Anomeric resonances, analysis

The DEPT spectrum (Figure 4.15) immediately confirms the seven-saccharide repeat, since there are seven anomeric resonances such a confirmation of the results of methylation analysis by NMR is important, because incomplete methylation can give the appearance of additional branch points. Moreover, the hydroxymethylene carbons at high field show inverted peaks, which identifies them as the C6 carbons of the various residues. 

The anomeric configuration of NeuSAc or NeuSGc can usually be inferred from the chemical shifts of the H3eq and H4 resonances. Analysis of a series of model... 

Nuclear magnetic resonance analysis of Rosa glauca araban has provided additional evidence that the arabinosyl residues are a-5-linked and in the furanose configuration. The C-1 resonance expected of a-arabinofuranosyl residues, but not the C-1 resonances expected of the P-arabinofuranosyl or a- and P-arabinopyranosyl residues, was detected by C-NMR analysis 68b). The proton NMR spectrum is consistent with a- or P-furanosyl residues and P-pyranosyl residues, but not with a-pyranosyl residues 68b). The a-anomeric nature of these linkages is confirmed by the negative optical rotations, from —181 to —108, exhibited by such arabans 11, 68b). 

In addition to well-resolved one-dimensional (ID) 1H and 13C spectra, which are usually sufficient for monitoring synthetic steps, HR-MAS techniques can be applied to two-dimensional (2D) homonuclear and heteronuclear experiments, which allow a wealth of structural information to be obtained. H,13C HMQC (heteronuclear multiple quantum coherence) spectra are particularly useful in the analysis of solid support-bound oligosaccharides, since the anomeric protons exhibit characteristic resonances. Such a spectrum of a polymer-bound trisaccharide glycal is shown in Figure 8.4. 

Limited carbonyl NMR data are available for these anomeric amides. However, carbonyl shifts for hydrazines 217 and 218 were on average 3 ppm higher than their hydroxamic ester precursors. This reflects a higher degree of residual amide resonance in the hydrazines relative to A-acyloxy-iV-alkoxyamides where the difference was closer to 8.0 ppm. As reported for A-acyloxy-A-alkoxyamides (see Section IV.B.2), analysis of variance in the hydrazine and hydroxamic ester shifts indicates that substituents affect the hydroxamic ester carbonyl shifts ( 2.6) more than those of the hydrazines ( 1.3 ppm). 

Further Justification for the Exo-anomeric Effect. Conformational Analysis Based in Nuclear Magnetic Resonance Spectroscopy of the B Human Blood Group Determinant, H. Thogersen, R. U. Lemieux, K. Bock, and B. Meyer, Can. J. Chem., 60 (1982) 44-57. 

Thogersen H, Lemieux RU, Boch K, Meyer B (1982) Further justification for the exo-anomeric effect. Conformational analysis based on nuclear magnetic resonance spectroscopy of oligosaccharides. Can J Chem 60 44-57... 

In addition to steric effects and electrostatic effects, the conformational analysis of carbohydrates requires two stereoelectronic effects to be taken into consideration the gauche effect and the anomeric effect, in its various manifestations. Both of these effects can be considered as aspects of no-bond resonance. 

---