Sugars decoupling

Comparison of the H-NMR spectrum of tjipanazole C3 (365) with that of tjipanazole Cl (362) indicated that these two alkaloids are C-2 -epimers. Proton decoupling and NOE experiments indicated that the sugar moiety was a p-rhamnosyl unit which was attached to N-11 of 3-chloroindolo[2,3-fl]carbazole. Based on these spectral data and the structural similarity to tjipanazole Cl (362), the structure 365 was assigned to tjipanazole C3 (329) (Scheme 2.94). 

The subtraction from the total l3C decoupled spectrum of the sugar 13C resonances allows to determine the aglycone contribution. 

Primary (e.g. C-6 of rhamnose), secondary (e.g. C-6 of glucose), tertiary (e.g. CHOH of a pyranose ring carbon atom) and quaternary carbons (e.g. of branched-chain sugars) can be distinguished by proton off-resonance decoupling [89, 676 678] or polarization transfer. 

The HMBC also incorporates a low-pass filter that tries to reject the one-bond correlations seen in HSQC/HMQC. Low pass means that only the low values of 7ch (0-10 Hz) are allowed to pass through and produce crosspeaks in the 2D spectrum. Because there is no 13 C decoupling, the one-bond correlations appear as wide doublets (7 150 Hz) centered on the XH peak position in F2 (Fig. 11.10—squares). They obscure the weak HMBC crosspeaks and can easily be misinterpreted as long-range correlations, especially if one of the two components of the doublet happens to fall at the position of another peak in the XH spectrum. The low-pass filter is set to reject a particular 7 value, typically 135 Hz for molecules dominated by saturated hydrocarbon (e.g., 3-heptanone, menthol, cholesterol), 142 Hz for sugars, and 170 Hz for molecules dominated by aromatic carbons. The same... 

Si NMR has been used to study some polytrimethylsilylated sugars. (537, 538) Selective decoupling, gated to remove the negative... 

A comprehensive review on measurement of proton-proton couplings in the sugar part of a DNA molecule has been given by Kennedy and co-workers. The authors explored the influence of stereo-selective deuteration, selective excitation, decoupling and /-doubling on the accuracy of the measured couplings. 

Unlike sugar phenylosotriazoles which possess a flexible side chain,130 some cyclitol phenylosotriazoles possess symmetrical structures, as indicated by the simplicity of their proton-decoupled 13C NMR spectra,392,532 Thus the 1H NMR spectra of inositol phenylosotriazoles, 292a, and their esters revealed the presence of a simple, twofold axis of symmetry and the ring protons were symmetrical about a midpoint (see 294), making them examples of four-nucleus AA BB systems. 

H. HMBC experiment performed on the derivatized saponin (256 experiments of 2K, reverse mode without decoupling in the carbon dimension experiment is not phased). Part of the map corresponding to the low field resonances (sugars) is shown below. The observed correlations allow sequencing of the chains of sugars. 

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