An efficient synthesis of selectively functionalized d-rhamnose derivatives

Automated Synthesis of C1-Functionalized Oligosaccharides

Automated glycan assembly (AGA) streamlines the synthesis of complex oligosaccharides. The reducing end of the oligosaccharide serves as an attachment site to the polymer support to liberate a free reducing end or an aminopentanol for ready conjugation to carrier proteins or surfaces. The facile installation of different aglycons on oligosaccharides has not been possible via AGA until now. Here, we describe a latent-active approach enabled by a traceless photolabile linker that allows for bidirectional AGA and ready introduction of various aglycons. Oligosaccharide thioglycosides, peptidoglycans, prototypical saponins, and click-chemistry-based conjugates are synthesized to illustrate the versatility of the method.

Progress toward the Total Synthesis of Nogalamycin Using a Benzyne Cycloaddition Strategy

Nogalamycin (NOG) is a member of the anthracycline glycoside natural products; no total syntheses have yet been reported, and there is minimal understanding of how the aglycone substitution pattern and identities of the A- and D-ring sugars impact the anticancer activity and toxicity. This paper reports progress toward a modular approach to NOG that could enable systematic structure-activity relationship studies. Key steps include a regioselective benzyne cycloaddition and reductive ring-opening to assemble a versatile AB core for analogue synthesis.

Synthesis and Immunogenicity of a Methyl Rhamnan Pentasaccharide Conjugate from Pseudomonas aeruginosa A-Band Polysaccharide

Pseudomonas aeruginosa was added to the World Health Organization's priority pathogen list for research and development of new antibiotics in 2017. Alongside the development of new antibiotics to fight antimicrobial-resistant P. aeruginosa, vaccines would be an appealing addition to the toolbox health professionals have against this bacteria, which causes life-threatening respiratory infections. Recently, the structure of a novel immunogenic terminal carbohydrate moiety on the cell surface of P. aeruginosa was elucidated, consisting of a 3-O-methyl (1→4)-α-d-rhamnan pentasaccharide. As isolating this oligosaccharide from P. aeruginosa in sufficient amounts for producing a conjugate vaccine is challenging, herein we describe the synthesis of 3-O-methyl d-rhamnose oligosaccharide. We also report the conjugation of the synthetic pentasaccharide to human serum albumin and its resulting immunogenicity.

Simple Carbohydrate Derivatives Diminish the Formation of Biofilm of the Pathogenic Yeast Candida albicans

The opportunistic human fungal pathogen Candida albicans relies on cell morphological transitions to develop biofilm and invade the host. In the current study, we developed new regulatory molecules, which inhibit the morphological transition of C. albicans from yeast-form cells to cells forming hyphae. These compounds, benzyl α-l-fucopyranoside and benzyl β-d-xylopyranoside, inhibit the hyphae formation and adhesion of C. albicans to a polystyrene surface, resulting in a reduced biofilm formation. The addition of cAMP to cells treated with α-l-fucopyranoside restored the yeast-hyphae switch and the biofilm level to that of the untreated control. In the β-d-xylopyranoside treated cells, the biofilm level was only partially restored by the addition of cAMP, and these cells remained mainly as yeast-form cells.

Epimerization of an Ascaroside-Type Glycolipid Downstream of the Canonical β-Oxidation Cycle in the Nematode Caenorhabditis nigoni

A species-specific ascaroside-type glycolipid was identified in the nematode Caenorhabditis nigoni using HPLC-ESI-(-)-MS/MS precursor ion scanning, HR-MS/MS, and NMR techniques. Its structure containing an l-3,6-dideoxy-lyxo-hexose unit was established by total synthesis. The identification of this novel 4-epi-ascaroside (caenorhabdoside) in C. nigoni along with the previous identification of 2-epi-ascarosides (paratosides) in Pristionchus pacificus indicate that nematodes can generate highly specific signaling molecules by epimerization of the ascarylose building block downstream of the canonical β-oxidation cycle.

A Simple Glycolipid Mimic of the Phosphatidylinositol Mannoside Core from Mycobacterium tuberculosis Inhibits Macrophage Cytokine Production

Glycolipids from Mycobacterium tuberculosis have a profound impact on the innate immune response of the host. Macrophage-inducible C-type lectin (Mincle) is a pattern-recognition receptor that has been shown to bind trehalose dimycolate (TDM) from the mycobacterium and instigate intracellular signalling in the immune cell. There are structural similarities between the structures of TDM and phosphatidyl inositol mannoside (PIM). We thus hypothesized that these latter structures might also modulate an immune response in a similar manner. To test this, we synthesized a series of new mannose derivatives modified with fatty esters at the 6-position and assessed the release of inflammatory cytokines in human U937 macrophages under the induction of lipopolysaccharides (LPS) after glycolipid treatment. The results showed that the amount of two major cytokines-tumour necrosis factor (TNF)-α and interleukin (IL)-6-released from LPS-stimulated U937 cells decreased significantly when compared to a control upon treatment with the prepared glycolipids, thus indicating a reduction in cytokine production by the macrophages.

Total Synthesis of the Glycosylated Macrolide Antibiotic Fidaxomicin

The first enantioselective total synthesis of fidaxomicin, also known as tiacumicin B or lipiarmycin A3, is reported. This novel glycosylated macrolide antibiotic is used in the clinic for the treatment of Clostridium difficile infections. Key features of the synthesis involve a rapid and high-yielding access to the noviose, rhamnose, and orsellinic acid precursors; the first example of a β-selective noviosylation; an effective Suzuki coupling of highly functionalized substrates; and a ring-closing metathesis reaction of a noviosylated dienoate precursor. Careful selection of protecting groups allowed for a complete deprotection yielding totally synthetic fidaxomicin.

Efficient routes toward the synthesis of the D-rhamno-trisaccharide related to the A-band polysaccharide of Pseudomonas aeruginosa

The present work describes efficient avenues for the synthesis of the trisaccharide repeating unit [α-D-Rhap-(1→3)-α-D-Rhap-(1→3)-α-D-Rhap] associated with the A-band polysaccharide of Pseudomonas aeruginosa. One of the key steps involved 6-O-deoxygenation of either partially or fully acylated 4,6-O-benzylidene-1-thiomannopyranoside by radical-mediated redox rearrangement in high yields and regioselectivity. The D-rhamno-thioglycosides so obtained allowed efficient access to the trisaccharide target via stepwise glycosylation as well as a one-pot glycosylation protocol. In a different approach, a 4,6-O-benzylidene D-manno-trisaccharide derivative was synthesized, which upon global 6-O-deoxygenation followed by deprotection generated the target D-rhamno-trisaccharide. The application of the reported regioselective radical-mediated deoxygenation on 4,6-O-benzylidene D-manno thioglycoside (hitherto unexplored) has potential for ramification in the field of synthesis of oligosaccharides based on 6-deoxy hexoses.