Synthesis of two hyaluronan trisaccharides

RGD Peptidomimetic MMAE-Conjugate Addressing Integrin αVβ3-Expressing Cells with High Targeting Index

Small molecule-drug conjugates (SMDCs) mimicking the RGD sequence (-Arg-Gly-Asp-) with a non-peptide moiety require a pharmacophore-independent attachment site. A library of 36 sulfonamide-modified RGD mimetics with nM to pM affinity for integrin α_V β₃ was synthesized and analysed via DAD mapping. The best structure of the conjugable RGD mimetic was used and a linker was attached to an aromatic ring by Negishi cross-coupling. The product retained high affinity and selectivity for integrin α_V β₃ . The conjugable RGD mimetic was then attached to an enzymatically cleavable GKGEVA linker equipped with a self-immolative PABC and the antimitotic drug monomethyl auristatin E (MMAE). The resulting SMDC preferred binding to integrin α_V β₃ over α₅ β₁ in a ratio of 1 : 4519 (ELISA) and showed selectivity for α_V β₃ -positive WM115 cells over α_V β₃ -negative M21-L cells in the in vitro cell adhesion assay as well as in cell viability assays with a targeting index of 134 (M21-L/WM115).

Synthetic Studies on the Incorporation of N-Acetylallosamine in Hyaluronic Acid-Inspired Thiodisaccharides

Two approaches for the synthesis of the thiodisaccharide β-S-GlcA(1→3)β-S-AllNAc are described here. The target disaccharide was a C-3 epimer and thio-analogue of the hyaluronic acid repetitive unit, tuned with a thiopropargyl anomeric group for further click conjugation. Thus, we analysed and tested two convenient sequences, combining the two key steps required to introduce the thioglycosidic bonds and consequently reach the target molecule: the SN2 substitution of a good leaving group (triflate) present at C-3 of a GlcNAc derivative and the introduction of the anomeric thiopropargyl substituent. The use of a 2-azido precursor showed to be a convenient substrate for the SN2 step. Nevertheless, further protecting group manipulation and the introduction of the thiopropargyl anomeric residue were then required. This approach showed to provide access to a variety of thiodisaccharide derivatives as interesting building blocks for the construction of neoglycoconjugates.

Stereoselective Synthesis of Diglycosyl Diacylglycerols with Glycosyl Donors Bearing a β-Stereodirecting 2,3-Naphthalenedimethyl Protecting Group

Diglycosyl diacylglycerols (DGDGs) are major components of Gram-positive bacterial plasma membranes and are involved in the immune response systems. The chemical synthesis of DGDGs has been highly demanded, as it will allow the elucidation of their biological functions at the molecular level. In this study, we have developed a novel β-stereodirecting 2,3-naphthalenedimethyl (NapDM) protecting group that is orthogonal to protecting groups commonly used in oligosaccharide synthesis. The NapDM group can be easily cleaved under TFA-mediated acidic conditions. Futhermore, we demonstrated the application of this protecting group to an acyl protecting-group-free strategy by utilizing the NapDM group for the synthesis of DGDGs. This strategy features the use of the β-stereodirecting NapDM group as an acid-cleavable permanent protecting group and late-stage glycosylation of monoglycosyl diacylglycerol acceptors, enabling the stereoselective synthesis of three different bacterial DGDGs with unsaturated fatty acid chain(s).

Synthesis of hyaluronic acid oligosaccharides with a GlcNAc-GlcA repeating pattern and their binding affinity with CD44

Hyaluronic acid (HA) is a ubiquitous glycosaminoglycan in the extracellular matrix and a ligand of CD44, a transmembrane glycoprotein that is important in cell migration. Crystal and NMR studies found a hexasaccharide of the pattern (GlcA-GlcNAc)3 as the shortest HA that could bind to CD44, but molecular dynamics simulations indicated that a tetrasaccharide of the pattern (GlcNAc-GlcA)2 is the key structure interacting with CD44. Access to oligomers with such a repeat pattern is crucial in binding studies with CD44. Here we developed a synthetic procedure to afford the HA oligosaccharides with the GlcNAc-GlcA repeating unit and measured the binding interaction between these sugars and human CD44 by isothermal titration calorimetry (ITC). During the chemical synthesis, we successfully generated the β-glycosidic bond in the absence of neighbouring group participation and overcome the issues in the oxidation step. In addition, ammonia-free dissolving metal reduction for debenzylation and azido reduction has been applied in carbohydrate synthesis for the first time. ITC analysis revealed that the HA tetrasaccharide (GlcNAc-GlcA)2 could indeed interact and bind to the human CD44.

Synthesis of (1→3) Thiodisaccharides of GlcNAc and the Serendipitous Formation of 2,3-Dideoxy-(1→2)-thiodisaccharides through a Vinyl Azide Intermediate

The syntheses of β-S-GlcA(1→3)GlcNAc and β-S-Gal(1→3)GlcNAc thiodisaccharides, which can be considered mimetics of the repeating units of hyaluronan and keratan respectively, were achieved by S_N2 displacement of a triflate group allocated at the 3-position of a convenient 2-azido-4,6-O-benzylidene-2-deoxy-β-d-allopyranose precursor by the corresponding nucleophilic suitable protected thioaldoses derived from glucuronic acid (GlcA) and galactose (Gal). The study of the reaction led to the finding that the vinyl azide formed by competitive E2 reaction of the mentioned triflate was an interesting precursor of a new kind of 2,3-dideoxy-2-azido-(1→2) thiodisaccharides through an addition reaction. Determination of the stereochemistry of the new stereocenter at C-2 was achieved by NOESY experiments. Final protecting group manipulation of the (1→3) thiodisaccharides led to a family of derivatives that could be used as building blocks for the synthesis of complex glycomimetics.

Constrained sialic acid donors enable selective synthesis of α-glycosides

Sialic acid is a sugar residue present in many biologically significant glycans of mammals, commonly as a terminal α-glycoside. The chemical structure of sialic acid, which features an anomeric center with carboxyl and methylene substituents, poses a challenge for synthesis of the α-glycoside, thus impeding biological and therapeutic studies on sialic acid–containing glycans. We present a robust method for the selective α-glycosidation of sialic acid using macrobicyclized sialic acid donors as synthetic equivalents of structurally constrained oxocarbenium ions to impart stereoselectivity. We demonstrate the power of our method by showcasing broad substrate scope and applicability in the preparation of diverse sialic acid–containing architectures.

Efficient one-pot per-O-acetylation–thioglycosidation of native sugars, 4,6-O-arylidenation and one-pot 4,6-O-benzylidenation–acetylation of S-/O-glycosides catalyzed by Mg(OTf)2

Sequential one-pot per-O-acetylation–S-/O-glycosidation under neat condition, regioselective 4,6-O-arylidenation and sequential one-pot benzylidenation–acetylation of Mg(OTf)₂as non-hygroscopic, recyclable catalyst are reported.

Direct Staudinger-Phosphonite Reaction Provides Methylphosphonamidates as Inhibitors of CE4 De-N-acetylases

De-N-acetylases of β-(1→6)-D-N-acetylglucosamine polymers (PNAG) and β-(1→4)-D-N-acetylglucosamine residues in peptidoglycan are attractive targets for antimicrobial agents. PNAG de-N-acetylases are necessary for biofilm formation in numerous pathogenic bacteria. Peptidoglycan de-N-acetylation facilitates bacterial evasion of innate immune defenses. To target these enzymes, transition-state analogue inhibitors containing a methylphosphonamidate have been synthesized through a direct Staudinger-phosphonite reaction. The inhibitors were tested on purified PgaB, a PNAG de-N-acetylase from Escherichia coli, and PgdA, a peptidoglycan de-N-acetylase from Streptococcus pneumonia. Herein, we describe the most potent inhibitor of peptidoglycan de-N-acetylases reported to date (Ki =80 μM). The minimal inhibition of PgaB observed provides insight into key structural and functional differences in these enzymes that will need to be considered during the development of future inhibitors.

Gaining insight into the catalysis by GH20 lacto-N-biosidase using small molecule inhibitors and structural analysis

Synthesis and structural analysis of rationally developed inhibitors.

Extending the glucosyl ceramide cassette approach: application in the total synthesis of ganglioside GalNAc-GM1b

The development of a novel cyclic glucosyl ceramide cassette acceptor for efficient glycolipid syntheses was investigated. p-Methoxybenzyl (PMB) groups were selected as protecting groups at C2 and C3 of the glucose residue with the aim of improving the functionality of the cassette acceptor. The choice of the PMB group resulted in a loss of β-selectivity, which was corrected by using an appropriate tether to control the spatial arrangement and the nitrile solvent effect. To investigate the effect of linker structure on the β-selectivity of intramolecular glycosylation, several linkers for tethering the glucose and ceramide moiety were designed and prepared, namely, succinyl, glutaryl, dimethylmalonyl, and phthaloyl esters. The succinyl ester linker was the best for accessing the cassette form. The newly designed glucosyl ceramide cassette acceptor was then applied in the total synthesis of ganglioside GalNAc-GM1b.

The first chemical synthesis of novel MeO-3-GlcUA derivative of hyaluronan-based disaccharide to elucidate the catalytic mechanism of hyaluronic acid synthases (HASs)

The first chemical synthesis of MeO-3-GlcUAβ(1→3)GlcNAc-UDP to elucidate the catalytic mechanism of hyaluronic acid synthases (HASs) is described. Construction of the desired β(1→3)-linked disaccharide 10 was achieved very efficiently by coupling MeO-3-GlcUA donor 3 with the suitable protected GlcNTroc acceptor 4 using BF(3(.) )Et(2)O as Lewis acid. Chemoselective removal of anomeric NAP, phosphorylation, hydrogenation, coupling with UMP-morpholidate and finally complete deprotection gave the target compound 1 in good yield.

A facile method for oxidation of primary alcohols to carboxylic acids and its application in glycosaminoglycan syntheses

A convenient two-step, one-pot procedure was developed for the conversion of primary alcohols to carboxylic acids. The alcohol was first treated with NaOCl and TEMPO under phase-transfer conditions, followed by NaClO2 oxidation in one pot. This reaction is applicable to a wide range of alcohols and the mild reaction conditions are compatible with many sensitive functional groups, including electron-rich aromatic rings, acid-labile isopropylidene ketal and glycosidic linkages, and oxidation-prone thioacetal, p-methoxybenzyl, and allyl moieties. Several glycosaminoglycans such as heparin, chondroitin, and hyaluronic acid oligosaccharides have been synthesized in high yields by using this new oxidation protocol.

Synthesis of a C-linked hyaluronic acid disaccharide mimetic

The synthesis of a C-disaccharide that is designed as a mimetic for the repeating unit disaccharide of hyaluronic acid is described. The target compound was obtained via the SmI2-promoted coupling reaction of the sulfone, 2-acetamido-4,6-O-benzylidene-3-O-tert-butyldimethylsilyl-1,2-dideoxy-1-pyridinylsulfonyl-beta-D-glucopyranose (6), and the aldehyde, p-methoxyphenyl 2,3-di-O-benzyl-4-deoxy-4-C-formyl-6-O-p-methoxybenzyl-beta-D-glucopyranoside (14).

Recent trends in the synthesis of O-glycosides of 2-amino-2-deoxysugars

The discovery of new methods for stereoselective glycoside synthesis and convergent oligosaccharide assembly has been critical for the area of glycosciences. At the heart of this account is the discussion of the approaches for stereoselective synthesis of glycosides of 2-amino-2-deoxysugars that have emerged during the past two decades. The introductory part provides general background information and describes the key features and challenges for the synthesis of this class of compounds. Subsequently, major approaches to the synthesis of 2-amino-2-deoxyglycosides are categorized and discussed. Each subsection elaborates on the introduction (or protection) of the amino functionality, synthesis of glycosyl donors by introduction of a suitable leaving group, and glycosidation. Wherever applicable, the deprotection of a temporary amino group substituent and the conversion onto the natural acetamido functionality is described. The conclusions part evaluates the current standing in the field and provides a perspective for future developments.

Highly efficient syntheses of hyaluronic acid oligosaccharides

Highly efficient syntheses of hyaluronic acid oligosaccharides have been accomplished through the pre-activation based iterative one-pot strategy. A series of oligosaccharides ranging from di- to hexasaccharides were rapidly assembled using only near stoichiometric amounts of the building blocks without aglycon adjustment or purifications of intermediate oligosaccharides. Deprotection and oxidation protocols were developed for protective group removal and oxidation-state adjustment. The availability of such structurally well defined synthetic hyaluronic acid oligosaccharides will greatly facilitate the establishment of detailed structure-function relationships.

An Expedient Synthesis of the Repeating Unit of the Acidic Polysaccharide of the Bacteriolytic Complex of Lysoamidase

The first synthesis of the trisaccharide repeating unit of the acidic polysaccharide of the bacteriolytic complex of lysoamidase is presented. The construction is based on a linear glycosylation strategy that starts from the reducing end and employs thio- and selenoglycosides in a highly stereoselective manner by a single set of activation conditions. The thus-formed trisaccharide is selectively deprotected and oxidised, after which a final deprotection step furnishes the desired repeating unit.

Gram-scale syntheses of the (1→3)-linked and (1→4)-linked hyaluronan disaccharides

The first gram-scale syntheses of two hyaluronan disaccharides are described. Construction of the (1-->4)-linked disaccharide 12 was achieved in 12% overall yield using 2,3-bis-dimethyl acetal protection in combination with chlorosilane-induced carbamate cleavage methodologies. The uronic acid functionality was installed using TEMPO oxidation with NaOCl as the hypochlorite source. The (1-->3)-linked disaccharide 18 was achieved in 7% overall yield utilizing acetonide protection in addition to the chlorosilane-induced carbamate cleavage methodology and the TEMPO oxidation.

A facile preparation of uronates via selective oxidation with TEMPO/KBr/Ca(OCl)2 under aqueous conditions

Addition of solid Ca(OCl)(2) as the terminal oxidant in the TEMPO-mediated selective oxidation has the benefit of easier operation. A variety of partially protected saccharide derivatives (1a-l) have been successfully converted into the corresponding uronate derivatives, including disaccharide building blocks for GAG fragments and precursors to saponins. The beneficial effect of Aliquat 336 was also disclosed in the oxidation of certain substrates.

Stereoselective synthesis of [13C]methyl 2-[15N]amino-2-deoxy-beta-D-glucopyranoside derivatives

Efficient syntheses of three [13C]methyl 2-[15N]amino-2-deoxy-beta-D-glucopyranoside derivatives are described. Amination of the D-glucal with (saltmen)Mn(15N) proceeded with 11:1 stereoselectivity favoring the gluco configuration; subsequent methylation of the [15N]lactol using [13C]iodomethane and silver(I) oxide afforded the doubly labeled beta glucoside in high yield. This compound served as the common precursor for three [13C]methyl 2-[15N]aminoglucosides: (2-[15N]trifluoroacetyl-), (2-[15N]acetyl-), and (2-[15N]azido-). Selected heteronuclear coupling constants are reported.

The mild cleavage of 2-amino-2-deoxy-D-glucoside methoxycarbonyl derivatives

The conversion of methyl carbamate to the corresponding free amine is described for a series of 2-amino-2-deoxy-D-glucosamine derivatives. Cleavage of methoxycarbonyl moiety with MeSiCl(3) and triethylamine in dry THF at 60 degrees C and subsequent aqueous hydrolysis yields the free amine in 54 to 93% yields. The selective cleavage of methyl carbamates with MeSiCl(3) in the presence of a 2,2,2-trichloroethoxycarbonyl group or 2-azido glycosides affords selectively, orthogonal N-deprotected carbohydrates.

A novel and effective procedure for the preparation of glucuronides

6-Phenylthio-6-deoxy-D-glucopyranosides were easily prepared from 6-hydroxy-D-glucopyranosides and employed as effective glycosyl donors or acceptors. And the resulting coupling products were then readily converted into the corresponding D-glucuronide-containing compounds.