Stereoselective synthesis of photoactivatable Man(ß1,4)GlcNAc-based bioorthogonal probes.

We report an operationally facile protocol to prepare photoactivatable probes of the bioactive mammalian disaccharide, Man(ß1,4)GlcNAc. Using conformationally restricted mannosyl hemi-acetal donors in a one-pot chlorination, iodination and glycosylation sequence, ß-mannosides were generated in excellent diastereoselectivities and yields. Upon accessing the disaccharide, we generated the corresponding photoactivatable probes by appending a diazirine-alkyne equipped linker via a condensation reaction between a diazirine-containing linker and C-1 and C-2 derivatized mannosylamines to furnish the desired C-1 and C-2 modified Man(ß1,4)GlcNAc-based probes. This new synthetic protocol greatly simplifies the preparation of this important bioactive disaccharide to enable future work to identify its protein binding partners in cells.

Stereoselective ß-Mannosylation via Anomeric O-Alkylation with L-Sugar-Derived Electrophiles.

A total synthesis of the trisaccharide repeat unit of Salmonella serogroup E1 O-antigen is reported. This synthesis features a key ß-mannosylation reaction via cesium carbonate-mediated anomeric O-alkylation of a partially protected D-mannose with an L-fucose-derived electrophile for the first time.

ß-Mannosylation via O-Alkylation of Anomeric Cesium Alkoxides: Mechanistic Studies and Synthesis of the Hexasaccharide Core of Complex Fucosylated N-Linked Glycans.

A number of 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 ß-mannosylation via 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 ß-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 ß-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 favour the formation of equatorial anomeric alkoxides, leading to the highly ß-selective anomeric O-alkylation. Based on experimental data and computational results, a revised mechanism for this ß-mannosylation is proposed. The utilization of this ß-mannosylation was demonstrated by an efficient synthesis of the hexasaccharide core of complex fucosylated N-linked glycans.

Stereoselective Construction of ß-Mannopyranosides by Anomeric O-Alkylation: Synthesis of the Trisaccharide Core of N-linked Glycans.

A new and efficient approach for direct and stereoselective synthesis of ß-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.

Recent Chemical and Chemoenzymatic Strategies to Complex-Type N-Glycans.

Glycosylation is one of the major forms of protein post-translational modification. N-glycans attached to proteins by covalent bonds play an indispensable role in intercellular interaction and immune function. In human bodies, most of the cell surface glycoproteins and secreted glycopeptides are modified with complex-type N-glycans. Thus, for analytical or medicinal purposes, efficient and universal methods to provide homogeneous complex-type N-glycans have been an urgent need. Despite the extremely complicated structures, tremendous progress in the synthesis of N-glycans has been achieved. On one hand, chemical strategies are shown to be effective to prepare core oligosaccharides of N-glycans by focusing on stereoselective glycosylations such as ß-mannosylation and α-sialylation, as well as the methodology of the N-glycan assembly. On the other hand, chemoenzymatic strategies have also become increasingly powerful in recent years. This review attempts to highlight the very recent advancements in chemical and chemoenzymatic strategies for eukaryotic complex-type N-glycans.