Sugar anomers, separation

In solution, most simple sugars and many of their derivatives occur as equilibrium mixtures of tautomers. The presence of a mixture of two anomers of the same ring size may be indicated in the name by the notation a,P-, e.g. a,P-D-gIucose. In formulae, the same situation can be expressed by separating the representation of the ligands at the anomeric centre from the a and P bonds [see examples (a) and (c)], or by use of a wavy line [(b) and (d)] (particularly if hydrogen atoms are omitted). 

Hultiple products are frequently observed for the separation of TMS-sugar derivatives. At equilibrium reducing sugars can exist in more than one isomeric form known as anomers. Formation of their THS derivatives followed by gas chromatography will result in multiple peaks corresponding in composition to the equilibriue anomeric mixture [436]. 

The insertion of a second oxygen atom in a sugar furanose ring in essence converts that moiety to an acetal. This modiftcation leads to another false substrate for viral reverse transcriptase. Glycosilation of the silylated purine (46-1) with chiral dioxolane (46-2) prepared in several steps from anhydromannose in the presence of ammonium nitrate affords the coupling product as a mixture of anomers (46-3). The mixture of products is then separated on a chromatographic column. The desired diastereomer (46-3) is reacted with ammonia to afford the product (46-4)... 

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. 

Analysis of Freshly Cyclized 2,3-di-O-methyl-D-arabinose. The system 3,4-di-O-methyl-D-mannitol (2)—sodium metaperiodate meets the foregoing specifications. 3,4-Di-O-methylmannitol was rapidly cleaved by periodate to 2,3-di-O-methyl-D-arabinose (3), which is resistant to further oxidation (19). We prepared the crystalline / -anomer of this sugar (the a-anomer had previously been crystallized (20)) and showed that on a suitable GLC column the trimethylsilyl derivatives of the four ring forms could be separated (Figure 9). The individual peaks were characterized by preparative GLC and mass spectrometry. This allowed the analysis of the mixtures of anomeric forms generated in kinetic experiments (see p. 34). 

More suitable for the determination of medium-and high-molecular-weight polysaccharides Shorter (50%) analysis times Higher recoveries with greater accuracy Directly applicable to sugar samples Far more sensitive Allows separation of a and / anomers Preferable for monosaccharides Derivatization required... 

Working at low temperatures (0-4°), h.p.l.c. on a cation-exchange resin in the calcium form will separate the pyranose anomers of most of the aldo-hexoses and -pentoses18 under these conditions, mutarotation is slower than separation. The furanoses are not separated, because they interconvert too rapidly, but, at—2 5 to—45 °, the two furanose forms of D-galac-tose and L-fucose have been separated.19 Attempts to separate the various forms of sugars on a preparative scale [p. 24] have not succeeded so far.20... 

In 1970, 1 was prepared by direct glycosylation of the silylated 5-azacytosine with acylated 1-halo sugars, but the yields were very low.20 In this procedure, 1,3,5-tri-O-acetyl-2-deoxy-D-ribofuranose (9) was converted into 3,5-di-0-acetyl-2-deoxy-D-ribofuranosyl chloride (10).21 The trimethylsilyl derivative of 5-azacytosine (11),22 prepared from 4-amino-6-pyrimidine by treatment with hexamethyldisilazane, was then allowed to react with intermediate 10 in acetonitrile over 7 days to give a mixture of anomers of l-(3,5-di-(9-acetyl-2-deoxy-D-ribofuranosyl)-5-azacytosine (12) in 10% overall yield. The anomeric mixture was treated with ethanolic ammonia to remove the acetyl groups. The resulting a and p anomers were separated by a combination of fractional crystallization and preparative layer chromatography on silica gel to give pure decitabine (1) and its a-anomer 13 in 7% and 52% yield, respectively. 

Efforts to improve the separation of closely related compounds are a frequent reason for using derivatives, and their application often makes it possible to separate compounds that otherwise cannot be separated. Chapter 5 gives various examples of this type, of which the separation of enantiomers of alcohols (p.90), carboxylic acids (p.125), amino acids (p.146) etc., are the most illustrative. The separation of sterols that differ in the position of the hydroxyl group may serve as another example. Isomers with a hydroxyl group in the a-position are not separated from 0-isomers on non-polar columns. However, if the hydroxyl group is converted into a suitable derivative, the two isomers can be separated well even on non-polar columns. The anomers of sugars can also be separated after their conversion into derivatives. 

For evaluation of the mutarotation constant, ki + k, Hudson and Lowry devised an ingenious method for the determination of the separate rate-constants. They showed that the final or maximum solubility of a sugar in a solvent in which it is difficultly soluble is + S, where S is the initial solubility of the a anomer, and is the... 

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