Ring size determination, monosaccharides

In the second procedure used to determine a monosaccharide s ring size, an acetal of the monosaccharide is oxidized with excess periodic acid. (Recall from Section 20.7 that periodic acid cleaves 1,2-diols.)... 

Although we now have powerful spectroscopic methods available to determine the sizes of the oxide rings formed by the simple monosaccharides, the way in which this was done chemically for glucose highlights the difference in reactivity between ether and alcohol functions. 

Using methods similar to Fischer s, the straight-chain form of any monosaccharide can be worked out. As we have seen, however, monosaccharides exist mostly as cyclic pyra-nose or furanose hemiacetals. These hemiacetals are in equilibrium with the open-chain forms, so sugars can react like hemiacetals or like ketones and aldehydes. How can we freeze this equilibrium and determine the optimum ring size for any given sugar Sir Walter Haworth (inventor of the Haworth projection) used some simple chemistry to determine the pyranose structure of glucose in 1926. 

In a useful variant of the classical methylation procedure for linkage analysis in the structural determination of oligo- and poly-sacccharides, the fully methylated oligomer is treated with triethylsilane and boron trifluoride etherate or triethylsilane and trimethylsilyl trifluoromethanesulfonate. This procedure produces partially methylated anhydroalditols. These are acetylated and analyzed by GLC-MS.222 223 This reductive-cleavage method makes possible simultaneous determination of identity, ratio, linkage position, and especially the ring size for each monosaccharide component. 

Two different procedures can be used to determine what size ring a monosaccharide forms. In the first procedure, treatment of the monosaccharide with excess methyl iodide and silver oxide converts all the OH groups to OCH3 groups (Section 22.12). Acid-catalyzed hydrolysis of the acetal then forms a hemiacetal, which is in equilibrium with its open-chain form. The size of the ring can be determined from the structure of the open-chain form because the sole OH group is the one that had formed the cyclic hemiacetal. 

Solution A trisaccharide is a carbohydrate that consists of three monosaccharides. Hydrolysis breaks the glycosidic bonds between the monosaccharides and we note that D-raffinose is made up of D-glucose, D-galactose and D-fructose. We can determine the ring size and the order of the component sugars by examining the products of D-raffinose s methylation and hydrolysis. These compounds are ... 

The structural elucidation of polysaccharides represents an even greater challenge. The individual monosaccharide subunits that constitute the unknown polysaccharide must first be identified. Then the ring size and position in the polysaccharide sequence must be elucidated for each monosaccharide. Finally, the nature of the gly-cosidic linkages that form the polysaccharide backbone must be defined. Toward this end, it is necessary to establish which hydroxyl group on one monosaccharide is involved in the formation of the acetal or ketal that forms the glycosidic bond to the adjoining monosaccharide the stereochemistry at this anomeric center must also be determined. 

Periodate oxidation has been used to determine structure, especially the position of the glycosidic linkage or the position of substitution on a monosaccharide residue. In Chapter 2, section 2.5 we saw how periodate oxidation was used to determine the size of the carbohydrate rings that were produced by the formation of intramolecular hemiacetals. As indicated in that section, periodate stoichiometri-cally cleaves carbon-carbon bonds of carbons that each have a hydroxyl group. 

Acyclic compounds decompose in a manner less complicated than that of the cyclic monosaccharides because of elimination of such factors (affecting the spectrum) as size of the ring, and because of an even smaller effect of the stereochemical differences. For this reason, the use of acyclic derivatives is most reasonable when the molecular weight of the monosaccharide, the position and size of specific groupings (for example, the position of the deoxy unit or acetamido grouping), and the position of the carbonyl group are to be determined. 

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