The First Synthesis of a Thioglycoside Analogue of the Immunostimulant KRN7000

Visible-light-mediated synthesis of non-anomeric S-aryl glycosides via a photoactive electron-donor-acceptor complex

A visible-light-mediated glycosylation reaction between glycosyl redox-active esters and disulfides has been reported, through which a series of S-aryl glycosides were obtained in good yields with satisfactory stereoselectivity. The preliminary mechanistic studies revealed that this transformation proceeded via an EDA complex. Moreover, the potential application value was demonstrated in the late-stage functionalisation of drug molecules and a gram-scale experiment.

New syntheses of thiosaccharides utilizing substitution reactions

Novel synthetic methods published since 2005 affording carbohydrates containing sulfur atom(s) are reviewed. The review is divided to subchapters based on the position of sulfur atom(s) in the sugar molecule. Only those methods that take advantage of substitution are discussed.

Regio and Stereoselective One-Pot Synthesis of 2-Deoxy-3-thio Pyranoses and Their O-Glycosides from Glycals

A reaction of glycals with two different types of nucleophiles in the presence of SnCl₄ enabled one-pot rapid access to 2-deoxy-3-thio pyranoses and their O-glycosides. The process involves thioaryl substitution at C-3 with stereoretention and α-selective O-glycosylation at C-1 from d-glycals, thus combining two reactions with three interventions. The present methodology features an attractive three-component coupling (1:1.2:1.5 ratio) with operational simplicity at 0 °C in 10-20 min. This stereoselective one-pot 1,3-difunctionalization approach of glycals is compatible with wide range of primary and secondary alcohols affording products in good to excellent yields. This methodology was successfully extended toward disaccharide synthesis. Several control experiments suggested a plausible reaction mechanism and rationale behind regio and stereoselectivity. The reaction strategy possesses an intrinsic ability for the synthesis of various natural products and drug molecules.

Glycoconjugates: Synthesis, Functional Studies, and Therapeutic Developments

Glycoconjugates are major constituents of mammalian cells that are formed via covalent conjugation of carbohydrates to other biomolecules like proteins and lipids and often expressed on the cell surfaces. Among the three major classes of glycoconjugates, proteoglycans and glycoproteins contain glycans linked to the protein backbone via amino acid residues such as Asn for N-linked glycans and Ser/Thr for O-linked glycans. In glycolipids, glycans are linked to a lipid component such as glycerol, polyisoprenyl pyrophosphate, fatty acid ester, or sphingolipid. Recently, glycoconjugates have become better structurally defined and biosynthetically understood, especially those associated with human diseases, and are accessible to new drug, diagnostic, and therapeutic developments. This review describes the status and new advances in the biological study and therapeutic applications of natural and synthetic glycoconjugates, including proteoglycans, glycoproteins, and glycolipids. The scope, limitations, and novel methodologies in the synthesis and clinical development of glycoconjugates including vaccines, glyco-remodeled antibodies, glycan-based adjuvants, glycan-specific receptor-mediated drug delivery platforms, etc., and their future prospectus are discussed.

Metal-free glycosylation with glycosyl fluorides in liquid SO2

Liquid SO₂ is a polar solvent that dissolves both covalent and ionic compounds. Sulfur dioxide possesses also Lewis acid properties, including the ability to covalently bind Lewis basic fluoride ions in a relatively stable fluorosulfite anion (FSO₂ ^-). Herein we report the application of liquid SO₂ as a promoting solvent for glycosylation with glycosyl fluorides without any external additive. By using various temperature regimes, the method is applied for both armed and disarmed glucose and mannose-derived glycosyl fluorides in moderate to excellent yields. A series of pivaloyl-protected O- and S-mannosides, as well as one example of a C-mannoside, are synthesized to demonstrate the scope of the glycosyl acceptors. The formation of the fluorosulfite species during the glycosylation with glycosyl fluorides in liquid SO₂ is proved by ¹⁹F NMR spectroscopy. A sulfur dioxide-assisted glycosylation mechanism that proceeds via solvent separated ion pairs is proposed, whereas the observed α,β-selectivity is substrate-controlled and depends on the thermodynamic equilibrium.

Natural and synthetic carbohydrate-based vaccine adjuvants and their mechanisms of action

Modern subunit vaccines based on homogeneous antigens offer more precise targeting and improved safety compared with traditional whole-pathogen vaccines. However, they are also less immunogenic and require an adjuvant to increase the immunogenicity of the antigen and potentiate the immune response. Unfortunately, few adjuvants have sufficient potency and low enough toxicity for clinical use, highlighting the urgent need for new, potent and safe adjuvants. Notably, a number of natural and synthetic carbohydrate structures have been used as adjuvants in clinical trials, and two have recently been approved in human vaccines. However, naturally derived carbohydrate adjuvants are heterogeneous, difficult to obtain and, in some cases, unstable. In addition, their molecular mechanisms of action are generally not fully understood, partly owing to the lack of tools to elucidate their immune-potentiating effects, thus hampering the rational development of optimized adjuvants. To address these challenges, modification of the natural product structure using synthetic chemistry emerges as an attractive approach to develop well-defined, improved carbohydrate-containing adjuvants and chemical probes for mechanistic investigation. This Review describes selected examples of natural and synthetic carbohydrate-based adjuvants and their application in synthetic self-adjuvanting vaccines, while also discussing current understanding of their molecular mechanisms of action.

Synthesis of polyfluoroalkyl sp2-iminosugar glycolipids and evaluation of their immunomodulatory properties towards anti-tumor, anti-leishmanial and anti-inflammatory therapies

Immunomodulatory glycolipids, among which α-galactosylceramide (KRN7000) is an iconic example, have shown strong therapeutic potential in a variety of conditions ranging from cancer and infection to autoimmune or neurodegenerative diseases. A main difficulty for those channels is that they often provoke a cytokine storm comprising both pro- and anti-inflammatory mediators that antagonize each other and negatively affect the immune response. The synthesis of analogues with narrower cytokine secretion-inducing capabilities is hampered by the intrinsic difficulty at controlling the stereochemical outcome in glycosidation reactions, particularly if targeting the α-anomer, which seriously hampers drug optimization strategies. Here we show that replacing the monosaccharide glycone by a sp²-iminosugar glycomimetic moiety allows accessing N-linked sp²-iminosugar glycolipids (sp²-IGLs) with total α-stereocontrol in a single step with no need of protecting groups or glycosidation promotors. The lipid tail has been then readily tailored by incorporating polyfluoroalkyl segments of varied lengths in view of favouring binding to the lipid binding site of the master p38 mitogen activated protein kinase (p38 MAPK), thereby polarizing the immune response in a cell-context dependent manner. The compounds have been evaluated for their antiproliferative, anti-leishmanial and anti-inflammatory activities in different cell assays. The size of the fluorous segment was found to be critical for the biological activity, probably by regulating the aggregation and membrane-crossing properties, whereas the hydroxylation profile (gluco or galacto-like) was less relevant. Biochemical and computational data further support a mechanism of action implying binding to the allosteric lipid binding site of p38 MAPK and subsequent activation of the noncanonical autophosphorylation route. The ensemble of results provide a proof of concept of the potential of sp²-IGLs as immunoregulators.

Synthesis and biological evaluation of S-simplexides and other analogues of simplexide

Simplexides are natural glycolipids isolated from the marine sponge Plakortis simplex, and contain alkyl 4-O-(α-D-glucopyranosyl)-β-D-galactopyranoside. Simplexides can release of cytokines (IL-6) and chemokines (CXCL-8) from human monocytes and cause the expansion of natural killer T-cells (iNKTs) in vitro, with iNKTs contributing to the sustenance of immune homeostasis. Herein, the stereoselective syntheses of S-glycosidic analogues, i.e. S-simplexides, are described. The routes included Lewis acid promoted anomerisation of glycosyl thiols and thioglycolipids, as well as anomeric S-alkylation. Synthesis of O-glycosidic analogues are included. Heptadecanyl O- and S-glycosides as well as the 17-tritriacontyl 4-O-(α-D-glucopyranosyl)-β-D-galactopyranoside, a component of the natural simplexide isolate, all induced IL-6 and CXCL-8 production at both 10 and 30 μg/mL concentrations from PBMCs whereas the two S-simplexides were inactive. It is speculated that the lack of activity for the S-disaccharide analogue could be due to inhibition of cellular α-glucosidase, preventing degradation of the simplex disaccharide to a simpler galactopyranoside, whereas lack of activity for the S-galactolipid analogue could be due to increased conformational flexibility of S-glycosides. On the other hand, simpler unbranched O- and S-glycolipid analogues were active. Natural simplexide, and a synthetic simplexide, the 18-pentatriacontanyl 4-O-(α-D-glucopyranosyl)-β-D-galactopyranoside, were more potent than the new compounds tested.

Metal-free photocatalytic thiol–ene/thiol–yne reactions

We report a metal-free photocatalytic approach to the synthesis of glycoconjugates, highlighting the mild nature of the reaction conditions.

Synthesis of aryl-thioglycopeptides through chemoselective Pd-mediated conjugation

We describe herein a Pd-catalyzed methodology for the thioglycoconjugation of iodoaryl peptides and aminoacids. This operationally simple process occurs under semi-aqueous conditions and displays wide substrate scope. The strategy has been successfully applied to both the thioglycosylation of unprotected peptides and the generation of thioglyco-aminoacid building blocks, including those suitable for solid phase peptide synthesis. To demonstrate the broad potential of this technique for late stage functionalization, we successfully incorporated challenging unprotected β-S-GlcNAc- and α-S-GalNAc-derivatives into very long unprotected peptides. This study opens the way to new applications in chemical biology, considering the well-recognized advantages of S-glycosides over O-glycosides in terms of resistance towards both enzymatic and chemical degradation.

Investigation of α-Thioglycoside Donors: Reactivity Studies toward Configuration-Controlled Orthogonal Activation in One-Pot Systems

The influence of anomeric configuration upon thioglycoside donors remains relatively unexplored. Utilizing methodology developed for the stereoselective and high-yielding synthesis of α-glycosyl thiols, a series of α-thioglycosides were synthesized, and their reactivity was compared to that of their β-counterparts. The highly selective activation observed for anomeric pairs containing a 2-O-acyl moiety and additional findings are reported. Application of a pair of "superarmed" thioglycosides to a one-pot oligosaccharide system is also described, in which selectivity is a result of configuration-based orthogonal activation.

Surface Modification of Polydivinylbenzene Microspheres with a Fluorinated Glycopolymer Using Thiol-Halogen Click Chemistry

Distillation-precipitation polymerization of divinylbenzene was applied to obtain uniform-sized polymeric microspheres. The microspheres were then modified with polypentafluorostyrene chains utilizing surface-initiated atom transfer radical polymerization techniques. The hydrophobic fluoropolymer-coated microsphere was then converted to a hydrophilic biopolymer by performing thiol-halogen click chemistry between polypentafluorostyrene and 1-thio-β-D-glucose sodium salt. The semi-fluorinated glycopolymer showed good binding ability with Concanavalin A as determined by confocal microscopy and turbidity experiments.

A Solvent-free Approach to Glycosyl Amides: Towards the Synthesis of α-N-Galactosyl Ceramides

A new, simple and efficient method for the synthesis of both α- and β-glycosyl amides using solvent-free conditions is described. This method involves the coupling of glycosyl amines with the p-nitrophenol esters of lipids as a key step.

Synthesis and Biological Activities of 5-Thio-α-GalCers

NKT cells, a unique subset of T cells that recognizes glycolipid antigens presented by CD1d molecules, are believed to produce key cytokines of both Th1 and Th2 T cells and are thus involved in the control of several types of immune response. As an active glycolipid antigen having α-galactosyl ceramide core structure, KRN7000 showed promising immunostimulation activity and was selected as an anticancer drug candidate for further clinical application. In this report, three new KRN7000 structural analogues were designed and synthesized, in which the ring oxygen of the galactopyranose residue is replaced by a sulfur atom along with the variation on the lipid chain. Their abilities for stimulating mouse NKT cells to produce IFN-γ and IL-4 were evaluated both in vivo and in vitro.

Expedient synthesis of an α-S-(1→6)-linked pentaglucosyl thiol

An α-S-(1→6)-linked pentaglucosyl thiol (1) is synthesized via the longest linear sequence of eleven steps from an α-glucosyl thiol (7).

Stimulation of natural killer T cells by glycolipids

Natural killer T (NKT) cells are a subset of T cells that recognize glycolipid antigens presented by the CD1d protein. The initial discovery of immunostimulatory glycolipids from a marine sponge and the T cells that respond to the compounds has led to extensive research by chemists and immunologists to understand how glycolipids are recognized, possible responses by NKT cells, and the structural features of glycolipids necessary for stimulatory activity. The presence of this cell type in humans and most mammals suggests that it plays critical roles in antigen recognition and the interface between innate and adaptive immunity. Both endogenous and exogenous natural antigens for NKT cells have been identified, and it is likely that glycolipid antigens remain to be discovered. Multiple series of structurally varied glycolipids have been synthesized and tested for stimulatory activity. The structural features of glycolipids necessary for NKT cell stimulation are moderately well understood, and designed compounds have proven to be much more potent antigens than their natural counterparts. Nevertheless, control over NKT cell responses by designed glycolipids has not been optimized, and further research will be required to fully reveal the therapeutic potential of this cell type.

Synthesis of α-O- and α-S-glycosphingolipids related to Sphingomonous cell wall antigens using anomerisation

Analogues of glycolipids from Spingomonadacaece with O- and S- and SO2-linkages have been prepared using chelation induced anomerisation promoted by TiCl4. Included are examples of the anomerisation of intermediates with O- and S-glycosidic linkages as well as isomerisation of β-thioglycuronic acids (β-glycosyl thiols). The β-O-glucuronide and β-O-galacturonide precursors were efficiently prepared using benzoylated trichloroacetimidates. β-Glycosyl thiols were precursors to β-S-derivatives. Triazole containing mimics of the natural glycolipids were prepared using CuI promoted azide-alkyne cycloaddition reactions in THF. The glycolipid antigens are being evaluated currently for their effects on iNKT cells.

Raising the roof: the preferential pharmacological stimulation of Th1 and th2 responses mediated by NKT cells

Natural killer T (NKT) cells serve as a bridge between the innate and adaptive immune systems, and manipulating their effector functions can have therapeutic significances in the treatment of autoimmunity, transplant biology, infectious disease, and cancer. NKT cells are a subset of T cells that express cell-surface markers characteristic of both natural killer cells and T cells. These unique immunologic cells have been demonstrated to serve as a link between the innate and adaptive immune systems through their potent cytokine production following the recognition of a range of lipid antigens, mediated through presentation of the major histocompatibility complex (MHC) class I like CD1d molecule, in addition to the NKT cell's cytotoxic capabilities upon activation. Although a number of glycolipid antigens have been shown to complex with CD1d molecules, most notably the marine sponge derived glycolipid alpha-galactosylceramide (α-GalCer), there has been debate as to the identity of the endogenous activating lipid presented to the T-cell receptor (TCR) via the CD1d molecule on antigen-presenting cells (APCs). This review aims to survey the use of pharmacological agents and subsequent structure-activity relationships (SAR) that have given insight into the binding interaction of glycolipids with both the CD1d molecules as well as the TCR and the subsequent immunologic response of NKT cells. These studies not only elucidate basic binding interactions but also pave the way for future pharmacological modulation of NKT cell responses.

Structure-activity relationship studies of novel glycosphingolipids that stimulate natural killer T-cells

KRN7000, an anticancer drug candidate developed by Kirin Brewery Co. in 1995, is an α-galactosyl ceramide. It is a ligand making a complex with CD1d protein, and it stimulates invariant natural killer T (NKT) cells, which are one of the lineages of immunocytes. NKT cells activated by recognition of the CD1d/KRN7000 complex with its invariant T-cell receptor (TCR) can induce both protective and regulatory immune responses. To determine the recognition and activation mechanisms of NKT cells and to develop drug candidates more effective than KRN7000, a large number of analogs of KRN7000 have been synthesized. Some of them show potent bioactivities and have the potential of being utilized as therapeutic agents. In this review, structure-activity relationship studies of novel glycolipids which stimulate NKT cells efficiently are summarized.

Activation of human invariant natural killer T cells with a thioglycoside analogue of α-galactosylceramide

Activation of CD1d-restricted invariant NKT (iNKT) cells with the glycolipid α-galactosylceramide (α-GalCer) confers protection against disease in murine models, however, clinical trials in humans have had limited impact. We synthesized a novel thioglycoside analogue of α-GalCer, denoted α-S-GalCer, and tested its efficacy for stimulating human iNKT cells in vitro. α-S-GalCer stimulated cytokine release by iNKT cells in a CD1d-dependent manner and primed CD1d(+) target cells for lysis. α-S-GalCer-stimulated iNKT cells induced maturation of monocyte-derived dendritic cells into antigen-presenting cells that released IL-12 and small amounts of IL-10. The nature and potency of α-S-GalCer and α-GalCer in human iNKT cell activation were similar. However, in contrast to α-GalCer, α-S-GalCer did not activate murine iNKT cells in vivo. Because of its enhanced stability in biological systems, α-S-GalCer may be superior to α-GalCer as a parent compound for developing adjuvant therapies for humans.