Syntheses of the 3- and 4-thio analogues of 4-nitrophenyl 2-acetamido-2-deoxy-beta-D-gluco- and galactopyranoside

Synthesis of Thiodisaccharides Bearing N-Acetylhexosamine Residues: Challenges, Achievements and Perspectives

Carbohydrate-protein interactions are involved in a myriad of biological processes. Thus, glycomimetics have arisen as one of the most promising synthetic targets to that end. Within the broad variety of glycomimetics, thiodisaccharides have proven to be excellent tools to study these processes, and even more, some of them unveiled interesting biological activities. This review brings together research made on the introduction of N-acetylhexosamine residues into thiodisaccharides to date, passing through classic substitution (as S_N 2, thioglycosylation and ring-opening reactions) and addition (as thiol-ene coupling and Michael-type additions) reactions. Recent and interesting developments regarding addition reactions to vinyl azides, cross-coupling reactions and novel chemoenzymatic methods are also discussed.

Protecting-group-free S-glycosylation towards thioglycosides and thioglycopeptides in water

A facile and green S-glycosylation method has been developed featuring protecting-group-free and proceeding-in-water like enzymatic synthesis. Glycosylation of fluoride donors with thiol sugar acceptors using Ca(OH)2 as a promoter afforded various thioglycosides in good yields with exclusive stereoselectivity. This method also enabled the successful production of S-linked oligosaccharides and S-linked glycopeptides.

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.

Scope of the DMC mediated glycosylation of unprotected sugars with phenols in aqueous solution

Activation of reducing sugars in aqueous solution using DMC and triethylamine in the presence of phenols allows direct stereoselective conversion to the corresponding 1,2-trans aryl glycosides without the need for any protecting groups.

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.

Facile Formation of β-thioGlcNAc Linkages to Thiol-Containing Sugars, Peptides, and Proteins using a Mutant GH20 Hexosaminidase

Thioglycosides are hydrolase‐resistant mimics of O‐linked glycosides that can serve as valuable probes for studying the role of glycosides in biological processes. The development of an efficient, enzyme‐mediated synthesis of thioglycosides, including S‐GlcNAcylated proteins, is reported, using a thioglycoligase derived from a GH20 hexosaminidase from Streptomyces plicatus in which the catalytic acid/base glutamate has been mutated to an alanine (SpHex E314A). This robust, easily‐prepared, engineered enzyme uses GlcNAc and GalNAc donors and couples them to a remarkably diverse set of thiol acceptors. Thioglycoligation using 3‐, 4‐, and 6‐thiosugar acceptors from a variety of sugar families produces S‐linked disaccharides in nearly quantitative yields. The set of possible thiol acceptors also includes cysteine‐containing peptides and proteins, rendering this mutant enzyme a promising catalyst for the production of thio analogues of biologically important GlcNAcylated peptides and proteins.

Monosaccharide inhibitors targeting carbohydrate esterase family 4 de-N-acetylases

The Carbohydrate Esterase family 4 contains virulence factors which modify peptidoglycan and biofilm-related exopolysaccharides. Despite the importance of this family of enzymes, a potent mechanism-based inhibition strategy has yet to emerge. Based on the postulated tridentate binding mode of the tetrahedral de-N-acetylation intermediate, GlcNAc derivatives bearing metal chelating groups at the 2 and 3 positions were synthesized. These scaffolds include 2-C phosphonate, 2-C sulfonamide, 2-thionoacetamide warheads as well as derivatives bearing thiol, amine and azide substitutions at the 3-position. The inhibitors were assayed against a representative peptidoglycan deacetylase, Pgda from Streptococcus pneumonia, and a representative biofilm-related exopolysaccharide deacetylase, PgaB from Escherichia coli. Of the inhibitors evaluated, the 3-thio derivatives showed weak to moderate inhibition of Pgda. The strongest inhibitor was benzyl 2,3-dideoxy-2-thionoacetamide-3-thio-β-d-glucoside, whose inhibitory potency showed an unexpected dependence on metal concentration and was found to have a partial mixed inhibition mode (K_i = 2.9 ± 0.6 μM).

Synthesis and conformational analysis of linear homo- and heterooligomers from novel 2-C-branched sugar amino acids (SAAs)

Sugar amino acids (SAAs), as biologically interesting structures bearing both amino and carboxylic acid functional groups represent an important class of multifunctional building blocks. In this study, we develop an easy access to novel SAAs in only three steps starting from nitro compounds in high yields in analytically pure form, easily available by ceric (IV) mediated radical additions. Such novel SAAs have been applied in the assembly of total nine carbopeptoids with the form of linear homo- and heterooligomers for the structural investigations employing circular dichroism (CD) spectroscopy, which suggest that the carbopeptoids emerge a well-extended, left (or right)-handed conformation similar to polyproline II (PPII) helices. NMR studies also clearly demonstrated the presence of ordered secondary structural elements. 2D-ROESY spectra were acquired to identify ⁱ⁺¹ NH ↔  ⁱ C ₁ H, ⁱ C ₂ H correlations which support the conformational analysis of tetramers by CD spectroscopy. These findings provide interesting information of SAAs and their oligomers as potential scaffolds for discovering new drugs and materials.

Synthesis of sulfated glucosaminides for profiling substrate specificities of sulfatases and fungal beta-N-acetylhexosaminidases

Systematic sulfation: Sulfated glycoconjugates are degraded either by desulfation followed by glycoside cleavage, or by glycoside cleavage followed by desulfation. To study these processes, here we report the synthesis of four regioisomerically sulfated p-nitrophenyl glucosaminides from the common precursor p-nitrophenyl N-acetyl-beta-D-glucosaminide. These substrates allowed the rapid analysis of the substrate preferences of a set of four sulfatases and 24 hexosaminidases.Sulfated carbohydrates are components of many glycoconjugates, and are degraded by two major processes: cleavage of the sulfate ester by a sulfatase, or en bloc removal of a sulfated monosaccharide by a glycoside hydrolase. However, these processes have proved difficult to study owing to a lack of homogeneous, defined substrates. We describe here the synthesis of a series of p-nitrophenyl beta-D-glucosaminides bearing sulfate esters at the 2-, 3-, 4- or 6-positions, by divergent routes starting with p-nitrophenyl 2-acetamido-2-deoxy-beta-D-glucopyranoside. The sulfated p-nitrophenyl beta-D-glucosaminides were used to study the substrate specificity of four sulfatases (from Helix pomatia, Patella vulgata, abalone, and Pseudomonas aeruginosa), and revealed significant differences in the preference of each of these enzymes for desulfation at different positions around the sugar ring. The 3-, 4- and 6-sulfated p-nitrophenyl 2-acetamido-2-deoxy-beta-D-glucosaminides were screened against a panel of 24 fungal beta-N-acetylhexosaminidases to assess their substrate specificity. While the 4- and 6-sulfates were substrates for many of the fungal enzymes investigated, only a single beta-N-acetylhexosaminidase, that from Penicillium chrysogenum, could hydrolyze the 3-sulfated p-nitrophenyl glycoside. Together these results demonstrate the utility of sulfated p-nitrophenyl beta-D-glucosaminides for the study of both sulfatases and glycoside hydrolases.

Convenient synthesis of branched-chain glycosamines by radical addition of nitromethane to glycals

Radical addition-reduction-acetylation is the simple three-step sequence for the synthesis of branched-chain glycosamines 3 from glycals 1 and nitromethane (2). The intermediary formed 2-C-nitromethyl-pyranosides are valuable precursors for the synthesis of C-2 branched disaccharides.