4,6-O-benzylidene-directed beta-mannopyranosylation and alpha-glucopyranosylation: the 2-deoxy-2-fluoro and 3-deoxy-3-fluoro series of donors and the importance of the O2-C2-C3-O3 interaction

Stereoselective synthesis of photoactivatable Man(β1,4)GlcNAc-based bioorthogonal probes

We report an operationally facile protocol to prepare photoactivatable probes of the bioactive mammalian disaccharide, Man(β1,4)GlcNAc. Using conformationally restricted mannosyl hemi-acetal donors in a one-pot chlorination, iodination and glycosylation sequence, β-mannosides were generated in excellent diastereoselectivities and yields. Upon accessing the disaccharide, we generated the corresponding photoactivatable probes by appending a diazirine-alkyne equipped linker via a condensation reaction between a diazirine-containing linker and C-1 and C-2 derivatized mannosylamines to furnish the desired C-1 and C-2 modified Man(β1,4)GlcNAc-based probes. This new synthetic protocol greatly simplifies the preparation of this important bioactive disaccharide to enable future work to identify its protein binding partners in cells.

Synthesis of the Branched Tetrasaccharide Fragment of Saccharomicin A

A synthesis of the branched tetrasaccharide fragment of saccharomicin A using 1-OTBS donors to stereoselectively synthesize both α- and β-linked disaccharides is reported. The disaccharides were united using BSP/Tf₂O to afford the tetrasaccharide fragment as a single α-anomer in 72% yield. This branched tetrasaccharide fragment can be used as donor and acceptor species to synthesize larger fragments of saccharomicin A.

Chemical Synthesis of Homogeneous Human E-Cadherin N-Linked Glycopeptides: Stereoselective Convergent Glycosylation and Chemoselective Solid-Phase Aspartylation

We report herein an efficient chemical synthesis of homogeneous human E-cadherin N-linked glycopeptides consisting of a heptapeptide sequence adjacent to the Asn-633 N-glycosylation site with representative N-glycan structures, including a conserved trisaccharide, a core-fucosylated tetrasaccharide, and a complex-type biantennary octasaccharide. The key steps are a chemoselective on-resin aspartylation using a pseudoproline-containing peptide and stereoselective glycosylation using glycosyl fluororide as a donor. This synthetic strategy demonstrates potential utility in accessing a wide range of homogeneous N-linked glycopeptides for the examination of their biological function.

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.

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.

Pondering the Structural Factors that Affect 1,2-trans-Galactosylation: A Lesson Learnt from 3-O-β-Galactosylation of Galactosamine

Stereoselective formation of glycosidic bonds remains one of the most challenging topics in carbohydrate chemistry. The predominant method for stereoselective construction of 1,2-trans-glycosidic bonds is through the neighboring group participation effect (NGPE), which proved to be less successful in synthesizing Galβ(1→3)GalNAc disaccharide. The steric effect that overshadows NGPE and the impacts of substituents at the 3-O- and 2-N-positions of donors and acceptors, respectively, on this synthesis were systematically examined to lead to some practical guidelines for choosing protecting groups towards the successful synthesis of Galβ(1→3)GalNAc and similar disaccharides.

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.

Is an acyl group at O-3 in glucosyl donors able to control α-stereoselectivity of glycosylation? The role of conformational mobility and the protecting group at O-6

The stereodirecting effect of a 3-O-acetyl protecting group, which is potentially capable of the remote anchimeric participation, and other protecting groups in 2-O-benzyl glucosyl donors with flexible and rigid conformations has been investigated. To this aim, an array of N-phenyltrifluoroacetimidoyl and sulfoxide donors bearing either 3-O-acetyl or 3-O-benzyl groups in combination with 4,6-di-O-benzyl, 6-O-acyl-4-O-benzyl, or 4,6-O-benzylidene protecting groups was prepared. The conformationally flexible 3-O-acetylated glucosyl donor protected at other positions with O-benzyl groups demonstrated very low or no α-stereoselectivity upon glycosylation of primary or secondary acceptors. On the contrary, 3,6-di-O-acylated glucosyl donors proved to be highly α-stereoselective as well as the donor having a single potentially participating acetyl group at O-6. The 3,6-di-O-acylated donor was shown to be the best α-glucosylating block for the primary acceptor, whereas the best α-selectivity of glycosylation of the secondary acceptor was achieved with the 6-O-acylated donor. Glycosylation of the secondary acceptor with the conformationally constrained 3-O-acetyl-4,6-O-benzylidene-protected donor displayed under standard conditions (-35°C) even lower α-selectivity as compared to the 3-O-benzyl analogue. However, increasing the reaction temperature essentially raised the α-stereoselectivities of glycosylation with both 3-O-acetyl and 3-O-benzyl donors and made them almost equal. The stereodirecting effects of protecting groups observed for N-phenyltrifluoroacetimidoyl donors were also generally proven for sulfoxide donors.

Synthesis and evaluation of 3-deoxy and 3-deoxy-3-fluoro derivatives of gluco- and manno-configured tetrahydropyridoimidazole glycosidase inhibitors

Three tetrahydropyridoimidazole-type glycosidase inhibitors have been synthesized with the 3-deoxy ribo- and arabino-, and 3-deoxy-3-fluoro gluco-configurations and two of them screened for activity against α- and β-gluco- and mannosidase enzymes. Only one substance, the 3-deoxy-3-fluoro-derivative of the gluco-configured tetrahydropyridoimidazole was found to have any activity against a single enzyme, sweet almond β-glucosidase, and even then at a level 100-fold lower than that of the corresponding simple gluco-configured tetrahydropyridoimidazole thereby underlining the importance of the 3-hydroxy group in the key substrate-enzyme interactions.

Fluorine-directed β-galactosylation: chemical glycosylation development by molecular editing

Validation of the 2-fluoro substituent as an inert steering group to control chemical glycosylation is presented. A molecular editing study has revealed that the exceptional levels of diastereocontrol in glycosylation processes by using 2-fluoro-3,4,6-tri-O-benzyl glucopyranosyl trichloroacetimidate (TCA) scaffolds are a consequence of the 2R,3S,4S stereotriad. This study has also revealed that epimerization at C4, results in a substantial enhancement in β-selectivity (up to β/α 300:1).

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.

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.

Novel galactosyl donor with 2-naphthylmethyl (NAP) as the non participating group at C-2 position: Efficient synthesis of alpha-galactosyl ceramide

Predominant alpha-linked products can be generated in glycosylation involving galactosyl trichloroacetimidate donors with 2-naphthylmethyl (NAP) as the non participating group at C-2 position. The above donor was successfully utilized for the synthesis of alpha-galactosyl ceramide.

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.

Stereodirecting effect of the pyranosyl C-5 substituent in glycosylation reactions

The stereodirecting effect of the glycosyl C-5 substituent has been investigated in a series of d-pyranosyl thioglycoside donors and related to their preferred positions in the intermediate (3)H(4) and (4)H(3) half-chair oxacarbenium ions. Computational studies showed that an axially positioned C-5 carboxylate ester can stabilize the (3)H(4) half-chair oxacarbenium ion conformer by donating electron density from its carbonyl function into the electron-poor oxacarbenium ion functionality. A similar stabilization can be achieved by a C-5 benzyloxymethyl group, but the magnitude of this stabilization is significantly smaller than for the C-5 carboxylate ester. As a result, the preference of the C-5 benzyloxymethyl to occupy an axial position in the half-chair oxacarbenium ions is much reduced compared to the C-5 carboxylate ester. To minimize steric interactions, a C-5 methyl group prefers to adopt an equatorial position and therefore favors the (4)H(3) half-chair oxacarbenium ion. When all pyranosyl substituents occupy their favored position in one of the two intermediate half-chair oxacarbenium ions, highly stereoselective glycosylations can be achieved as revealed by the excellent beta-selectivity of mannuronate esters and alpha-selectivity of 6-deoxygulosides.

Does neighboring group participation by non-vicinal esters play a role in glycosylation reactions? Effective probes for the detection of bridging intermediates

Neighboring group participation in glycopyranosylation reactions is probed for esters at the 3-O-axial and -equatorial, 4-O-axial and -equatorial, and 6-O-sites of a range of donors through the use tert-butoxycarbonyl esters. The anticipated intermediate cyclic dioxanyl cation is interrupted for the axial 3-O-derivative, leading to the formation of a 1,3-O-cyclic carbonate ester, with loss of a tert-butyl cation, providing convincing evidence of participation by esters at that position. However, no evidence was found for such a fragmentation of carbonate esters at the 3-O-equatorial, 4-O-axial and -equatorial, and 6-O positions, indicating that neighboring group participation from those sites does not occur under typical glycosylation conditions. Further probes employing a 4-O-(2-carboxy)benzoate ester and a 4-O-(4-methoxybenzoate) ester, the latter in conjunction with an (18)O quench designed to detect bridging intermediates, also failed to provide evidence for participation by 4-O-esters in galactopyranosylation.

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

2,3-Di-O-benzyl-4,6-O-benzylidene-thiohexopyranosides, on activation with 1-benzenesulfinyl piperidine and triflic anhydride, react with allyl silanes and stannanes, and with silyl enolethers to give C-glycosides. In the mannose series the beta-isomers are formed selectively whereas the glucose series provides the alpha-anomers. This selectivity pattern parallels that of O-glycoside formation and eliminates the need to consider donor-acceptor hydrogen bonding in the formation of the O-glycosides.

Continuum of mechanisms for nucleophilic substitutions of cyclic acetals

The effect of nucleophile strength on diastereoselectivity in the nucleophilic substitution of cyclic acetals was explored. Stereoselectivity remained constant and high as nucleophilicity increased until a threshold value was reached. Beyond this point, however, selection of Lewis acid determined whether stereochemical inversion or erosion was observed.

Convergent synthesis of a beta-(1-->3)-mannohexaose

An as yet unknown beta-(1-->3)-mannohexaose has been synthesized by a block route involving the coupling of two trisaccharides. Comparison of three closely related attempted mannohexaose syntheses reinforces the influence of subtle matching and/or mismatching interactions on the outcome of convergent oligosaccharide synthesis.