Synthesis of fluoro- and seleno-containing D-lactose and D-galactose analogues.

Synthetic deoxy-fluoro-carbohydrate derivatives and seleno-sugars are useful tools in protein-carbohydrate interaction studies using nuclear magnetic resonance spectroscopy because of the presence of the 19F and 77Se reporter nuclei. Seven saccharides containing both these atoms have been synthesized, three monosaccharides, methyl 6-deoxy-6-fluoro-1-seleno-ß-D-galactopyranoside (1) and methyl 2-deoxy-2-fluoro-1-seleno-α/ß-D-galactopyranoside (2α and 2ß), and four disaccharides, methyl 4-O-(ß-D-galactopyranosyl)-2-deoxy-2-fluoro-1-seleno-ß-D-glucopyranoside (3), methyl 4-Se-(ß-D-galactopyranosyl)-2-deoxy-2-fluoro-4-seleno-ß-D-glucopyranoside (4), and methyl 4-Se-(2-deoxy-2-fluoro-α/ß-D-galactopyranosyl)-4-seleno-ß-D-glucopyranoside (5α and 5ß), the three latter compounds with an interglycosidic selenium atom. Selenoglycosides 1 and 3 were obtained from the corresponding bromo sugar by treatment with dimethyl selenide and a reducing agent, while compounds 2α/2ß, 4, and 5α/5ß were synthesized by the coupling of a D-galactosyl selenolate, obtained in situ from the corresponding isoselenouronium salt, with either methyl iodide or a 4-O-trifluoromethanesulfonyl D-galactosyl moiety. While benzyl ether protecting groups were found to be incompatible with the selenide linkage during deprotection, a change to acetyl esters afforded 4 in a 17% overall yield and over 9 steps from peracetylated D-galactosyl bromide. The synthesis of 5 was performed similarly, but the 2-fluoro substituent led to reduced stereoselectivity in the formation of the isoselenouronium salt (α/ß âˆ¼ 1 : 2.3). However, the ß-anomer of the uronium salt could be obtained almost pure (∼98%) by precipitation from the reaction mixture. The following displacement reaction occurred without anomerisation, affording, after deacetylation, pure 5ß.

Synthesis of a library of 2-fluoro-2-deoxy-derivatives of the trimannoside methyl α-D-Man-(1 → 3)-[α-D-Man-(1 → 6)]-α-D-Man and the dimannosides methyl α-D-Man-(1 → 3)-α-D-Man and methyl α-D-Man-(1 → 6)-α-D-Man.

A library of sixteen compounds, 1-16, comprising all (mono-, di-, and tri-) 2-fluoro-2-deoxy-derivatives of the N-glycan core trimannoside α-D-Man-(1 â†’ 3)-[α-D-Man-(1 â†’ 6)]-D-Man, including the corresponding 2-fluoro-2-deoxy disaccharide part structures and the non-fluorinated parent compounds, have been synthesized as their α-methyl glycosides for use as tools in 19F NMR-based lectin-carbohydrate interaction studies. Two methyl 2-fluoro-2-deoxy-mannoside acceptors, 21 (3-OH) and 22 (6-OH), were constructed and used in combination with the corresponding non-fluorinated mannose acceptors, 24 (6-OH) and 25 (3-OH), the 2-fluoro-2-deoxy mannosyl bromide donor 18 and the non-fluorinated bromide donor 23 to efficiently afford the target di-and trisaccharides (disaccharides, 2-3 steps, 47-66% overall yield; trisaccharides, 4 steps, 25-40% overall yield).

Fluorinated Carbohydrates as Lectin Ligands: Synthesis of OH/F-Substituted N-Glycan Core Trimannoside and Epitope Mapping by 2D STD-TOCSYreF†NMR spectroscopy.

Glycan-protein interactions play an important role in a broad range of physiological processes, raising interest to elucidate the structural interplay. Yet, their dynamic nature limits the analysis by crystallography, whereas NMR spectroscopy suffers from the low 1 H dispersion of glycans. Therefore, their sparse fluorination and NMR screening by 1D Saturation Transfer Difference with relay to 19 F (STDreF) was previously proposed to exploit the superior dispersion in 19 F NMR spectroscopy. A new 2D STD-TOCSYreF experiment is presented here that enables comprehensive epitope mapping of fluorinated glycans by combining the spectral resolution of 19 F with the spatial resolution and coverage of 1 H. For an illustration, the 2-deoxy-2-fluoro derivative of the N-glycan core trimannoside was synthesised and its recognition of Pisum sativum agglutinin by either of the two terminal mannose residues was confirmed. Going beyond the crystallographic information, the 2D STD-TOCSYreF spectrum moreover visualised collateral contacts from the branching mannose and allowed to assess the ratio of both co-existing binding modes through the α1,3- (67 %) and α1,6-linked (33 %) terminal mannose moieties.

Fluorinated carbohydrates as lectin ligands: dissecting glycan-cyanovirin interactions by using 19F†NMR spectroscopy.

NMR spectroscopy and isothermal titration calorimetry (ITC) are powerful methods to investigate ligand-protein interactions. Here, we present a versatile and sensitive fluorine NMR spectroscopic approach that exploits the (19)F nucleus of (19)F-labeled carbohydrates as a sensor to study glycan binding to lectins. Our approach is illustrated with the 11 kDa Cyanovirin-N, a mannose binding anti-HIV lectin. Two fluoro-deoxy sugar derivatives, methyl 2-deoxy-2-fluoro-α-D-mannopyranosyl-(1→2)-α-D-mannopyranoside and methyl 2-deoxy-2-fluoro-α-D-mannopyranosyl-(1→2)-α-D-mannopyranosyl-(1→2)-α-D-mannopyranoside were utilized. Binding was studied by (19)F NMR spectroscopy of the ligand and (1)H-(15)N HSQC NMR spectroscopy of the protein. The NMR data agree well with those obtained from the equivalent reciprocal and direct ITC titrations. Our study shows that the strategic design of fluorinated ligands and fluorine NMR spectroscopy for ligand screening holds great promise for easy and fast identification of glycan binding, as well as for their use in reporting structural and/or electronic perturbations that ensue upon interaction with a cognate lectin.

Cyclic glycopeptidomimetics through a versatile sugar-based scaffold.

Cyclic peptidomimetics are attracting structures to obtain a distinct, bioactive conformation. Even more attractive are sugar-containing cyclic peptidomimetics which present turn structures induced by the pyranose ring when incorporated in cyclic peptides. The use of a new and versatile saccharidic scaffold to achieve sugar-based peptidomimetics is here reported together with the successful synthesis of diastereomerically pure cyclic SAA peptidomimetics 15 and 16.