Search

A total synthesis of the trisaccharide repeat unit of Salmonella serogroup E1 O‐antigen is reported. This synthesis features a key β‐mannosylation reaction through a cesium carbonate‐mediated anomeric O‐alkylation of a partially protected D‐mannose with an L‐fucose‐derived electrophile for the first time. [ABSTRACT FROM AUTHOR]

Several structurally diverse d‐mannose‐derived lactols, including various deoxy‐d‐mannoses and conformationally restricted bicyclic d‐mannoses, have been synthesized and investigated in mechanistic studies of β‐mannosylation through Cs2CO3‐mediated anomeric O‐alkylation. It was found that deoxy mannoses or conformationally restricted bicyclic d‐mannoses are not as reactive as their corresponding parent mannose. This type of β‐mannosylation proceeds efficiently when the C2‐OH is left free, and protection of that leads to inferior results. NMR studies of d‐mannose‐derived anomeric cesium alkoxides indicated the predominance of the equatorial β‐anomer after deprotonation. Reaction progress kinetic analysis suggested that monomeric cesium alkoxides be the key reactive species for alkylation with electrophiles. DFT calculations supported that oxygen atoms at C2, C3, and C6 of mannose promote the deprotonation of the anomeric hydroxyl group by Cs2CO3 and chelating interactions between Cs and these oxygen atoms favor the formation of equatorial anomeric alkoxides, leading to the highly β‐selective anomeric O‐alkylation. Based on experimental data and computational results, a revised mechanism for this β‐mannosylation is proposed. The utilization of this β‐mannosylation was demonstrated by an efficient synthesis of the hexasaccharide core of complex fucosylated N‐linked glycans. [ABSTRACT FROM AUTHOR]

Synthesis of β-D-Xyl-(l→2)-β-D-Man-(1→4)-α-D-Glc-OMe (1), a tr-isaccharide oligomer of the Hyriopsis schlegelii glycosphingolipid is described. The synthesis involves a key β-mannosylation via cesium carbonate-mediated anomeric O-alkylation for direct synthesis of partially protected disaccharide β-D-Man-(1→4)-α-D-Glc-OMe (4) bearing a free C2˭OH in the mannose moiety. In addition, a silver triflate-promoted glycosylation of 4 with 2,3,4-tri-O-benzoyl-α-D-xylopyranosyl bromide (5) followed by deprotection affords the desired trisaccharide component (1) of the Hyriopsis schlegelii glycosphingolipid. [ABSTRACT FROM AUTHOR]

Development of efficient, mild, and easily operable stereoselective glycosylations is of critical importance to access sufficient amounts of pure and structurally well-defined carbohydrates for studies of their biological functions. Such studies will facilitate our understanding of the role of complex oligosaccharides and glycoconjugates in biological processes as well as the development of carbohydrate-based effective therapeutic agents. This review highlights recent advances in the transition metal catalyzed stereoselective glycosylations for the synthesis of a variety of structurally complex and biologically significant O-, N-, C-, and S-glycosides. [ABSTRACT FROM AUTHOR]

Synthesis of the 2-deoxy trisaccharide glycal of antitumor antibiotics landomycins A and E has been described. The synthesis involves an anomeric O-alkylation for the synthesis of 2-deoxy β -linked disaccharide, a tert -butyldimethylsilyl triflate-catalyzed α-selective L-rhodinosylation, and a lithium 4,4′-di- tert -butylbiphenyl-mediated reductive debenzylation and concomitant reductive lithiation-elimination for the production of the 2-deoxy trisaccharide glycal. [ABSTRACT FROM AUTHOR]

A new and efficient approach for direct and stereoselective synthesis of β-mannopyranosides by anomeric O-alkylation has been developed. This anomeric O-alkylation of mannopyranose-derived lactols is proposed to occur under synergistic control of a kinetic anomeric effect and metal chelation. The presence of a conformationally flexible C6 oxygen atom in the sugar-derived lactol donors is required for this anomeric O-alkylation to be efficient, probably because of its chelation with cesium ion. In contrast, the presence of a C2 oxygen atom plays a minor role. This glycosylation method has been successfully utilized for the synthesis of the trisaccharide core of complex N-linked glycans. [ABSTRACT FROM AUTHOR]

Take control! An unprecedented sulfenylation of stereochemically defined 2‐deoxyglycosyl lithium species with asymmetric sugar‐derived disulfide acceptors enabled the stereoselective synthesis of both α‐ and β‐S‐linked 2‐deoxyoligosaccharides. Reductive lithiation of 2‐deoxyglycosyl phenylsulfides at −78 °C provides predominantly axial glycosyl lithium species, which upon warming isomerize to predominantly equatorial lithium species (see scheme). [ABSTRACT FROM AUTHOR]

Gold-catalyzed glycosylation usingS-but-3-ynyl andgem-dimethylS-but-3-ynyl thioglycoside donors has been investigated for the synthesis of various types of complex oligosaccharides. It was found that 2,2-dimethylS-but-3-ynyl thioglycoside donors are more reactive than theirS-but-3-ynyl thioglycoside counterparts and gave the glycoside products in relatively higher yields. [ABSTRACT FROM AUTHOR]

This review summarizes recent accomplishments involving transition metal catalysis in efficient and stereoselective O-glycosylation reactions. In addition, proposed mechanisms of each catalytic O-glycosylation are discussed respectively. [ABSTRACT FROM AUTHOR]

Review: [196 refs. [ABSTRACT FROM AUTHOR]

Review: 122 refs. [ABSTRACT FROM AUTHOR]

[Display omitted] Stereoselective construction of a variety of β-glycosides can be achieved using abundant and inexpensive K 2 CO 3 -mediated stereoselective anomeric O-alkylation of sugar lactols with primary electrophiles. In addition, application of this methodology to the synthesis of various azido-modified glycosphingolipids has been accomplished in good yields and excellent anomeric selectivity using sphingosine-derived primary triflate. [ABSTRACT FROM AUTHOR]