Monosaccharides, acetals size

Additionally, H and NMR spectroscopy provide valuable means for ascertaining the ring size and a preferred conformation of functionalized protected monosaccharides such as benzylidene acetals [27,28]. 

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. 

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.)... 

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. 

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