C-Disaccharides as Probes for Carbohydrate Recognition – Investigation of the Conformational Requirements for Binding of Disaccharide Mimetics of Sialyl Lewis X

Synthesis of C-linked α-Gal and α-GalNAc-1'-hydroxyalkanes by way of C2 functionality transfer

Inspired by reports of water sculpted Tn antigen (α-GalNAc-O-Ser/Thr) epitopes and our interest in producing metabolically more stable C-linked analogs of Tn, we explored the utility of C2 functionality on α-Gal-C-alkenes to deliver hydroxyl to the pendant alkenyl chain. Toward this end, a cyclic carbonate approach gave rise to a single C-linked α-Gal-1'(S)-hydroxyethane in 3 steps, and use of a 2-(hydroxyimino)galactoside cyclization transferred an oxygen to a pendant cis-substituted C-linked alkene affording the R-configuration at the newly formed stereocenter (7:1 dr). Reduction and acetylation of the resultant isoxazoline demonstrated this approach as a viable route to C-linked α-GalNAc-1'-hydroxyalkanes.

Turning-Off Signaling by Siglecs, Selectins, and Galectins: Chemical Inhibition of Glycan-Dependent Interactions in Cancer

Aberrant glycosylation, a common feature associated with malignancy, has been implicated in important events during cancer progression. Our understanding of the role of glycans in cancer has grown exponentially in the last few years, concurrent with important advances in glycomics and glycoproteomic technologies, paving the way for the validation of a number of glycan structures as potential glycobiomarkers. However, the molecular bases underlying cancer-associated glycan modifications are still far from understood. Glycans exhibit a natural heterogeneity, crucial for their diverse functional roles as specific carriers of biologically relevant information. This information is decoded by families of proteins named lectins, including sialic acid-binding immunoglobulin (Ig)-like lectins (siglecs), C-type lectin receptors (CLRs), and galectins. Siglecs are primarily expressed on the surface of immune cells and differentially control innate and adaptive immune responses. Among CLRs, selectins are a family of cell adhesion molecules that mediate interactions between cancer cells and platelets, leukocytes, and endothelial cells, thus facilitating tumor cell invasion and metastasis. Galectins, a family of soluble proteins that bind β-galactoside-containing glycans, have been implicated in diverse events associated with cancer biology such as apoptosis, homotypic cell aggregation, angiogenesis, cell migration, and tumor-immune escape. Consequently, individual members of these lectin families have become promising targets for the design of novel anticancer therapies. During the past decade, a number of inhibitors of lectin–glycan interactions have been developed including small-molecule inhibitors, multivalent saccharide ligands, and more recently peptides and peptidomimetics have offered alternatives for tackling tumor progression. In this article, we review the current status of the discovery and development of chemical lectin inhibitors and discuss novel strategies to limit cancer progression by targeting lectin–glycan interactions.

Recent advances toward the development of inhibitors to attenuate tumor metastasis via the interruption of lectin-ligand interactions

Aberrant glycosylation is a well-recognized phenomenon that occurs on the surface of tumor cells, and the overexpression of a number of ligands (such as TF, sialyl Tn, and sialyl Lewis X) has been correlated to a worse prognosis for the patient. These unique carbohydrate structures play an integral role in cell-cell communication and have also been associated with more metastatic cancer phenotypes, which can result from binding to lectins present on cell surfaces. The most well studied metastasis-associated lectins are the galectins and selectins, which have been correlated to adhesion, neoangiogenesis, and immune-cell evasion processes. In order to slow the rate of metastatic lesion formation, a number of approaches have been successfully developed which involve interfering with the tumor lectin-substrate binding event. Through the generation of inhibitors, or by attenuating lectin and/or carbohydrate expression, promising results have been observed both in vitro and in vivo. This article briefly summarizes the involvement of lectins in the metastatic process and also describes different approaches used to prevent these undesirable carbohydrate-lectin binding events, which should ultimately lead to improvement in current cancer therapies.

Synthesis of the C-glycoside of α-(D)-mannose-(1 → 6)-(D)-myo-inositol

The dimannosylatedinositol pseudotrisaccharide phospholipid of the lipoarabinomannan (LAM) component of the mycobacterial cell wall has attracted interest as a therapeutic target because of its uniqueness to mycobacteria, its assembly at an early stage in LAM biosynthesis and the immunological activity of oligosaccharides containing this subunit. Accordingly, analogues of this pseudotrisaccharide, α-d-mannose-(1 → 2)-α-d-mannose-(1 → 6)-d-myo-inositol are of interest as mechanistic probes and drug leads. C-glycosides are of special interest because of their hydrolytic stability and conformational differences compared to O-glycosides. Herein, as a prelude to C-glycoside analogues of this pseudotrisaccharide, we describe the synthesis of the C-glycoside of α-d-mannose-(1 → 6)-d-myo-inositol. The synthetic strategy centers on the elaboration of a C1-linked glycal-inositol, the glycone segment of which is assembled via an oxocarbenium ion cyclization on a thioacetal-enol ether precursor that originates from "glycone" and "aglycone" components.

C-Galactosylceramide: Synthesis and Immunology

A synthetic C-glycoside, α-C-galactosylceramide, is an active immunostimulant in mice. It displays better activity than α-O-galactosylceramide in several disease models. Syntheses of several α-C-galactosylceramides are described. Experiments that probe its immunostimulant activity are outlined. Possible explanations for its superior activity are discussed.

A strategy for constructing C-sialosides based on Ireland-Claisen rearrangement and its application for synthesis of CF2-linked ganglioside GM4 analog

Sialidase-resistant ganglioside analogs having similar biological activities to natural gangliosides are expected to be important probes for clarifying the biological functions of gangliosides. Focusing on difluoromethylene-linked (CF2-linked) and methylene-linked (CH2-linked) α(2,3)sialylgalactose as a core structure of sialidase-resistant ganglioside mimics, we have developed novel, stereocontrolled, and efficient methodologies to synthesize C-sialosides based on Ireland-Claisen rearrangement. These methods were employed to synthesize CF2-linked GM4. The CF2-linked GM4 inhibited human sialidases NEU2 and NEU4 with IC50 values of 754 and 930 μM, respectively, and strongly inhibited human lymphocyte proliferation in the same concentration range as natural GM4.

New and general synthesis of β-C-glycosylformaldehydes from easily available β-C-glycosylpropanones

A highly effective method for the introduction of a formyl group at the anomeric position of pyranosides was developed via enolisation of beta-C-D-glycopyranosylpropan-2-one using thermodynamic conditions then oxidative cleavage of the more substituted double bond. This sequence affords the desired aldehydes that are conveniently protected as aminals for purification and storage and easily regenerated using Dowex resin H+. In this paper, the syntheses of nine differently protected aldehydes derived from d-glucose, d-galactose, lactose and N-acetyl-d-glucosamine are presented. Our strategy proved to be very efficient in most cases excepted in the D-mannose series.

Synthesis and conformational behavior of the difluoromethylene linked C-glycoside analog of beta-galactopyranosyl-(1<-->1)-alpha-mannopyranoside

C-Glycosides in which the pseudoglycosidic substituent is a methylene group have been advertised as hydrolytically stable mimetics of their parent O-glycosides. While this substitution assures greater stability, the lower polarity and increased conformational flexibility in the intersaccharide linker brought about by this change may compromise biological mimicry. In this regard, C-glycosides, in which the pseudoanomeric methylene is replaced with a difluoromethylene group, are interesting because the CF2 group is more of an isopolar replacement for oxygen than CH2. In addition, the CF2 residue is expected to instill conformational bias into the intersaccharide torsions. Herein is described the synthesis and conformational behavior of the difluoromethylene linked C-glycoside of beta-D-galactopyranosyl-(1<-->1)-alpha-D-mannopyranoside. The synthesis centers on the formation of the galactose residue via an oxocarbenium ion-enol ether cyclization. Conformational analysis, using a combination of molecular mechanics, dynamics, and NMR spectroscopy, suggests that the difluoro-C-glycoside populates the non-exo-Gal/exo-Man conformer to a major extent (ca 50%), with a minor contribution ( approximately 15%) from the exo-Gal/exo-Man conformer that corresponds to the ground sate of the parent O-glycoside.

The conformational behaviour and P-selectin inhibition of fluorine-containing sialyl LeX glycomimetics

A combination of experimental J/NOE NMR data with molecular mechanics and dynamics calculations has been used to examine the conformational behaviour and assign the configuration of synthetically prepared epimeric 3-carboxymethyl-O-Gal-(1-->1)-alpha-Man-fluoro-C-glycosides. It is shown that the population distributions around the glycosidic linkages strongly depend on the configuration at the fluorinated carbon of the pseudoacetal residue. It is also shown that these compounds resemble the inhibition ability of sialyl LeX towards P-selectin.