Towards the synthesis of a Yersinia pestis cell wall polysaccharide: enantioselective synthesis of an L-glycero-D-manno-heptose building block

Orthogonal Glycosylation with Phosphate Acceptors for Expeditious Synthesis of Bacterial Inner Core Oligosaccharides

We report a practical one-pot glycosylation strategy for synthesis of bacterial inner core oligosaccharides that composed of unavailable L-glycero-D-manno and D-glycero-D-manno-heptopyranose components. The glycosylation method features a new orthogonal glycosylation procedure; whereby a phosphate acceptor is coupled with a thioglycosyl donor producing a disaccharide phosphate, which can be engaged in another orthogonal glycosylation procedure to couple with a thioglycosyl acceptor. The phosphate acceptors used in above one-pot procedure are directly prepared from thioglycosyl acceptors via the in-situ phosphorylation. Such phosphate acceptor preparation protocol eliminates the traditional protection and deprotection procedures. Based on the new one-pot glycosylation strategy, two partial inner core structures of Yersinia pestis lipopolysaccharide and Haemophilus ducreyi lipooligosaccharide were acquired.

Stereoselective Synthesis of β-d-Manno-heptopyranoside via Cs2CO3-Mediated Anomeric O-Alkylation: Synthesis of a Tetrasaccharide Repeat Unit of Bacillus thermoaerophilus Surface-Layer Glycoprotein

Stereoselective synthesis of d-glycero- and l-glycero-β-d-mannoheptosides has been achieved by cesium carbonate-mediated β-selective anomeric O-alkylation of the corresponding d-mannoheptoses. In addition, this method has been utilized in the total synthesis of a tetrasaccharide repeat unit of Bacillus thermoaerophilus surface-layer glycoprotein.

Pathogen-Associated Molecular Patterns: The Synthesis of Heptose Phosphates and Derivatives

Lipopolysaccharide biosynthesis metabolites, such as d-glycero-β-d-manno-heptopyranosyl 1,7-diphosphate, d-glycero-β-d-manno-heptopyranosyl phosphate, and adenosine 5′-(l-glycero-β-d-manno-heptopyranosyl)diphosphate, have been found to activate NF-κB via alpha-kinase 1 and TRAF-interacting protein with forkhead associated domain. This axis has been determined as a novel pathway of innate immunity yet to be targeted for immunomodulatory treatment approaches. Key in understanding this new axis has been the ability to synthesize these metabolites. The design of synthetic analogues and probes have also been published not only to design new drugs, but also to gain insight into the mechanism of action for these compounds. The focus of the present review is the synthesis of heptose phosphate metabolites­ as well as synthetic analogues and probes.

1 Introduction

2 Synthesis of d-glycero-d-manno-Heptose

2.1 Using d-Mannose as Starting Material

2.2 Using d-Ribose as Starting Material

2.3 Using 2,2-Dimethyl-1,3-dioxan-5-one as Starting Material

3 Synthesis of l-glycero-d-manno-Heptose

3.1 Using d-Mannose as Starting Material

3.2 Using 2,2-Dimethyl-1,3-dioxan-5-one as Starting Material

3.3 Using l-Lyxose as Starting Material

4 Synthesis of Heptose Phosphates

4.1 Synthesis of d-glycero-β-d-manno-Heptose 1,7-Diphosphate

4.2 Synthesis of Heptose Phosphate Derivatives

4.2.1 Development of Scaffolds for Conjugation

4.2.2 Development of Heptose Phosphates Derivatives for Cell Intake and Metabolic Stability

5 Conclusion and Outlook

Promoter-Assisted Stereoselective Synthesis of the 6-Deoxy-β-d-manno-heptopyranose Oligosaccharides

We report a promoter-assisted glycosidation approach for the stereoselective synthesis of the 6-deoxy-β-d-manno-heptopyranose oligosaccharides. SphosAuNTf₂-promoted glycosidation of 6-deoxy-d-manno-heptopyranosyl o-hexynylbenzoate with common alcohols afforded a range of 6-deoxy-d-manno-heptosides with good to excellent β-selectivities. The counterion and the ligand of SPhosAuNTf₂ were found to have a dramatic effect on the formation of the 1,2-cis-β-linked 6-deoxy-d-manno-heptosides. This approach was effectively applied to the stereocontrolled synthesis of the 6-deoxy-β-d-manno-heptopyranose oligosaccharides relevant to Burkholderia pseudomallei and Burkholderia mallei.

Cascade In Situ Phosphorylation and One-Pot Glycosylation for Rapid Synthesis of Heptose-Containing Oligosaccharides

We report a one-pot glycosylation strategy for achieving rapid syntheses of heptose (Hep)-containing oligosaccharides. The reported procedure was designed to incorporate an in situ phosphorylation step into an orthogonal one-pot glycosylation. Hep-containing oligosaccharides were assembled directly from building blocks with minimal effort expended on manipulation of protecting and aglycone leaving groups. The utility of our one-pot procedure was illustrated by synthesizing partial core oligosaccharide structure present in the lipopolysaccharide of Ralstonia solanacearum.

Synthesis of l-glycero- and d-glycero-d-manno-Heptose Building Blocks for Stereoselective Assembly of the Lipopolysaccharide Core Trisaccharide of Vibrio parahemolyticus O2

Synthesis of bacterial cell surface l-glycero-d-manno-heptose (l,d-Hep)- and d-glycero-d-manno-heptose (d,d-Hep)-containing higher carbon sugars is a challenging task. Here, we report a convenient and efficient approach for the synthesis of the l,d-Hep and d,d-Hep building blocks. Using l-lyxose and d-ribose as starting materials, this approach features diastereoselective Mukaiyama-type aldol reactions as the key steps. On the basis of the synthetic l,d-Hep and d,d-Hep building blocks, we achieved the first stereoselective synthesis of the unique α-l,d-Hep-(1→3)-α-d,d-Hep-(1→5)-α-Kdo core trisaccharide of the lipopolysaccharide of Vibrio parahemolyticus O2.

Straight Forward and Versatile Differentiation of the l-glycero and d-glycero-d-manno Heptose Scaffold

Bacterial lipopolysaccharides (LPS) are important bio-medical structures, playing a major role in the interaction with human immune systems. Their core regions, containing multiple units of l-glycero-d-manno heptoses (l,d-heptose), are highly conserved structurally (with O3 and O7 glycosidic bonds), making them an epitope of high interest for the potential development of new antibiotics and vaccines. Research in this field has always been restricted by the limited availability of the parent l,d-heptose as well as its biochemical epimeric precursor d-glycero-d-manno heptose (d,d-heptose). This problem of availability has recently been solved by us, through a rapid and efficient practical synthesis of l,d-manno-heptose peracetate demonstrated at scale. Herein we report an optimized, technically simple and versatile synthetic strategy for the differentiation of both the l-glycero and d-glycero-d-manno heptose scaffolds. Our approach is based on an orthoester methodology for the differentiation of all three positions of the sugar core using a O6, O7-tetraisopropyl disiloxyl (TIPDS) protecting group for the exocyclic positions. Furthermore, the regioselective opening toward 7-OH acceptors (6O-FTIPDS ethers) differentiates the exocyclic diol which has been demonstrated with a broader set of substrates and for both manno-heptoses for the first time.

Total Synthesis of the Trisaccharide Antigen of the Campylobacter jejuni RM1221 Capsular Polysaccharide via de Novo Synthesis of the 6-Deoxy-d- manno-heptose Building Blocks

A de novo approach utilizing the d-proline-catalyzed and LDA-promoted aldol reactions as key steps for the preparation of differentiated-protected 6-deoxy-d- manno-heptose building blocks was developed. PPh₃AuBAr₄^F-catalyzed glycosylation with the 6-deoxy-d- manno-heptosyl o-hexynylbenzoate as donor was demonstrated as a direct and practical method for the stereoselective synthesis of the β-linked 6-deoxy-d- manno-heptoside as the major product. Coupling of the 6-deoxy-α-d- manno-heptosyl H-phosphonate with the 3-hydroxyl disaccharide acceptor based on H-phosphonate chemistry was described for the construction of the trisaccharide skeleton with the acid-labile phosphodiester linkage. Finally, first total synthesis of the unique trisaccharide antigen of the capsular polysaccharide of Campylobacter jejuni RM1221 that belongs to HS:53 serotype complex was accomplished for further evaluation as vaccine candidate against C. jejuni RM1221 infection.

Synthesis of D-manno-heptulose via a cascade aldol/hemiketalization reaction

A [4 + 3] synthesis of D-manno-heptulose is described. The cascade aldol/hemiketalization reaction of a C₄ aldehyde with a C₃ ketone provides the differentially protected ketoheptose building block, which can be further reacted to furnish target D-manno-heptulose.

Large-Scale Synthesis of Crystalline 1,2,3,4,6,7-Hexa-O-acetyl-l-glycero-α-d-manno-heptopyranose

The higher-carbon sugar l-glycero-d-manno-heptose is a major constituent of the inner core region of the lipopolysaccharide (LPS) of many Gram-negative bacteria. All preparative routes used to date require multiple steps, and scalability has been rarely addressed. Here a highly practical synthesis of crystalline 1,2,3,4,6,7-hexa-O-acetyl-l-glycero-α-d-manno-heptopyranose by a simple four-step sequence starting from l-lyxose is disclosed. Only two recrystallisations are required and the process was demonstrated on a >100 mmol scale, yielding 41 g of the target compound.

Diversity-oriented synthesis of inner core oligosaccharides of the lipopolysaccharide of pathogenic Gram-negative bacteria

Lipopolysaccharide (LPS) is a potent virulence factor of pathogenic Gram-negative bacteria. To better understand the role of LPS in host-pathogen interactions and to elucidate the antigenic and immunogenic properties of LPS inner core region, a collection of well-defined L-glycero-D-manno-heptose (Hep) and 3-deoxy-α-D-manno-oct-2-ulosonic acid (Kdo)-containing inner core oligosaccharides is required. To address this need, we developed a diversity-oriented approach based on a common orthogonal protected disaccharide Hep-Kdo. Utilizing this new approach, we synthesized a range of LPS inner core oligosaccharides from a variety of pathogenic bacteria including Y. pestis, H. influenzae, and Proteus that cause plague, meningitis, and severe wound infections, respectively. Rapid access to these highly branched core oligosaccharides relied on elaboration of the disaccharide Hep-Kdo core as basis for the elongation with various flexible modules including unique Hep and 4-amino-4-deoxy-β-L-arabinose (Ara4N) monosaccharides and branched Hep-Hep disaccharides. A regio- and stereoselective glycosylation of Kdo 7,8-diol was key to selective installation of the Ara4N moiety at the 8-hydroxyl group of Kdo moiety of the Hep-Kdo disaccharide. The structure of the LPS inner core oligosaccharides was confirmed by comparison of (1)H NMR spectra of synthetic antigens and isolated fragments. These synthetic LPS core oligosaccharides can be covalently bound to carrier proteins via the reducing end pentyl amine linker, to explore their antigenic and immunogenic properties as well as potential applications such as diagnostic tools and vaccines.

De novo synthesis of D- and L-fucosamine containing disaccharides

The availability of rare monosaccharides that cannot be isolated from natural sources is currently limiting the access to the synthesis and the biological evaluation of complex bacterial cell-surface glycans. Here, we report the synthesis of D- and L-fucosamine building blocks by a de novo approach from L- and D-Garner aldehydes. These differentially protected monosaccharide building blocks were utilized to prepare disaccharides present on the surface of Pseudomonas aeruginosa bacteria.

De novo synthesis of L-colitose and L-rhodinose building blocks

A divergent, practical, and efficient de novo synthesis of fully functionalized L-colitose (3,6-dideoxy-L-galactose), 2-epi-colitose (3,6-dideoxy-L-talose), and L-rhodinose (2,3,6-trideoxy-L-galactose) building blocks has been achieved using inexpensive, commercially available (S)-ethyl lactate as the starting material. The routes center around a diastereoselective Cram-chelated allylation that provides a common homoallylic alcohol intermediate. Oxidation of this common intermediate finally resulted in the synthesis of the three monosaccharide building blocks.