The Cyclic Structure of Monosaccharides

Haworth projection common way of representing the cyclic structure of monosaccharides using a three-dimensional perspective. 

Since in aqueous solutions the cyclic form of monosaccharides is in equilibrium with their corresponding open forms, the a and P structures continually interconvert. At equilibrium, one form usually predominates. For instance, glucose dissolved in water consists of about a 2 1... 

Since in aqueous solutions the cyclic form of monosaccharides is in equilibrium with their corresponding open forms, the a. and p structures continually interconvert. At equilibrium, one form usually predominates. For instance, glucose dissolved in water consists of about a 2 1 ratio of p-D-glucose to a-D-glucose. Although their chemical constituents are identical, the biochemical properties between the a and the P forms can be quite different. Monosaccharides linked together to form disaccharides and polysaccharides cannot continue to interconvert and are therefore frozen in the a or p forms. Changing one monosaccharide in a complex carbohydrate to its opposite... 

The actual structure of monosaccharides such as o-glucose is not what has been shown up to this point. We previously learned that aldehydes and ketones react reversibly with alcohols to form hemiacetals and hemiketals, respectively. (Refer back to Sec. 14.7 to review this reaction.) This reaction takes place intramolecularly with monosacchardes since both OH and C = O are present in the same molecule. The structures of aldoses and ketoses are cyclic hemiacetals and hemiketals, respectively. 

Pyranose and Furanose Names Cyclic structures of monosaccharides are named according to their five- or six-membered rings. A six-membered cyclic hemiacetal is called a pyranose, derived from the name of the six-membered cyclic ether pyran. A five-membered cyclic hemiacetal is called a furanose, derived from the name of the five-membered cyclic ether ran. The ring is still numbered as it is in the sugar, not beginning with the heteroatom as it would be in the heterocyclic nomenclature. These structural names are incorporated into the systematic names of sugars. 

It is convenient at times to represent the cyclic structures of a monosaccharide without specifying whether the configuration of the anomeric carbon atom is a or j8. When we do this, we shall use formulas such as the following ... 

Cyclic Structures of Monosaccharides + 24.13 Provide a complete name for the following compound ... 

Even though monosaccharides exist largely as cyclic hemiacetals or hemiketals, rapid equilibrium, with trace quantities of carbonyl-containing compounds, means that discussion of the reactions of most sugars must consider both the acyclic and the cyclic structures. Common monosaccharides can be reduced to alkanepolyols (D-manitol, sorbitol) or oxidized to aldaric, aldonic, or uronic acids. 

In a similar manner, ketones can react with alcohols to form hemiketals. The analogous intramolecular reaction of a ketose sugar such as fructose yields a cyclic hemiketal (Figure 7.6). The five-membered ring thus formed is reminiscent of furan and is referred to as a furanose. The cyclic pyranose and fura-nose forms are the preferred structures for monosaccharides in aqueous solution. At equilibrium, the linear aldehyde or ketone structure is only a minor component of the mixture (generally much less than 1%). 

Figure 1.10 Examples of the cyclic and straight chain structures of monosaccharides. The carbon of the carbonyl group has the lowest locant...

In the second group are the fascinating researches directed towards the elucidation of the precise structures of some of these cyclic derivatives of the polyhydric alcohols and thence towards the provision of data concerning the size, position and stability of the ring form favored by each particular carbonyl compound. This information cannot necessarily be deduced from any which may already be available for the corresponding derivatives of the monosaccharides, because in the latter case, but not the former, free rotation in the carbon skeleton of the carbohydrate moiety is restricted owing to the presence of a pyranose or furanose ring system. 

This chapter begins with a discussion of the structure and stereochemistry of monosaccharides. Then the formation of cyclic structures front monosaccharides is discussed. This is followed by the presentation of a small number of reactions of these compounds. The classic series of experiments that was used to establish the structure of glucose is presented next. Finally, the structures of disaccharides, polysaccharides, and a few other types of carbohydrate-containing compounds are introduced. 

The building blocks for polysaccharides are monosaccharides (sugars) such as glucose or fructose (QH O ). Two cyclic structures of D-glucose are reproduced below (a third, open-chain structure is not shown). 

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. 

When monosaccharides are in aqueous solution, they exist in both open-chain and cyclic structures. The cyclic structures are more stable and are the predominant form of monosaccharides at equilibrium. Note in Figure 24-8 that the carbonyl groups are present only in the open-chain structures. In the cyclic structures, they are converted to hydroxyl groups. 

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