Reactions of Monosaccharides

Because monosaccharides contain only two kinds of functional gronps, hydroxyls and carbonyls, most of the chemistry of monosaccharides is the familiar chemistry of these two groups. As we ve seen, alcohols can he converted to esters and ethers and can be oxidized carbonyl compounds can react with nucleophiles and can be reduced. 

Monosaccharides behave as simple alcohols in much of their chemistry. For example, carbohydrate -OH groups can be converted into esters and ethers, which are often easier to work with than the free sugars. Because of their many hydroxyl groups, monosaccharides are usually soluble in water but insoluble in organic solvents such as ether. They are also difficult to purify and have a tendency to form syrups rather than crystals when water is removed. Ester and ether derivatives, however, are soluble in organic solvents and are easily purified and crystallized. 

Esterification is normally carried out by treating the carbohydrate with an acid chloride or acid anhydride in the presence of a base (Sections 16.4 and 16.5). All the -OH groups react, including the anomeric one. For example. 

Draw the products you would obtain by reaction of j8-D-ribofuranose with (a) CH3l,Ag20 (b) (CH3C0)20, p rridine 

We saw in Section 14.8 that treatment of a hemiacetal with an alcohol and an acid catalyst yields an acetal  

Thomson lu a Click Organic Interactive to predict products from reactions of simple monosaccharides. 

Draw /3-L-glucopyranose in its more stable chair conformation. 

Problem 25.11 Ribose exists largely in a furanose form, produced by addition of the C4 -OH group to the Cl aldehyde. Draw D-ribose in its furanose form. 

Problem 25.13 j Draw /S-D-galactop Tanose and /3-D-mannopyranose in their more stable chair con-i formations. Label each ring substituent as either axial or equatorial. Which would you expect to be more stable, galactose or mannose  

Problem 25.14 Draw /8-[.-galactopyranose in its more stable chair conformation, and label the substituents as either axial or equatorial. 

Problem 25.15 Identify the following monosaccharide, write its full name, and draw its open-chain form in Fischer projection. 

Biochemically important sugars usually contain five or six carbon atoms their structure includes a carbonyl group (either the aldehyde or the ketone form) and several hydroxyl groups. 

Optical isomerism is of paramount importance in the structure of simple sugars. Most of the important sugars found in nature have the D configuration, based on the standard compound D-glyceraldehyde. 

Most sugars exist in cyclic forms with five- or six-membered rings. The cyclization process involves the carbonyl group and gives rise to another chiral center in addition to the ones already present in the sugar molecule. The two possible cyclic isomers, called anomers, are designated a and p. 

What are some oxidation-reduction reactions of sugars  

When the carbonyl group of a sugar is reduced to a hydroxyl group, the resulting compound is one of the polyhydroxy alcohols known as alditols. Two compounds of this kind, xylitol and sorbitol, derivatives of the sugars xylulose and sorbose, respectively, have commercial importance as sweeteners in sugarless chewing gum and candy. 

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Reactions of monosaccharides (for example, D-ribose, D-xylose, D-man-nose) with cyclohexene and tetracyanoethylene in liquid HF have been studied. 

REACTIONS OF MONOSACCHARIDES WITH BE TA -KETONIC ESTERS AND RELATED SUBSTANCES... 

REACTIONS OF MONOSACCHARIDES WITH BETA-KETONIC ESTERS 101... 

REACTIONS OF MONOSACCHARIDES WITH BBTA-KETONIC ESTERS 105... 

REACTIONS OF MONOSACCHARIDES WITH BF7U-KETONIC ESTERS 107... 

REACTIONS OF MONOSACCHARIDES WITH AETA-KETONIC ESTERS 129... 

REACTIONS OF MONOSACCHARIDES WITH RETA-KETONIC ESTERS 137... 

This acid-catalyzed cleavage of the glycosidic bonds is rather complex and often suffers from a lack of selectivity mainly due to side dehydration or recombination reactions of monosaccharides. In the existing literature, four different classes of solid catalysts are reported (1) cation-exchange resins, (2) siliceous-based materials, (3) metal oxides, and (4) sulfonated amorphous carbons. 

For most of the reactions of monosaccharides that involve the aldehyde or ketone functional group, the presence of open chain form is crucial, as only in this form do these functional groups exist. A sugar solution contains two cyclic anomers and the open chain form in an equilibrium. Once the aldehyde or ketone group of the open chain form is used up in a reaction, the cyclic forms open up to produce more open chain form to maintain the equilibrium. 

The search for new catalysts that would be effective for acetalations still stimulates interest. Better yields, and increased rates, have been claimed for the reaction of monosaccharides with acetone in the presence of ferric chloride.48 (Ferric chloride was already known to catalyze acetalation in other series.47) Not yet applied in the carbohydrate field, but potentially useful for sugars, is the separate use of two different catalysts. The first one recommended is pyridinium p-toluenesul-fonate as a mild and efficient catalyst for the tetrahydropyranylation of alcohols.48 The main interest in this catalyst lies in the excellent yields... 

GarcIa, GonzAlez, F., Reactions of Monosaccharides with fteto-Ketonic Esters and Related Substances, 11, 97-143... 

Similar enol ethers probably serve as intermediates in another common modification-reaction of monosaccharide units especially characteristic of exocellular polysaccharides, namely, the formation of cyclic acetals of pyruvic acid. 

A. G. Fadnis, Metal-ion oxidation reactions of monosaccharides a kinetic study, Carbohydr. Res., 146 (1986) 97-105. 

C. M. Brands and M. A. J. S. van Boekel, Reactions of monosaccharides during heating of sugar-casein systems building a reaction network model, J. Agric. Food Chem., 2001, 49, 4661-4615. 

Perhaps the most important reaction of monosaccharides is the conversion of the hemiacetal to an acetal in the presence of an alcohol and acid. The products are known as glycosides, and the bond between the oxygen of the alcohol and the acetal carbon is called a glycosidic bond. 

Click Coached Tutorial Problems for more practice with Reactions of Monosaccharides. 

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