity. Employing ethanol as the nucleophile resulted in a 1:1 (alpha:beta) ratio of diastereomers under S(N)1-like reaction conditions. Stereoselective formation of the 2-deoxy-alpha-O-glycoside was only observed when weaker nucleophiles, such as trifluoroethanol, were employed. The lack of stereoselectivity observed in reactions of common oxygen nucleophiles can be attributed to reaction rates of the stereochemistry-determining step that approach the diffusion limit. In this scenario, both faces of the prochiral oxocarbenium ion are subject to nucleophilic addition to afford a statistical mixture of diastereomeric products. Control experiments confirmed that all nucleophilic substitution reactions were performed under kinetic control.

Correlations between nucleophilicities and selectivities in the substitutions of tetrahydropyran acetals

Selectivities that deviate from S(N)1 stereoelectronic models in the nucleophilic substitutions of tetrahydropyran acetals were investigated. When weak nucleophiles were employed, stereoselectivities conformed to known S(N)1 stereoelectronic models. In contrast, stereoselectivities in the substitutions of acetals with strong nucleophiles depended on reaction conditions. Erosions in selectivities were observed when strong nucleophiles were employed in the absence of coordinating counterions. These erosions in selectivities are attributed to rates of nucleophilic additions to oxocarbenium ion intermediates that approach the diffusion limit. When triflate counterions were present, however, S(N)2-like pathways became accessible with strong nucleophiles. In most cases examined, the major stereoisomers formed from reactions that proceeded through S(N)2-like pathways were opposite to the major stereoisomers formed from the analogous reactions that proceeded through S(N)1 pathways.

Stereocontrolled synthesis of D- and L-beta-rhamnopyranosides with 4-O-6-S-alpha-cyanobenzylidene-protected 6-thiorhamnopyranosyl thioglycosides

The synthesis of both enantiomers of a 4-O-6-S-alpha-cyanobenzylidene-protected 6-thiorhamnopyranosyl thioglycoside is described starting from D-mannnose and L-arabinose derivatives for the D- and L-series, respectively. This donor is effective in the preparation of the corresponding beta-glycosides using the 1-benzenesulfinyl piperidine/trifluoromethanesulfonic anhydride protocol. Following desulfurization and concomitant debenzylation with Raney nickel, the so-formed 6-thio-beta-mannosides are converted in high yield to the beta-rhamnopyranosides.