Anomeric configuration, of glycosides

With the help of ESI MS, one can also determine anomeric configuration of glycosides [39], Such studies were performed on sugar phosphate derivatives of glucopyranose, fucopyranose, galactopyranose, mannopyranose, ribofur-anose, and xylopyranose. Collision studies (tandem mass spectrometry) allowed detailed identification of compounds including information on their anomeric configuration. 

The results in Table 12.1 show that the anomeric configuration of the products of random galactosylation can be biased toward the a-glycosides by use of 2 or 10 as donor, or all of the products can be obtained in a single run using 11 or 12 as donor. While all disaccharides are clearly present in significant amounts in the product, they are not formed in equal amounts, and it is hoped that further studies on galactosyl and other donors will lead to an improvement in the product distributions. 

If both anomers of the glycoside are obtained in a reaction, it is necessary to separate them. One separation method that has been used is the preferential extraction101 of the anomers with a volatile solvent. After extraction of the individual glycoside, the solvent is removed by evaporation, and the glycoside may be obtained in crystalline form. A second method of separation utilizes fractional recrystallization, and it may be possible by proper selection of the solvent to obtain both anomers in crystalline form. A third method utilizes chromatography for separating the anomers, and the pure anomers may be obtained from appropriate fractions from the column. The anomeric configuration of the anomers which have been obtained in pure form will need to be determined. Such determinations can be made by measurement of physical constants, from the n.m.r. spectra, and from the susceptibility of the anomer to enzymes of known specificity. 

The anomeric configuration of the newly formed bond should be reasonably well controlled (i.e., major 1,2-cis or 1,2-trans glycosides). 

There are presumably two configurational inversions in such a mechanism, so that the anomeric configuration of the newly formed glycosidic bond is the same as that of the donor. 

The chemical shifts were influenced also by the anomeric configuration. In the nonulosonic acids with an equatorial OH-4, the difference between the 111 NMR resonances for H-3ax and H-3eq was 0.86 1.02 ppm for the anomer with an axial carboxy group but only 0.21 0.46 ppm for the other anomer, independently of whether NH-5 was axial or equatorial. When OH-4 was axial and NH-5 equatorial, a typical difference of 0.71-0.75 or 0.05 ppm was observed for the anomers with an axial or an equatorial carboxyl group, respectively. In the glycosidically linked nonulosonic acids, the differences were not the same but in most examples were useful for determination of the anomeric configuration of naturally occurring isomers in OPSs and LPSs.14 22 23 25 30 31 34 36 40... 

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