Acetal-ketal linkage

In the previous question, the first three, a,b,c, are reducing sugars. The d structure has both saccharides involved in an acetal-ketal linkage. 

Similarly, ketose sugars participate in polysaccharide formation by reaction of their ano-meric carbon with a hydroxyl of another monosaccharide to create a ketal linkage. The acetal and ketal bonds within polysaccharides are termed o-glycosidic linkages. 

Acetal and ketal linkages are widely foimd in nat-inal sugars and polysaccharides. The structure of sucrose is a splendid example. Sucrose is a disaccharide, composed of two linked monosaccharide imits, glncose in pyranose ring form and frnctose in fmanose ring form. As we have seen above,... 

Poly(vinyl acetates) produced at low conversion or at low temperatures may be subjected to this process without significant changes in the molecular weight distribution. As indicated before. Marvel and Inskeep [45] had postulated that acetal (or ketal) linkages were ruptured during hydrolysis. The existence of acetal bonding in poly(vinyl acetate) is, however, difficult to visualize. 

Disaccharides are sugars composed of two monosaccharide nnits linked together by the acetal or ketal linkages described earlier. They can be hydrolyzed to yield their monosaccharide building blocks by boiling with dilute acid or reacting them with appropriate enzymes. Nutritionally, the most important members of this gronp are maltose, lactose, and sucrose. 

Alcohol Dehydrogenase Surfactants with acetal or ketal linkages Oxidation of fatty alcohol to long-chain aldehyde or ketone Orlich et al., 2000... 

I60C-Hydroxy Derivatives of Gorticoids and their Acetonides. The preparation of 16a-hydroxy-9a-fluoroprednisolone (48) from the 3,20-bisethylene ketal of hydrocortisone acetate (49) has been reported (73). The latter was dehydrated with thionyl chloride in pyridine to yield the 4,9(11),16-triene (50). The 16,17-unsaturated linkage was selectively hydroxylated with OsO /pyridine to yield the 16a,17a-diol (51), which was converted... 

Because carbohydrates are so frequently used as substrates in kinetic studies of enzymes and metabolic pathways, we refer the reader to the following topics in Ro-byt s excellent account of chemical reactions used to modify carbohydrates formation of carbohydrate esters, pp. 77-81 sulfonic acid esters, pp. 81-83 ethers [methyl, p. 83 trityl, pp. 83-84 benzyl, pp. 84-85 trialkyl silyl, p. 85] acetals and ketals, pp. 85-92 modifications at C-1 [reduction of aldehydes and ketones, pp. 92-93 reduction of thioacetals, p. 93 oxidation, pp. 93-94 chain elongation, pp. 94-98 chain length reduction, pp. 98-99 substitution at the reducing carbon atom, pp. 99-103 formation of gycosides, pp. 103-105 formation of glycosidic linkages between monosaccharide residues, 105-108] modifications at C-2, pp. 108-113 modifications at C-3, pp. 113-120 modifications at C-4, pp. 121-124 modifications at C-5, pp. 125-128 modifications at C-6 in hexopy-ranoses, pp. 128-134. 

The following abbreviations are used in this and the subsequent tables Ac for acetyl, Pr for propionyl, Bu for butyryl, Bz for benzoyl, Ts for tosyl, My for methylene, Ed for ethylidene, Bd for benzylidene, Fd for furfurylidene. Id for isopropylidene, Me for methyl, Et for ethyl, Be for benzyl, Tr for trityl and Az for azoyl. Where the linkages of acetals and ketals are known, they are shown by different type fonts.)... 

Fig. 3 Hydrolyzable, acid-sensitive and reducible bonds. Efficient and biocompatible high molecular weight polymers are created by reversible linkage of small molecular weight compounds. Thus, programmed biodegradation due to, for example, hydrolyzable ester bonds [92] (a), acid-sensitive ketal (b) or acetal linkages (c) [98] is possible. The reducing cytosolic environment can also be taken advantage of in order to create biodegradable polymers by introduction of disulfide bonds as shown in (d) [105, 106]...

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. 

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