Glycosides ring size, determination

This reaction has been used m an imaginative way to determine the ring size of glycosides Once all the free hydroxyl groups of a glycoside have been methylated the glycoside is subjected to acid catalyzed hydrolysis Only the anomeric methoxy group IS hydrolyzed under these conditions—another example of the ease of carbocation for matron at the anomeric position... 

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. 

Oxidation by periodic acid, HI04-2H20 or H IOg, has proven useful in structure determinations of carbohydrates, particularly in determining the size of glycoside rings. Recall from Section 10.8E that periodic acid cleaves the carbon-carbon bond... 

Glycol cleavage by oxidation with sodium periodate, lead tetraacetate, or sodium bismuthate, is a useful method for determination of the ring size of glycosides as well as for the assignment of anomeric configuration and position of substituents on the ring. (See Chapters VI and I.)... 

The dialdehydes obtained in the periodate oxidation of glycosides and utilized for the determination of ring size (p. 32, 216), were also employed by Jackson and Hudson 94) for the configurational correlation of the anomeric carbons of aldohexopyranosides and aldopentopyranosides. The dialdehydes were oxidized to the dibasic acids and these were isolated as... 

The structures of disaccharides and disaccharide alditols were revealed by periodate oxidation, reduction and analysis of the products by e.i.- and FAB-m.s. as their peracetyl or permethyl derivatives. The compounds gave simple m.s. with intense and characteristic ions from which the glycosidic linkage positions and ring size could be determined. ... 

Proton-decoupled C-n.m.r. spectra of partially hydroxy-deuterated carbohydrates show signals in which the multiplicities depend on the environment of the hydroxy group. Five types of multi-plet were identified as being characteristic, e.g., C-6 of a hexo-pyranose gives a broad doublet, while C-1 of a glycoside gives a narrow doublet, etc. The ring sizes of benzoylated L-rhamnono-lactones as determined by C-n.m.r. spectroscopy are discussed in Chapter 16. 

There are several well-established methods for the determination of the ring size, such as oxidative degradation of the methylated sugars and a comparison with the lactones of the sugar acids or oxidation of the glycoside by periodate and characterisation of the products of oxidation . 

The use of periodic acid oxidation in structure determination can be illustrated by a case in which a previously unknown methyl glycoside was obtained by the reaction of D-arabinose with methanol and hydrogen chloride. The size of the ring was identified as five-membered because only one mole of periodic acid was consumed per mole of glycoside and no formic acid was produced. Were the ring six-membered, two moles of periodic acid would be required per mole of glycoside and one mole of formic acid would be produced. 

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