Simple Synthesis of 1,2-Dideoxy-2-Vinyl Carbohydrates by Tin-Free Radical Reactions of Xanthates

Vinyl-substituted carbohydrates have been synthesized from glycals derived from hexoses and pentoses. Key step is the radical reaction of xanthates in the presence of triethylborane, a non-toxic reagent. The mechanism has been investigated by isolation of various side products, which speak for a reversibility of the cyclopropylmethyl radical ring-opening. Compared to reactions with tributyltin hydride, higher regioselectivities in favor of the 2-vinyl-substituted sugars have been obtained. Yields are slightly lower with triethylborane, but all products have been isolated in analytically pure form. The new reaction is applicable to benzyl- and silyl-protected carbohydrates, which makes free sugars accessible as well. Overall, more than 15 1,2-dideoxy-2-vinyl carbohydrates have been synthesized from simple precursors in only few steps.

First synthesis of an ABCE ring substructure of daphnicyclidin A

The ABCE tetracyclic ring system of daphnicyclidin A was prepared using an intramolecular (4 + 3) cycloaddition of an oxidopyridinium ion as the key step.

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

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

Guidelines for O-Glycoside Formation from First Principles

With a view to reducing the notorious complexity and irreproducibility of glycosylation reactions, 12 guidelines for the choice of concentration, temperature, and counterions are adumbrated.

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.

The Cyclic Nitronate Route to Pharmaceutical Molecules: Synthesis of GSK's Potent PDE4 Inhibitor as a Case Study

An efficient asymmetric synthesis of GlaxoSmithKline’s potent PDE4 inhibitor was accomplished in eight steps from a catechol-derived nitroalkene. The key intermediate (3-acyloxymethyl-substituted 1,2-oxazine) was prepared in a straightforward manner by tandem acylation/(3,3)-sigmatropic rearrangement of the corresponding 1,2-oxazine-N-oxide. The latter was assembled by a (4 + 2)-cycloaddition between the suitably substituted nitroalkene and vinyl ether. Facile acetal epimerization at the C-6 position in 1,2-oxazine ring was observed in the course of reduction with NaBH₃CN in AcOH. Density functional theory (DFT) calculations suggest that the epimerization may proceed through an unusual tricyclic oxazolo(1,2)oxazinium cation formed via double anchimeric assistance from a distant acyloxy group and the nitrogen atom of the 1,2-oxazine ring.

Total synthesis of theHelicobacter pyloriserotype O2 O-antigen α-(1 → 2)- and α-(1 → 3)-linked oligoglucosides

Unique α-(1 → 2)- and α-(1 → 3)-linked oligoglucosides from theH. pyloriserotype O2 O-antigen were synthesized with exclusive α-selectivity using remote participation effects.

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.

Stereocontrolled Synthesis of α-Xylofuranosides Using a Conformationally Restricted Donor

A number of biologically relevant glycoconjugates possess 1,2- cis-furanosidic linkages, a class of glycosidic bond that remains challenging to introduce with high stereoselectivity. In this paper, we report an approach to one family of such linkages, α-xylofuranosides, via the use of thioglycoside donors possessing a conformationally restricting xylylene protecting group. The method was shown to provide the desired targets in good to excellent yield and stereoselectivity. Computational investigations support the proposal that the protecting group locks the electrophilic intermediate in these reactions into a conformation that leads to the high selectivity. The power of the methodology was demonstrated through the synthesis of a complex hexasaccharide motif from lipoarabinomannan, an immunomodulatory polysaccharide from mycobacteria.

Stereodivergent Mannosylation Using 2- O-( ortho-Tosylamido)benzyl Group

We report a novel strategy for obtaining both anomers from a single mannosyl donor equipped with a C2- o-TsNHbenzyl ether (2- O-TAB) by switching reaction conditions. In particular, the formation of various β-mannosides was achieved with high selectivity by using a mannosyl phosphite in the presence of ZnI₂.

Bimodal Glycosyl Donors Protected by 2- O-( ortho-Tosylamido)benzyl Group

A glucosyl donor equipped with C2- o-TsNHbenzyl ether was shown to provide both α- and β-glycosides stereoselectivity, by changing the reaction conditions. Namely, β-glycosides were selectively obtained when the trichloroacetimidate was activated by Tf₂NH. On the other hand, activation by TfOH in Et₂O provided α-glycosides as major products. This "single donor" approach was employed to assemble naturally occurring trisaccharide α-d-Glc-(1→2)-α-d-Glc-(1→6)-d-Glc and its anomers.

Stereocontrolled Synthesis of α-Xylofuranosides Using a Conformationally Restricted Donor

A number of biologically relevant glycoconjugates possess 1,2-cis-furanosidic linkages, a class of glycosidic bond that remains challenging to introduce with high stereoselectivity. In this paper, we report an approach to one family of such linkages, α-xylofuranosides, via the use of thioglycoside donors possessing a conformationally restricting xylylene protecting group. The method was shown to provide the desired targets in good to excellent yield and stereoselectivity. Computational investigations support the proposal that the protecting group locks the electrophilic intermediate in these reactions into a conformation that leads to the high selectivity. The power of the methodology was demonstrated through the synthesis of a complex hexasaccharide motif from lipoarabinomannan, an immunomodulatory polysaccharide from mycobacteria.

Synthesis and intramolecular glycosylation of sialyl mono-esters of o-xylylene glycol. The importance of donor configuration and nitrogen protecting groups on cyclization yield and selectivity; isolation and characterization of a N-sialyl acetamide indicative of participation by acetonitrile

The synthesis and cyclization reactions, leading to spirocyclic medium ring-sized diolides, of o-(hydroxymethyl)xylylene monoesters of sialyl thioglycosides is described. Cyclization yields and stereoselectivities are found to vary as a function of the anomeric stereochemistry of the thioglycoside and of the N5 protecting group, and these effects are discussed in terms of the reaction mechanism. Cyclization in the presence of acetonitrile results in the isolation and characterization of a Ritter-type N-sialyl acetamide, which affords strong evidence for the participation of acetonitrile in the form of sialyl nitrilium ions.

Mechanism of Glycosylation of Anomeric Sulfonium Ions

Anomeric sulfonium ions are attractive glycosyl donors for the stereoselective installation of 1,2-cis glycosides. Although these donors are receiving increasing attention, their mechanism of glycosylation remains controversial. We have investigated the reaction mechanism of glycosylation of a donor modified at C-2 with a (1S)-phenyl-2-(phenylsulfanyl)ethyl chiral auxiliary. Preactivation of this donor results in the formation of a bicyclic β-sulfonium ion that after addition of an alcohol undergoes 1,2-cis-glycosylation. To probe the importance of the thiophenyl moiety, analogs were prepared in which this moiety was replaced by an anisoyl or benzyl moiety. Furthermore, the auxiliaries were installed as S- and R-stereoisomers. It was found that the nature of the heteroatom and chirality of the auxiliary greatly influenced the anomeric outcome and only the one containing a thiophenyl moiety and having S-configuration gave consistently α-anomeric products. The sulfonium ions are sufficiently stable at a temperature at which glycosylations proceed indicating that they are viable glycosylation agents. Time-course NMR experiments with the latter donor showed that the initial rates of glycosylations increase with increases in acceptor concentration and the rate curves could be fitted to a second order rate equation. Collectively, these observations support a mechanism by which a sulfonium ion intermediate is formed as a trans-decalin ring system that can undergo glycosylation through a bimolecular mechanism. DFT calculations have provided further insight into the reaction path of glycosylation and indicate that initially a hydrogen-bonded complex is formed between sulfonium ion and acceptor that undergoes SN2-like glycosylation to give an α-anomeric product.

Design of α-Selective Glycopyranosyl Donors Relying on Remote Anchimeric Assistance

Oligosaccharides have a variety of versatile biological effects, but unlike peptides and oligonucleotides, investigation of the roles of oligosaccharides is not easy. Since biosynthesis of oligosaccharides does not comply with direct genetic control, their isolation from natural sources and biotechnological preparation are complicated, due to the heterogeneous composition of raw carbohydrates. Oligosaccharide synthesis is needed for the establishment or confirmation of the structure of natural glycocompounds. Also, synthetically prepared, defined oligosaccharides and their derivatives are becoming increasingly important tools for many biological and immunological research projects. The key step of oligosaccharide synthesis involves glycosylation, a reaction that builds glycosidic bonds. Usually, construction of 1,2-trans-bonds is easy, and therefore, this reaction can even be included into automated syntheses. At this time, installation of the 1,2-cis-bond remains simultaneously frustrating and compelling. In our and other laboratories, a strategy of α-selective (1,2-cis) glycosylation, relying on remote anchimeric assistance with acyl groups, is studied. The state of the art in this work is summarized in this review.

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.

Achiral 2-Hydroxy Protecting Group for the Stereocontrolled Synthesis of 1,2-cis-α-Glycosides by Six-Ring Neighboring Group Participation

Glycosylation of a fully armed donor bearing a 2-O-(trimethoxybenzenethiol) ethyl ether protecting group is completely α-selective with a range of carbohydrate alcohol acceptors. Low-temperature NMR studies confirm the intermediacy of cyclic sulfonium ion intermediates arising from six-membered β-sulfonium ring neighboring group participation. Selective protecting group removal is achieved in high yield in a single operation by S-methylation and base-induced β-elimination.

Modulating glycosylation with exogenous nucleophiles: an overview

The major challenge in carbohydrate synthesis is stereochemical control of glycosidic bond formation. Different glycosylation methods have been developed that are based on the modulation effect of external nucleophiles. This review highlights the development, synthetic application, challenges and outlook of the modulated glycosylation methods.

Hydrogen bond mediated aglycone delivery: synthesis of linear and branched α-glucans

A Hydrogen bond mediated aglycone delivery (HAD) method was applied to the synthesis of α-glucans, which are abundant in nature, but as targets represent a notable challenge to chemists. The synthesis of linear oligosaccharide sequences was accomplished in complete stereoselectivity in all glycosylations. The efficacy of HAD may diminish with the increased bulk of the glycosyl acceptor, and may be an important factor for the syntheses of oligomers beyond pentasaccharides. The synthesis of a branched structure proved more challenging, particularly with bulky trisaccharide acceptors.

Methylthioxylose--a jewel in the mycobacterial crown?

Ten years ago an unusual sugar was discovered in a cell wall polysaccharide of Mycobacterium tuberculosis. Structural elucidation revealed the presence of the first thiosugar in a bacterial polysaccharide. Synthetic studies have helped to define its relative and absolute configuration as α-D-methylthioxylofuranosyl. While its biosynthetic origins remain the subject of speculation, work has begun to define its possible biological roles.

Glycosidation of thioglycosides in the presence of bromine: mechanism, reactivity, and stereoselectivity

Elaborating on previous studies by Lemieux for highly reactive "armed" bromides, we discovered that β-bromide of the superdisarmed (2-O-benzyl-3,4,6-tri-O-benzoyl) series can be directly obtained from the thioglycoside precursor. When this bromide is glycosidated, α-glycosides form exclusively; however, the yields of such transformations may be low due to the competing anomerization into α-bromide that is totally unreactive under the established reaction conditions.

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.

Direct synthesis of diastereomerically pure glycosyl sulfonium salts

It is reported that stable glycosyl sulfonium salts can be generated via direct anomeric S-methylation of ethylthioglycosides. Mechanistically, this pathway represents the first step in the activation of thioglycosides for glycosidation; however, it can further allow for the synthesis and isolation of quasi-stable sulfonium ions, representing a new approach for studying these key intermediates.

Ribosylation of adenosine: an orthogonally protected building block for the synthesis of ADP-ribosyl oligomers

A method to ribosylate adenosine on the 2' hydroxyl function in an α-selective fashion and in good yield is presented. Protective groups chosen for the acceptor and donor used in this glycosylation not only direct α-selectivity but also allow the construction of a fully orthogonally protected building block for the future assembly of oligo-ADP-ribosylated peptides and proteins.