Exploring and Exploiting the Reactivity of Glucuronic Acid Donors

Unexpected Nucleophile Masking in Acyl Transfer to Sterically Crowded and Conformationally Restricted Galactosides

Design and synthesis of orthogonally protected monosaccharide building blocks are crucial for the preparation of well-defined oligosaccharides in a stereo- and regiocontrolled manner. Selective introduction of protecting groups to partially protected monosaccharides is nontrivial due to the often unpredictable electronic, steric, and conformational effects of the substituents. Abolished reactivity toward a commonly used Lewis base-catalyzed acylation of O-2 was observed in conformationally restricted 4,6-O-benzylidene-3-O-Nap galactoside. Investigation of analogous systems, crystallographic characterization, and quantum chemical calculations highlighted the overlooked conformational and steric considerations, the combination of which produces a unique passivity of the 2-OH nucleophile. Evaluating the role of electrophile counterion and auxiliary base in the acylation of the sterically crowded and conformationally restricted galactoside system revealed an alternative Brønsted base-driven reaction pathway via nucleophilic activation. Insights gained from this model system were utilized to access the target galactoside intermediate within the envisioned synthetic route. The acylation strategy described herein can be implemented in future syntheses of key monomeric building blocks with unique protecting group hierarchies.

Semi-Synthetic Glycoconjugate Vaccine Lead Against Acinetobacter baumannii 17978

Acinetobacter baumannii is a opportunistic bacterial pathogen responsible for serious nosocomial infections that is becoming increasingly resistant against antibiotics. Capsular polysaccharides (CPS) that cover A. baumannii are a major virulence factor that play an important role in pathogenesis, are used to assign serotypes and provide the basis for vaccine development. Synthetic oligosaccharides resembling the CPS of A. baumannii 17978 were printed onto microarray slides and used to screen sera from patients infected with A. baumannii as well as a monoclonal mouse antibody (mAb C8). A synthetic oligosaccharide emerged from glycan array screening as lead for the development of a vaccine against A. baumannii 17978. Tetrasaccharide 20 is a key epitope for recognition by an antibody and is a vaccine lead.

Orthogonal One-Pot Synthesis of Oligosaccharides Based on Glycosyl ortho-Alkynylbenzoates

One of the most popular one-pot glycosylation strategies is orthogonal one-pot synthesis, which was mainly based on thioglycosides. Despite its successful application, shortcomings of thioglycosides including aglycon transfers, interference of departing species and unpleasant odor restrict its application scope. Herein, we report a new and efficient orthogonal one-pot synthesis of oligosaccahrides based on glycosyl ortho-alkynylbenzoate, which solves the issues of thioglycoside-based orthogonal one-pot synthesis. Over a dozen of oligosaccharides have been efficiently synthesized by this method.

Mapping the Relationship between Glycosyl Acceptor Reactivity and Glycosylation Stereoselectivity

The reactivity of both coupling partners-the glycosyl donor and acceptor-is decisive for the outcome of a glycosylation reaction, in terms of both yield and stereoselectivity. Where the reactivity of glycosyl donors is well understood and can be controlled through manipulation of the functional/protecting-group pattern, the reactivity of glycosyl acceptor alcohols is poorly understood. We here present an operationally simple system to gauge glycosyl acceptor reactivity, which employs two conformationally locked donors with stereoselectivity that critically depends on the reactivity of the nucleophile. A wide array of acceptors was screened and their structure-reactivity/stereoselectivity relationships established. By systematically varying the protecting groups, the reactivity of glycosyl acceptors can be adjusted to attain stereoselective cis-glucosylations.

Synthesis of silodosin glucuronide and its deuterated counterpart: solving a problematic O-glycosylation of a nitrogen-containing molecule

We report here the first chemical synthesis of silodosin glucuronide, a metabolite of the α1A-adrenoceptor antagonist silodosin, and its deuterium-labeled counterpart. As a key synthetic step, the incorporation of a glucuronosyl unit onto silodosin invariably led to either an undesired orthoester or a complex mixture under an array of standard glycosylation conditions. This problematic O-glycosylation may be attributed to the presence of multiple basic groups that could neutralize the acidic activators, decrease the nucleophilicity of a hydroxy group via hydrogen bond or even facilitate acyl migration side reactions. After elaborate tuning of reaction conditions, success was eventually achieved by using perbenzoylated d-glucuronosyl N-phenyltrifluroacetimidate (PTFA) as donor in combination with a procedure of sequential addition of TMSOTf. This protocol is potentially general for the glycosylation of other nitrogen-containing small molecule drugs.

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.

Total synthesis of a Streptococcus pneumoniae serotype 12F CPS repeating unit hexasaccharide

The Gram-positive bacterium Streptococcus pneumoniae causes severe disease globally. Vaccines that prevent S. pneumoniae infections induce antibodies against epitopes within the bacterial capsular polysaccharide (CPS). A better immunological understanding of the epitopes that protect from bacterial infection requires defined oligosaccharides obtained by total synthesis. The key to the synthesis of the S. pneumoniae serotype 12F CPS hexasaccharide repeating unit that is not contained in currently used glycoconjugate vaccines is the assembly of the trisaccharide β-D-GalpNAc-(1→4)-[α-D-Glcp-(1→3)]-β-D-ManpNAcA, in which the branching points are equipped with orthogonal protecting groups. A linear approach relying on the sequential assembly of monosaccharide building blocks proved superior to a convergent [3 + 3] strategy that was not successful due to steric constraints. The synthetic hexasaccharide is the starting point for further immunological investigations.

Automated glycan assembly of a S. pneumoniae serotype 3 CPS antigen

Vaccines against S. pneumoniae, one of the most prevalent bacterial infections causing severe disease, rely on isolated capsular polysaccharide (CPS) that are conjugated to proteins. Such isolates contain a heterogeneous oligosaccharide mixture of different chain lengths and frame shifts. Access to defined synthetic S. pneumoniae CPS structures is desirable. Known syntheses of S. pneumoniae serotype 3 CPS rely on a time-consuming and low-yielding late-stage oxidation step, or use disaccharide building blocks which limits variability. Herein, we report the first iterative automated glycan assembly (AGA) of a conjugation-ready S. pneumoniae serotype 3 CPS trisaccharide. This oligosaccharide was assembled using a novel glucuronic acid building block to circumvent the need for a late-stage oxidation. The introduction of a washing step with the activator prior to each glycosylation cycle greatly increased the yields by neutralizing any residual base from deprotection steps in the synthetic cycle. This process improvement is applicable to AGA of many other oligosaccharides.

A Convergent Synthesis of 6-O-Branched β-Glucan Oligosaccharides

β-Glucans are important carbohydrate antigens on the surface of fungal cells useful for antifungal vaccine development. This paper has described a highly convergent and efficient strategy for the synthesis of structurally defined branched β-glucan oligosaccharides that can be used for detailed studies of β-glucans and for the design of β-glucan-based vaccines. The strategy was highlighted by assembling the title compounds via preactivation-based glycosylation with thioglycosides as glycosyl donors. It was used to successfully prepare β-glucan oligosaccharides that had a β-1,3-linked nonaglucan backbone with β-1,6-glucotetraose, β-1,3-glucodiose and β-1,3-glucotetraose branches at the 6-O-position of the nonaglucan central sugar unit. The structure and size of the glycosyl donors and acceptors used in the syntheses did not significantly affect the glycosylation efficiency, suggesting that the strategy can be generally useful for the synthesis of more complex structures.

Synthesis and immunological studies of linear oligosaccharides of β-glucan as antigens for antifungal vaccine development

Antifungal vaccines have recently engendered considerable excitement for counteracting the resurgence of fungal infections. In this context, β-glucan, which is abundantly expressed on all fungal cell surfaces, functionally necessary for fungi, and immunologically active, is an attractive target antigen. Aiming at the development of effective antifungal vaccines based on β-glucan, a series of its oligosaccharide derivatives was designed, synthesized, and coupled with a carrier protein, keyhole limpet hemocyanin (KLH), to form new semisynthetic glycoconjugate vaccines. In this article, a convergent and effective synthetic strategy using preactivation-based iterative glycosylation was developed for the designed oligosaccharides. The strategy can be widely useful for rapid construction of large oligo-β-glucans with shorter oligosaccharides as building blocks. The KLH conjugates of the synthesized β-glucan hexa-, octa-, deca-, and dodecasaccharides were demonstrated to elicit high titers of antigen-specific total and IgG antibodies in mice, suggesting the induction of functional T cell-mediated immunity. Moreover, it was revealed that octa-, deca-, and dodeca-β-glucans were much more immunogenic than the hexamer and that the octamer was the best among these. The results suggested that the optimal oligosaccharide sequence of β-glucan required for exceptional immunogenicity was a hepta- or octamer and that longer glucans are not necessarily better antigens, a finding that may be of general importance. Most importantly, the octa-β-glucan-KLH conjugate provoked protective immunity against Candida albicans infection in a systemic challenge model in mice, suggesting the great potential of this glycoconjugate as a clinically useful immunoprophylactic antifungal vaccine.

Synthesis of chondroitin/dermatan sulfate-like oligosaccharides and evaluation of their protein affinity by fluorescence polarization

Here, we present a novel approach for the chemical synthesis of chondroitin and dermatan sulfate oligosaccharides. A key point of this strategy is the preparation and use of an N-trifluoroacetyl galactosamine building block containing a 4,6-O-di-tert-butylsilylene group. Glycosylation reactions proceeded in good yields (74-91%) with our protecting group distribution. Using this approach, we have synthesized, for the first time, a chondroitin/dermatan sulfate-like tetrasaccharide that contains both types of uronic acids, D-glucuronic and L-iduronic acid. Moreover, we have employed a fluorescence polarization competition assay to evaluate the interactions between the synthesized oligosaccharides and FGF-2 (basic fibroblast growth factor). Our results show that this method, using standard instrumentation and minimal sample consumption, is a powerful tool for the rapid analysis of the glycosaminoglycan affinity for proteins in solution.

First gram-scale synthesis of a heparin-related dodecasaccharide

The first example of a gram-scale synthesis of a structurally defined, heparin-related dodecasaccharide is reported. An iterative 14-step process using an iduronate donor disaccharide delivers >1g quantities of the dodecasaccharide sequence [GlcNS-IdoA2S](6)-OMe in 15% overall yield from the reducing terminal disaccharide, a 2 orders of magnitude increase in scale for access to synthetic heparanoid dodecasaccharide mimetics. The synthesis also delivers multigram amounts of the protected oligosaccharides from tetra- through to dodecasaccharide.