Monosaccharides ring structures

Hexose monosaccharides can form both five- and six-membered rings. In most cases, the six-membered ring structure is more stable, but fructose is an important example of a hexose that is more stable as a five-membered ring. The structure ofy6-fhictose is shown in Figure 13-17. Notice that there are — CH2 OH fragments bonded to two positions of this five-membered ring. Examples and explore the structures of monosaccharides in more detail. 

Constitutional studies have broadly followed the pattern of methyla-tion of the unsubstituted alcoholic groups in a carbohydrate, hydrolytic or oxidative disruption of the molecule and identification of the resulting methylated fragments. In this way methyl ethers have been invaluable in the determination of the ring structures of the monosaccharides and in the elucidation of the constitutions of the more complex saccharides. 

Figure 3.24. Monosaccharides that commonly constitute the carbohydrate portion of glycoproteins. Refer to text for details. Note that individual hydrogen atoms attached to the core ring structure are omitted for clarity of presentation...

For simplicity, we have thus far represented the structures of aldoses and ketoses as straight-chain molecules (Figs 7-3, 7-4). In fact, in aqueous solution, aldotet-roses and all monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl... 

Monomers and polymers of carbohydrates, (a) The most common carbohydrates are the simple six-carbon (hexose) and five-carbon (pentose) sugars. In aqueous solution, these sugar monomers form ring structures, (b) Polysaccharides are usually composed of hexose monosaccharides covalently linked together by glycosidic bonds to form long straight-chain or branched-chain structures. 

Monosaccharides tend to form ring structures known as intramolecular hemiacetals, especially when the product is a five-member (furanose) ring or a six-member (pyranose) ring. Depending on which way the ring forms about the reactive carbon, the structure is called an a- or /3-hemiacetal. 

An extra chiral center is produced at the hemiacetal (or hemiketal) carbon (former carbonyl carbon). The hydroxyl group can be either below (a) or above ( 3) the plane of the ring structure. Monosaccharides that differ only in the configuration of the groups at the hemiacetal (hemiketal) carbon are known as anomers. The hemiacetal (hemiketal) carbon is known as the anomeric carbon (Fig. 122). 

The Haworth projection formulas are neater ways of writing the ring forms shown in the equilibria above and yet preserving the configuration shown at each chiral carbon. It is not difficult to translate the open-chain structure for a monosaccharide into the Haworth ring structure. 

The Haworth structure tells us that a substituent that is above the general plane of the monosaccharide ring must also appear above the general plane of the chair whether this is axial or equatorial will depend on the carbon atom being considered. For example, looking at each carbon of 0-D-glucopyranose in turn, we find... 

Question In what respect does a glycoside ring differ from the ring structure of a monosaccharide ... 

Wen et al. (1993) showed the ROA spectra of 15 monosaccharides in aqueous solution. They found three main regions of interest, were ROA spectra should provide insights into local stereochemistry. The first is the region of 750 to 950 cm reflecting anomeric configuration, the second at 950 to 1200 cm gives information about ring structure and... 

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