Monosaccharides with alcohols

In this section, we discuss reactions of monosaccharides with alcohols, reducing agents, and oxidizing agents. In addition, we examine how these reactions are useful in our everyday lives. 

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%). 

We saw in Section 25.6 that reaction of a monosaccharide with an alcohol yields a glycoside in which the anomeric -OH group is replaced by an -OR substituent. If the alcohol is itself a sugar, the glycosidic product is a disaccharide. 

Acetic anhydride, electrostatic potential map of, 791 reaction with alcohols, 807 reaction with amines, 807 reaction with monosaccharides, 988... 

The polyhydric alcohols arising formally from the replacement of a carbonyl group in a monosaccharide with a CHOH group are termed alditols (see 2-Carb-19). 

Industrial applications of glycosides are predetermined by their aglycon. The condensation of a monosaccharide with a long-chain linear alcohol... 

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. 

Abstract The telomerization of butadiene with alcohols is an elegant way to synthesize ethers with minimal environmental impact since this reaction is 100% atom efficient. Besides telomerization of butadiene with methanol and water that is industrially developed, the modification of polyols is still under development. Recently, a series of new substrates has been involved in this reaction, including diols, pure or crude glycerol, protected or unprotected monosaccharides, as well as polysaccharides. This opens up the formation of new products having specific physicochemical properties. We will describe recent advances in this field, focusing on the reaction of renewable products and more specifically on saccharides. The efficient catalytic systems as well as the optimized reaction conditions will be described and some physicochemical properties of the products will be reported. 

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... 

Benzoyl chlorides, like other acyl chlorides, react readily with alcohols to form the corresponding esters (259). The reaction requires 19 h to be completed, and it has been studied in the analysis of carbohydrates, aminosugars, and glycosides (260). Because the preparation of acyl derivatives of monosaccharides is difficult, phenyldimethylsilyl chloride has been also investigated as an alternative label (261). After dissolving the analytes in dimethylformamide and addition of... 

Less than one percent of each of the monosaccharides with five or more carbons exists in the open-chain (acyclic) form. Rather, they are predominantly found in a ring form, in which the aldehyde (or ketone) group has reacted with an alcohol group on the same sugar. 

Reaction XC. Hydrolysis of the Disaccharides. (J. pr., [2] 2, 1, 245 B., 13, 1761 28, 1429.)—When the disaccharides, and in fact all the glucosides, are heated with mineral acids, they are hydrolysed into their component monosaccharides or into their component monosaccharides and alcohols. 

Monosaccharides react with alcohols (H+ catalyst) to give glycosides. The -OH group at the anomeric carbon is replaced by an -OR group. The product is an acetal. Alcohols and phenols often occur in nature combined with sugars as glycosides, which renders them water-soluble. 

We saw in the previous chapter that aldehydes react with alcohols to form hemi-acetals, and hemiacetals react with another alcohol to afford acetals. Well, the intramolecular reaction of the aldehyde carbon of an aldose monosaccharide with one of the hydroxyl groups in the same molecule affords a cyclic hemiac-etal. An mferaiolecular reaction of this hemiacetal with the hydroxyl group of another sugar molecule provides an acetal functional group. This kind of acetal linkage connects monosaccharides into polysaccharides. Biochemists refer to these acetal (and ketal) bonds as glycoside bonds. 

The reactivity of the anomeric hydroxyl group is illustrated by the ease with which monosaccharides react with alcohols and with amines. The normal hydroxyl groups in the molecule do not react, but the anomeric hydroxyl does. The process is known as glycosylation (of the alcohol or amine), and the products as O-glycosides and N-glycosides. 

Like D-(+)-glucose, other monosaccharides exist in anomeric forms capable of mutarotation, and react with alcohols to yield anomeric glycosides. 

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