NIS/TFA: a general method for hydrolyzing thioglycosides

Total Synthesis and Structural Studies of Zwitterionic Bacteroides fragilis Polysaccharide A1 Fragments

Zwitterionic polysaccharides (ZPSs) are exceptional carbohydrates, carrying both positively charged amine groups and negatively charged carboxylates, that can be loaded onto MHC-II molecules to activate T cells. It remains enigmatic, however, how these polysaccharides bind to these receptors, and to understand the structural features responsible for this "peptide-like" behavior, well-defined ZPS fragments are required in sufficient quantity and quality. We here present the first total synthesis of Bacteroides fragilis PS A1 fragments encompassing up to 12 monosaccharides, representing three repeating units. Key to our successful syntheses has been the incorporation of a C-3,C-6-silylidene-bridged "ring-inverted" galactosamine building block that was designed to act as an apt nucleophile as well as a stereoselective glycosyl donor. Our stereoselective synthesis route is further characterized by a unique protecting group strategy, built on base-labile protecting groups, which has allowed the incorporation of an orthogonal alkyne functionalization handle. Detailed structural studies have revealed that the assembled oligosaccharides take up a bent structure, which translates into a left-handed helix for larger PS A1 polysaccharides, presenting the key positively charged amino groups to the outside of the helix. The availability of the fragments and the insight into their secondary structure will enable detailed interaction studies with binding proteins to unravel the mode of action of these unique oligosaccharides at the atomic level.

Synthetic Zwitterionic Streptococcus pneumoniae Type 1 Oligosaccharides Carrying Labile O-Acetyl Esters

We herein report the first total synthesis of the Streptococcus pneumoniae serotype 1 (Sp1) oligosaccharide, a unique zwitterionic capsular polysaccharide carrying labile O-acetyl esters. The target oligosaccharides, featuring rare α-2,4-diamino-2,4,6-trideoxy galactose (AAT) and α-galacturonic acids, were assembled up to the 9-mer level, in a highly stereoselective manner using trisaccharide building blocks. The lability of the O-acetyl esters imposed a careful deprotection scheme to prevent migration and hydrolysis. The migration was investigated in detail at various pD values using NMR spectroscopy, to show that migration and hydrolysis of the C-3-O-acetyl esters readily takes place under neutral conditions. Structural investigation showed the oligomers to adopt a right-handed helical structure with the acetyl esters exposed on the periphery of the helix in close proximity of the neighboring AAT residues, thereby imposing conformational restrictions on the AATα1-4GalA(3OAc) glycosidic linkages, supporting the helical shape of the polysaccharide, that has been proposed to be critical for its unique biological activity.

α-Selective Glucosylation Can Be Achieved with 6-O-para-Nitrobenzoyl Protection

A systematic study of the effect of various 6-O-acyl groups on anomeric selectivity in glucosylations with thioglycoside donors was conducted. All eight different esters were found to induce moderate-to-high α-selectivity in glucosylation with l-menthol with the best being 6-O-p-nitrobenzoyl. The effect appears to be general across various glucosyl acceptors, glucosyl donor types, and modes of activation. No evidence was found in favor of distal participation.

An efficient synthetic route to O-(2-O-benzyl-3,4-di-O-acetyl-α/β-l-fucopyranosyl)-trichloroacetimidate

An efficient synthetic route to prepare O-(2-O-benzyl-3,4-di-O-acetyl-α/β-l-fucopyranosyl)-trichloroacetimidate from l-fucose was developed by introducing the thiophenyl group at the anomeric center and the benzylidene functional group to protect the 3 and 4 positions. Although three approaches were considered, the best result was obtained when, after the 2-hydroxyl benzylation, both protective groups were simultaneously removed by using acetic anhydride and perchloric acid supported on silica as catalyst. Selective deacetylation of the obtained tri-O-acetate followed by the reaction of the resultant hemiacetal with trichloroacetonitrile and DBU afforded the trichloroacetimidate with an overall yield of 56% from the l-fucose.

Synthesis of the tetrasaccharide repeating unit of the cryoprotectant capsular polysaccharide from Colwellia psychrerythraea 34H

Synthesis of the threonine-decorated tetrasaccharide repeating unit of a cryoprotectant polysaccharide with a glycosaminoglycan-like structure.

Crystal structure of racemic 2-[(β-arabino-pyran-osyl)-sulfanyl]-4,6-diphenylpyridine-3-carbo-nitrile

In the racemic title compound, C23H20N2O4S, the sulfur atom is attached equatorially to the sugar ring with unequal S-C bonds, viz.: S-Cs = 1.808 (2) and S-Cp = 1.770 (2) Å (s = sugar, p = pyrid-yl). The dihedral angles between the pyridine ring and its attached phenyl groups are 42.24 (8) and 6.37 (14)°. In the crystal, a system of classical O-H⋯O and O-H⋯(O,O) hydrogen bonds links the mol-ecules to form tube-like assemblies propagating parallel to the c-axis direction. Weak C-H⋯N inter-actions are also observed.

OFox imidates as versatile glycosyl donors for chemical glycosylation

Previously we communicated 3,3-difluoroxindole (HOFox) - mediated glycosylations wherein 3,3-difluoro-3H-indol-2-yl (OFox) imidates were found to be key intermediates. Both the in situ synthesis from the corresponding glycosyl bromides and activation of the OFox imidates could be conducted in a regenerative fashion. Herein, we extend this study with the main focus on the synthesis of various OFox imidates and their investigation as glycosyl donors for chemical 1,2-cis and 1,2-trans glycosylation.

Nucleic acid components and their analogs: Design and synthesis of novel cytosine thioglycoside analogs

The synthesis of a new category of novel cytosine 4-thioglycoside analogs has been first accomplished. The main step of this strategy is the synthesis of sodium pyrimidine-4-thiolate through the condensation of 2-cyano-N-arylacetamides with sodium cyanocarbonimidodithioate, followed by coupling with α-bromo-sugars to afford the corresponding cytosine 4-thioglycoside analogs. The free thioglycosides were also prepared. Subsequent studies on the application of this strategy for the preparation of other potent pyrimidine thioglycosides are reported.

Antimetabolites: A First Synthesis of a New Class of Cytosine Thioglycoside Analogs

A first synthesis of a new class of novel cytosine thioglycoside analogs from readily available starting materials has been described. The key step of this protocol is the formation of sodium pyrimidine-4-thiolate via condensation of N'-arylidene-2-cyanoacetohydrazides with sodium cyanocarbonimidodithioate salt, followed by coupling with halo sugars to give the corresponding cytosine thioglycoside analogs. Ammonolysis of the latter compounds afforded the free thioglycosides.

O-Benzoxazolyl imidates as versatile glycosyl donors for chemical glycosylation

Herein, we report a new class of glycosyl donors, benzoxazolyl imidates, for chemical glycosylation. The O-benzoxazolyl (OBox) leaving group was designed with an aim to compare the relative reactivity and stability of similarly structured S-benzoxazolyl (SBox) glycosides (thioimidates) developed in our lab and glycosyl trichloroacetimidates (TCAI, O-imidates) developed by Schmidt. Novel OBox donors can be activated under catalytic conditions and provided excellent yields in glycosylation. The OBox imidates were found to be more reactive than either SBox or TCAI donors. The high reactivity profile was confirmed in direct competitive experiments and was found beneficial for HPLC-assisted solid-phase synthesis.

Trichloroisocyanuric acid (TCCA): an efficient green reagent for activation of thioglycosides toward hydrolysis

Trichloroisocyanuric acid (TCCA), an inexpensive, commercially available, and non-toxic reagent has been used for the activation of thioglycosides toward their hydrolysis to the corresponding hemiacetals in high to excellent yields. The methodology provides a mild reaction condition for dealing with compounds containing acid sensitive functional groups.

A practical synthesis of capped 4-methylumbelliferyl hyaluronan disaccharides and tetrasaccharides as potential hyaluronidase substrates

The synthesis of hyaluronan dimers and tetramers equipped with a 4-methylumbelliferyl group at the reducing end to potentially allow monitoring of hyaluronidase activities is described. The 4-OH at the non-reducing glucuronate in the presented series is either removed or methylated to prohibit transglycosylase reactions, leading to a total of four probes.

Synthesis of Hyaluronic Acid Oligomers Using Ph2SO/Tf2O-Mediated Glycosylations

The synthesis of hyaluronic acid (HA) oligomers using a stepwise glycosylation strategy is described. This method employs protected 1-hydroxyuronic acid and 1-phenylthio glucosamine donors, both of which are activated with the Ph2SO/Tf2O activator system.

NIS/H2SO4–Silica: a mild and efficient reagent system for the hydrolysis of thioglycosides

Chemoselective hydrolysis of a variety of thioglycosides in the presence of a wide range of protecting groups has been achieved by using N-iodosuccinimide and H(2)SO(4) immobilized on silica in good to excellent yields.