Hemiketals cyclic, sugars

Cyclic hemiacetals or hemiketals of sugars with a five-membered (tetrahydrofuran) ring are called furanoses, those with a six-membered (tetrahydropyran) ring pyran-oses. For sugars with other ring sizes see 2-Carb-5. 

Sugars, especially those with five or six carbon atoms, normally exist as cyclic molecules rather than as the open-chain forms we have shown so far. The cycUzation takes place as a result of interaction between the functional groups on distant carbons, such as G-1 and G-5, to form a cyclic hemiacetal (in aldohexoses). Another possibility (Figure 16.5) is interaction between G-2 and G-5 to form a cyclic hemiketal (in ketohexoses). In either case, the carbonyl carbon becomes a new chiral center called the anomeric carbon. The cyclic sugar can take either of two different forms, designated a and P, and are called anomers of each other. 

Toward the end of the nineteenth century, it was realized that the free sugars exist as cyclic hemiacetals or hemiketals. Individual sugars react with methanol under acid catalysis to give stable products termed methyl glycosides, which are mixed full acetals. These have a new asymmetric center at the original carbonyl atom and were thus isolated as pairs of isomers that were designated as the ot and p forms, later termed anomers. In the d series, the a anomer was defined as the one having the more-positive specific rotation, and in the l series, the a anomer... 

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%). 

Towards the end of the nineteenth century it was realized that the free sugars (not only the glycosides) existed as cyclic hemiacetals or hemiketals. Mutarotation, discovered in 1846 by Dubrunfaut, was now interpreted as being due to a change... 

A vast range of natural sugars exempMly these cyclic addition products. A typical sugar exists predominantly in the form of a hemiacetal or hemiketal in solution, although this is an equilibrium reaction, and the open chain carbonyl form is always present to a small extent (<1%). The formation of a six-membered cyclic hemiacetal from glucose is achieved by attack of the C-5 hydroxyl onto the protonated carbonyl (conjugate acid). 

The cyclic hemiacetal and hemiketal forms of monosaccharides are capable of reacting with an alcohol to form acetals and ketals (see Section 7.2). The acetal or ketal product is termed a glycoside, and the non-carbohydrate portion is referred to as an aglycone. In the nomenclature of glycosides we replace the suffix -ose in the sugar with -oside. Simple glycosides may be synthesized by treating an alcoholic solution of the monosaccharide with an acidic catalyst, but the reaction mixture usually then contains a mixture of products. This is an accepted problem with many carbohydrate reactions it is often difficult to carry out selective transformations because of their multifunctional nature. 

Simple sugars in aqueous solution usually form cyclic structures, either hemi-acetals or hemiketals (Figure 4—5). 

However, it is important to keep in mind that the carbonyl groups of sugars usually are combined with one of the hydroxyl groups in the same molecule to form a cyclic hemiacetal or hemiketal. These structures once were written as follows, and considerable stretch of the imagination is needed to recognize that they actually represent oxacycloalkane ring systems ... 

Monosaccharides, polyhydroxy aldehydes or ketones, are either aldoses or ketoses. Sugars that contain four or more carbons primarily have cyclic forms. Cyclic aldoses or ketoses are hemiacetals and hemiketals, respectively. 

The cyclic hemiacetal (or hemiketal) can react with an alcohol to form an acetal (or ketal), called a glycoside. If the name pyranose or furanose is used, the acetal is called a pyranoside or a furanoside. The bond between the anomeric carbon and the alkoxy oxygen is called a glycosidic bond. The preference for the axial position by certain substituents bonded to the anomeric carbon is called the anomeric effect. If a sugar has an aldehyde, ketone, hemiacetal, or hemiketal group, it is a reducing sugar. 

Fructose is a ketohexose sugar. The cyclic structure formed is called a hemiketal, and the five membered ring that is formed is called furanose ring. The straight chain structure and the cyclic structures of fructose are shown in Figure 29-5. 

Cyclic hemiacetals and hemiketals are much more stable than the open-chain compounds discussed earlier. In Chapter 7, we ll see that sugars such as glucose and fructose exist predominantly in the form of cyclic hemiacetals and hemiketals. 

Although carbohydrate cyclic hemiacetals and hemiketals are in equilibrium with open-chain forms of the monosaccharides, the glycosides (acetals and ketals) are much more stable and do not exhibit open-chain forms. Therefore, glycosides of monosaccharides are not reducing sugars. 

Monosaccharides are sweet-tasting solids that are very soluble in water. Noncarbohydrate low-calorie sweeteners such as aspartame have been developed as sugar substitutes. Pentoses and hexoses form cyclic hemiacetals or hemiketals whose structures can be represented by Haworth structures. Two isomers referred to as anomers (the a and p forms) are produced in the cyclization reaction. All monosaccharides are oxidized by Benedict s reagent and are called reducing sugars. Monosaccharides can react with alcohols to produce acetals or ketals that are called glycosides. 

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