Monosaccharides carbon-13 chemical shifts

Analysis of the monosaccharide signals in the C-NMR spectrum (103.1, 77.9, 75.0, 71.3 and 66.9) and further assignment of all proton and carbon chemical shifts using H- H COSY and HMQC experiments indicated the presence of a xylose residue. The presence of xylose was confirmed by acid hydrolysis of 44 with aqueous 2N trifluoroacetic acid followed by GC analysis of the corresponding peracetylated alditol. The D-configuration was determined by GC analysis of the l-[(S)-A-acetyl-(2-hydroxypropylamino)]-l-deoxy alditol acetate derivative as for... 

The coupling constants of H-1 and H-2 and the carbon chemical shifts of each monosaccharide showed that GluAMe, Xyl. and Api were in p, while Rha. and Ara. were in a configurations [7, 8]. Therefore, the structure of compound 2 was determined as methyl ester of 3 -0-P-D-glucuronyl-16a-hydroxyolean-12-en-28-oic acid-28-0-P-D-xylopyranosyl( 1 -3)-a-L-arabinopyranosyl(l-4)-P-D-apiofuranosyl(l-3)-a-L-rhamnopyranosyl(l-2)-P-D-xylopyranoside. 

Fig. 1 also may be used to note some general characteristics of C spectra of carbohydrate polymers ( -11) Chemical shifts of anomeric carbons (C-l), in the region of 100-110 p.p.m., are typically well separated from other signals. As compared with C-l of the related monosaccharides (12-15)j the anomeric carbon is strongly deshielded (commonly by 7-10 p.p.m.) through glycoside formation (9)> i.e., by the change from 0-H to an 0-C bond. 

The most important, practical application of 13C-n.m.r. spectroscopy is probably the simple characterization and identification of organic compounds. Because of the simplicity of proton-decoupled carbon spectra, and the sensitivity of carbon-13 chemical-shifts towards structural changes, carbon spectra are extremely well suited for this purpose (see, for example, Ref. 84), and it is for this reason that the emphasis of the present article has been placed on presenting chemical-shift data of monosaccharides and their derivatives. Such data are also important for structural studies of oligo- and poly-saccharides,3 and for the investigation of such mixtures as those arising from mutarotation85-87 (see Section 11,4) or from other reactions.34... 

Carbon-13 chemical-shifts have been used to study the interaction of monosaccharides with such complexing agents as borates124,125 and calcium ions.126,127 Paramagnetic complexing-agents are mentioned in Section III,5. 

Experience has indicated that the chemical shifts of the monosaccharides are similar to those of the monosaccharide residues within the polysaccharide, except for substituent effects.37,38 These effects, caused by the attachment of any substituent to a sugar moiety, cause an increase in the chemical shift of the carbon atom directly involved in the linkage this increase is usually accompanied by a decrease of smaller magnitude (or, sometimes, an increase) in the chemical shifts... 

A very useful source on various NMR techniques and chemical shifts of functionalized carbon atoms that contains tabulated data on monosaccharides is Reference [23]. Specifically correlating functional groups with chemical shifts of protons and the associated carbon is an important tool for elucidating the stmcture of the monosaccharide moiety of many complex oligosaccharides. Since monosaccharides contain one or more chiral centers, absolute configuration must be known in order to predict the conformational shape of the molecules. The outcome of many stereoselective functionalizations of free and partially functionalized monosaccharides depends on conformation. 

The limited dispersion of internuclear orientations of CH vectors between directly bonded carbon and proton atoms in monosaccharide rings means that additional types of RDCs such as DHh> "D< h, and Dcc are required for proper analysis of RDCs. The demand on the number and accuracy of measured RDCs increases for flexible carbohydrates. Attention must be paid to higher order effects caused by the narrow range of 1H chemical shifts of carbohydrates. 

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