The assignment of carbonyl resonances in 13C-n.m.r. spectra of peracetylated mono- and oligo-saccharides containing D-glucose and D-mannose: an alternative method for structural determination of complex carbohydrates

Tetranuclear zinc cluster: a dual purpose catalyst for per-O-acetylation and de-O-acetylation of carbohydrates

A tetranuclear Zn cluster-catalyzed per-O-acetylation and de-O-acetylation of carbohydrates has been reported.

Stable isotope-enhanced two- and three-dimensional diffusion ordered 13C NMR spectroscopy (SIE-DOSY 13C NMR)

The feasibility of obtaining high quality homonuclear or heteronuclear diffusion-ordered (13)C NMR data is shown to be greatly improved by using (13)C isotopically-enriched samples. Stable isotope-enhanced diffusion ordered (SIE-DOSY) (13)C NMR has been applied to (13)C-enriched carbohydrates, and has been used to determine diffusion coefficients for pentose and hexose monosaccharides, and a disaccharide and trisaccharide. These 2D spectra were obtained with as little as 8 min of acquisition time. Fully resolved 3D DOSY-HMQC NMR spectra of [U-(13)C]xylose, [U-(13)C]glucose, and [1-(13)C(gal)]lactose were obtained in 5h. Sample derivatization with [carbonyl-(13)C]acetate (peracetylation) extends the usefulness of the technique to included non-labeled sugars; the (13)C-carbonyl - carbohydrate ring proton (1)H-(13)C correlations also provide additional structural information, as shown for the 3-D DOSY-HMQC analysis of a mixture of maltotriose and lactose per-[carbonyl-(13)C]acetates.

Mild and efficient method for the cleavage of benzylidene acetals by using erbium (iii) triflate

Er(OTf)3 is proposed as new efficient Lewis acid catalyst in a mild deprotection protocol of benzylidene derivatives. In a modified procedure, where acetic anhydride is used as the reaction solvent, the simultaneous cleavage of the benzylidene acetal and the peracetylation of the substrates is obtained in quantitative yields and very short reaction times.

Nuclear magnetic resonance spectroscopy of peracetylated oligosaccharides having 13C-labeled carbonyl groups in lieu of permethylation analysis for establishing linkage substitutions of sugars

Peracetylation of free hydroxyl groups in model saccharides with [13C-carbonyl]acetic anhydride resulted in additional splittings of sugar ring proton signals in NMR spectra, due to 3-bond J couplings between each acetyl carbonyl carbon and a sugar ring proton at that position. Quantification of 144 of these 3-bond coupling constants in different saccharide structures showed a range between 2.5 and 4.7 Hz, whereas all possible 4-bond couplings between sugar ring protons and acetyl carbonyl carbons were within linewidth (< 0.5 Hz). Therefore, further splitting of sugar ring proton signals in the range of 2.5-4.7 Hz upon acetylation with a [13C-carbonyl]acetyl group identifies that position as (formerly) having a free hydroxyl group. This performs the same basic function as permethylation analysis, but does not require hydrolysis of glycosidic linkages. Additionally, proton-detected 2D heteronuclear multiple bond correlation (HMBC) experiments or proton-detected heteronuclear correlation spectroscopy (hetCOSY) enabled ring proton-carbonyl-13C 3-bond J connectivities to be correlated with high sensitivity. Modified NMR pulse sequences are reported that include frequency selective decoupling schemes to enable coupling constants to be determined from 2D data. The tailored pulse sequences resulted in higher spectral resolution and sensitivity for [13C-carbonyl]-ring proton correlations.

The application of NMR-pattern-recognition methods to the classification of reduced, peracetylated oligosaccharide residues

In the present paper homo- and hetero-nuclear correlation spectroscopies have been used to assign proton and carbonyl carbon resonances of a number of reduced, peracetylated mono- and oligo-saccharide derivatives. Each of the native structures for which assignments were made represent residues or substructures typically found in N- or O-linked glycans. Using the assigned NMR parameters as a basis, residues contained in parent structures were classified according to their residue type and glycosidic substitution sites using a relatively simple K-Nearest Neighbor pattern recognition approach. The method was able to correctly assign 99% of 77 "test residues" to their correct structural class using the full set of 19 assigned parameters as a basis. Similar correlations made between data and structure were less successful when reduced variable sets selected on the basis of SIMCA optimization were used.

NMR spectroscopy in the structural elucidation of oligosaccharides and glycosides

The potential of one- and two-dimensional NMR techniques for the identification of individual sugar residues, their anomeric configuration, interglycosidic linkages, sequencing and the site of any appended group, in establishing the structures of naturally occurring oligosaccharides and glycosides is presented.