Solid-phase synthesis of alpha-glucosamine sulfoforms with fragmentation analysis by tandem mass spectrometry

Thiourea-Cu(OTf)2/NIS-synergistically promoted stereoselective glycoside formation with 2-azidoselenoglycosides or thioglycosides as donors

A novel promoter system for glycosylation is described. A catalytic amount of thiourea and Cu(OTf)2 together with a slight excess of N-iodosuccinimide synergistically promotes glycosylation at room temperature. The combination of reagents applies to some 2-azidoselenoglycoside and thioglycoside donors. A wide range of alcoholic acceptors underwent smooth conversion to O-(2-azido)glycosides with good stereoselectivities. In addition, the value of this method has been highlighted by its convenient operation and outstanding functional group compatibility.

Synthesis and reactivity of 4'-deoxypentenosyl disaccharides

4-Deoxypentenosides (4-DPs) are versatile synthons for rare or higher-order pyranosides, and they provide an entry for structural diversification at the C5 position. Previous studies have shown that 4-DPs undergo stereocontrolled DMDO oxidation; subsequent epoxide ring-openings with various nucleophiles can proceed with both anti or syn selectivity. Here, we report the synthesis of α- and β-linked 4'-deoxypentenosyl (4'-DP) disaccharides, and we investigate their post-glycosylational C5' additions using the DMDO oxidation/ring-opening sequence. The α-linked 4'-DP disaccharides were synthesized by coupling thiophenyl 4-DP donors with glycosyl acceptors using BSP/Tf2O activation, whereas β-linked 4'-DP disaccharides were generated by the decarboxylative elimination of glucuronyl disaccharides under microwave conditions. Both α- and β-linked 4'-DP disaccharides could be epoxidized with high stereoselectivity using DMDO. In some cases, the α-epoxypentenosides could be successfully converted into terminal l-iduronic acids via the syn addition of 2-furylzinc bromide. These studies support a novel approach to oligosaccharide synthesis, in which the stereochemical configuration of the terminal 4'-DP unit is established at a post-glycosylative stage.

Solid-Phase Synthesis of 2-Aminoethyl Glucosamine Sulfoforms

Mono- and disaccharides of sulfonated glucosamines (GlcN sulfoforms) conjugated to 2-aminoethyl linkers were generated by solid-phase synthesis. Orthogonally protected intermediates were tethered onto tritylated polystyrene resin beads, subjected to a modular sequence of deprotection and sulfonation steps, then cleaved from solid support without degradation of N- or O-sulfate esters using solvolytic conditions, and finally purified by reverse-phase HPLC to afford the title compounds.

Sulfoform generation from an orthogonally protected disaccharide

An orthogonally protected disaccharide (GlcN(α1→4)Glc) with a β-linked 2'-aminoethyl linker was used to generate a series of sulfated derivatives (sulfoforms), with a 6-O-sulfate on the glucose residue and one or more sulfate esters on the terminal glucosamine. Deprotection and sulfonation steps were performed in solution and in variable order, with isolated yields of 36-54% (85-90% per operation) after HPLC purification. The modular deprotection-sulfonation sequences can be performed with efficient recovery of the polysulfate products, and avoids complications associated with heterogeneous reactivity in solid-phase synthesis.

ESI-MS differential fragmentation of positional isomers of sulfated oligosaccharides derived from carrageenans and agarans

We have prepared a number of isomeric red seaweed galactan-derivative sulfated oligosaccharides to determine whether there were diagnostic differences among the isomeric mass spectra obtained using ESI CID MS/MS (triple quadrupole instrument). Fragmentation of the single or multicharged molecular ions from di-, tetra-, and hexasaccharides indicated that the relative positioning of the sulfate groups and type of monosaccharide unit affect the rate of cleavage of the glycosidic bonds. We also performed a comparative [M-Na](-) fragmentation study of positional isomers of sulfated disaccharides that present all four monosulfation possibilities on the galactopyranosidic ring. In this case, negative-ion ESI CID MS/MS approach gave diagnostic product ions from cross-ring cleavages along with the same main B(1) ion (from sulfated Galp), at m/z 241, for all isomers. The isomeric disaccharides were also submitted to increased spray energy conditions inducing in-source fragmentation; preformed B(1) ions were then fragmented to give similar product ions as those found in [M-Na](-) analysis. Evaluation of the relative abundances mainly for cross-ring fragment ions at m/z 138, 139, 151, 153 allowed clear distinction among the members of the disaccharide series. The different ratios for m/z 151/153 ions were consistent with the predominance of m/z 153 being related to the cases when the bond involved in the cleavage process links a sulfated carbon. A quadrupole ion trap instrument (MS(n) analysis) was also utilized to compare the results obtained with the triple quadrupole instrument.