Carbohydrate anomers, separation

Recently, graphitized-carbon stationary phases have become available for reversed-phase carbohydrate analysis. They exhibit a stronger hydrophobic interaction than C,g bonded phases. They are also compatible with a high-pH mobile phase, which speeds up mutarotation and prevents peak splitting due to anomer separation. 

Gas-liquid chromatography may be the method of choice when it is necessary to identify or to quantitate one or more carbohydrates especially when they are present in small amounts. Although this technique is often used because it is possible to resolve carbohydrates with very similar structures, the fact that a and /3 anomers and pyranose and furanose forms of the same carbohydrate all give separate peaks is sometimes a disadvantage. 

The allyl glycosides have been synthesized from allyl alcohol and the free carbohydrate, or the per-O-acetylglycosyl halide in the presence of acid, or mercury cyanide, as the catalyst. The condensation of the alcohol occurs with both anomers of the carbohydrate, and a mixture of the a- and /3-glycosides is obtained. The anomers can be separated by preferential, solvent extraction,12 or by chromatography on... 

The majority of the many methods used to study the composition of equilibrium solutions of carbohydrates examine the mixture without separating the individual components. With the discovery that the anomeric forms of sugars could be readily separated by gas chromatography of their tri-methylsilyl ethers, a new approach to the problem was found. A protocol was developed for the direct gas chromatographic analysis of the amount of each anomer present in an aqueous solution. The protocol can be used on the micro scale and can be used in enzyme assays such as that for mutarotase. The method has been made more effective by combining gas chromatography with mass spectrometry. It is shown how mass spectral intensity ratios can be used to discriminate anomers one from another. The application of these methods to the study of complex mutarotations is discussed. 

From carbohydrates, several diphosphines have been derived and used as ligands in hydroformy-lation reactions (Section D.l. 5.8.). a,a -TREDFP (a,a -40) is prepared from commercially available a, a -trehalose (39) by the reaction sequence outlined42,43 similarly, the / ,/f-anomer is obtained from / ,/ -trehalose44. L-Iditol is commercially available but expensive, but may be prepared by catalytic reduction of L-sorbose and separation from the isomeric L-sorbitol by fractionated crystallization of the hexaacetate45. Conversion to the diphosphine 42 involves a further fractional crystallization for the separation of isomeric ditosylates46. The reactions for the synthesis of the precursors employ techniques described in Section 4.4. for the derivatization of carbohydrates. 

---