Synthesis of Branched Dithiotrisaccharides via Ring-Opening Reaction of Sugar Thiiranes

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.

Synthesis of thiodisaccharides related to 4-thiolactose. Specific structural modifications increase the inhibitory activity against E. coli β-galactosidase

In the search for new glycosidase inhibitors, a set of benzyl β-D-Gal-S-(1→4)-3-deoxy-4-thio-α-D-hexopyranosides was synthesized. Diverse configurations were installed at C-2 and C-4 of the glucose residue. The benzyl glycosidic group was kept intact or substituted by an electron-donating or electron-withdrawing group that could also participate in hydrogen bonding. All thiodisaccharides were found to be inhibitors of E. coli β-galactosidase. In general, benzyl thiodisaccharides were better inhibitors than those substituted (NO₂ or NH₂) on the benzyl ring. Thiodisaccharides containing a hexopyranoside, instead of a pentopyranoside, showed a weaker inhibitory activity, except for those having the α-D-xylo configuration, which exhibited inhibition constants of the same order of magnitude. These and previous results indicated that the inhibition process by thiodisaccharides is strongly dependent on the configuration of the 3-deoxy-4-thiopyranoside, as well as its substitution pattern (such as the presence of a benzyl glycoside). The enzyme-inhibitor interaction during the hydrolysis process involves a conformational selection resulting from rotation around the thioglycosidic bond and the flexibility of the terminal six-membered ring. Thus, the mentioned structural features of the inhibitor could give rise to favorable ground state conformations for the interaction with the enzyme, similar to those found for selected thiodisaccharides in the bound state. These studies demonstrated that the performance of thiodisaccharides as enzyme inhibitors could be increased by selecting the appropriate configuration and substitution of the hexopyranoside replacing the glucose moiety of 4-thiolactose.

Synthesis of Oligosaccharides Containing the S-Galp(α1 → 3)Galp Unit, Glycomimetic of the Epitope Recognized by Lytic Antibodies

Two important activities take place in the surface of Trypanosoma cruzi, the agent of Chagas disease: the trans-sialidase (TcTS) catalyzes the transfer of sialic acid from the host glycoconjugates to the mucin-like glycoproteins from the parasite and the presence of lytic antibodies recognize the epitope α-Galp(1 → 3)-β-Galp(1 → 4)-α-GlcNAcp. This antigenic structure is known to be present in the parasite mucins; however, in order to be substrates of trans-sialidase, some of the galactose residues should be in the β-Galp configuration. To study the interaction between both activities, it is important to count the synthetic structures as well as the structural-related glycomimetics. With this purpose, we addressed the synthesis of a trisaccharide and two isomeric tetrasaccharides containing the 1-S-α-Galp(1 → 3)-β-Galp motif, the thio analog of the epitope recognized by lytic antibodies. Starting with a common lactose precursor, the sulfur function was incorporated by double inversion of the configuration of the galactose residue that was further glycosylated using different activated donors. Both tetrasaccharides were good acceptors of sialic acid in the reaction catalyzed by TcTS, as determined by high-performance anion exchange chromatography.

The disulfide bond as a key motif for the construction of multivalent glycoclusters

S-Glycosylated dendrons having a thioacetate group in their focal points led to multivalent glycoclusters by spontaneous O2-oxidation of sulfides.

Photoinitiated Thiol-ene Reactions of Enoses: A Powerful Tool for Stereoselective Synthesis of Glycomimetics with Challenging Glycosidic Linkages

Thioglycosides and C-glycosides represent pharmacologically useful classes of glycomimetics that possess a high degree of biological stability. One emerging tool for the stereoselective synthesis of thioglycosides is the photoinitiated addition of thiols to unsaturated sugars. Moreover, thiyl radical-mediated reactions of exo-glycals and 1-substituted endo-glycals offer facile routes to β-C-glycosidic structures. This Concept article summarizes the thiol-ene coupling strategies developed recently by our group and Somsák's group for the synthesis of several kinds of glycomimetics which are difficult to synthesize by conventional methods. One unusual characteristic of the thiol-ene reactions of endo-glycals is that heating inhibits, whereas cooling promotes the reaction. This unique temperature dependence as well as the effects of the enose structures and thiol configurations on the efficacy and stereoselectivity of the reactions are also discussed.

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 triflic acid-catalyzed α-1,2-cis-thio glycosylations: scope and application to the synthesis of S-linked oligosaccharides, glycolipids, sublancin glycopeptides, and TN/TF antigens

Studies of S-linked glycoconjugates have attracted growing interest because of their enhanced chemical stability and enzymatic resistance over O-glycoside counterparts.

Studies of S-linked glycoconjugates have attracted growing interest because of their enhanced chemical stability and enzymatic resistance over O-glycoside counterparts. We here report a facile approach to access α-1,2-cis-S-linked glycosides using triflic acid as a catalyst to promote the glycosylation of a series of thiols with d-glucosamine, galactosamine, glucose, and galactose electrophiles. This method is broadly applicable for the stereoselective synthesis of S-linked glycopeptides, oligosaccharides and glycolipids in high yield and excellent α-selectivity. Many of the synthetic limitations associated with the preparation of these S-linked products are overcome by this catalytic method.

Promoter free allylation of trichloroacetimidates with allyltributylstannanes under thermal conditions to access the common 1,1'-diarylbutyl pharmacophore

1,1'-Diarylbutyl groups are a common pharmacophore found in many biologically active small molecules. To access these systems under mild conditions, the reaction of diarylmethyl trichloroacetimidates with allyltributylstannanes was explored. Simply heating allyltributylstannane with the trichloroacetimidate resulted in substitution of the imidate with an allyl group. Unlike other methods used to access these systems, no strong base, transition metal catalyst, Brønsted acid or Lewis acid promoter was required to affect the transformation. Conversions are best with electron rich benzylic trichloroacetimidate systems, where excellent yields are achieved just by refluxing the reactants together in nitromethane.

Synthesis of 1,1'-Diarylethanes and Related Systems by Displacement of Trichloroacetimidates with Trimethylaluminum

Benzylic trichloroacetimidates are readily displaced by trimethylaluminum under Lewis acid promoted conditions to provide the corresponding methyl substitution product. This method is a convenient way to access 1,1'-diarylethanes and related systems, which play a significant role in medicinal chemistry, with a number of systems owing their biological activity to this functionality. Most benzylic substrates undergo ready displacement, with electron deficient systems being the exception. The use of an enantiopure imidate showed significant racemization, implicating the formation of a cationic intermediate.

Synthesis of Enantiomeric Polyhydroxyalkylpyrrolidines from 1,3-Dipolar Cycloadducts. Evaluation as Inhibitors of a β-Galactofuranosidase

Enantiomeric 2,3,4-tris(hydroxyalkyl)-5-phenylpyrrolidines have been synthesized from the major cycloadducts obtained by the 1,3-dipolar cycloaddition of sugar enones with azomethine ylides derived from natural amino acids. Reduction of the ketone carbonyl group of the cycloadducts, which possess a basic structure of bicyclic 6-(menthyloxy)hexahydropyrano[4,3-c]pyrrol-7(6H)one, afforded a number of pyrrolidine-based bicyclic systems. A sequence of reactions, which involved hydrolysis of the menthyloxy substituent, reduction, N-protection, and degradative oxidation, afforded varied pyrrolidine structures having diverse configurations and patterns of substitution; in particular, polyhydroxylated derivatives have been obtained. The unprotected products were isolated as pyrrolidinium trifluoroacetates. Because of the furanose-like nature of the target trihydroxyalkyl pyrrolidines, these molecules have been evaluated as inhibitors of the β-galactofuranosidase from Penicillium fellutanum. The compounds showed practically no inhibitory activity for concentration of pyrrolidines in the range of 0.1-1.6 mM.

Synthesis of galactofuranosyl-(1 → 5)-thiodisaccharide glycomimetics as inhibitors of a β-d-galactofuranosidase

Description of the synthesis, molecular modeling and inhibitory properties of furanosyl thiodisaccharides that are mimetics of the motif β-d-Galf-(1 → 5)-d-Galf, found in glycoconjugates of pathogenic microorganisms.

Synthesis of D-galactofuranose-containing molecules: design of galactofuranosyl acceptors

D-Galactofuranose (D-Galf) is present in glycoconjugates of several pathogenic microorganisms but is absent in mammals, so it is a good target for the development of chemotherapeutic agents for the treatment of microbial infections. This fact has increased interest in the synthesis of D-Galf-containing molecules for corresponding glycobiological studies. The synthesis of oligosaccharides, glycoconjugates, and mimetics of D-Galf requires specific methods for the preparation of galactose derivatives in the furanosic configuration, the synthesis of appropriate acceptors, and efficient glycosylation methods for the construction of α- and β-D-Galf linkages. This review summarizes the different strategies developed for the preparation of partially protected derivatives of D-Galf, suitable as acceptors for the construction of (1→2), (1→3), (1→5), and (1→6) link- ages, and describes recent applications.

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).