Hemiacetal monosaccharides

Glycoside An acetal derived from a monosaccharide hemiacetal (27.7)... 

In the same way, treatment of a monosaccharide hemiacetal with an alcohol and an acid catalyst yields an acetal in which the anomeric -OH has been replaced by an -OR group. For example, reaction of -D-glucopyranose with methanol gives a mixture of cv and / methyl u-glucopyranosides ... 

Section 22.10 Cyclic Structure of Monosaccharides Hemiacetal Formation... 

In Section 4.3, we learned that hemiacetals and hemiketals can react with alcohols in acid solutions to yield acetals and ketals, respectively. Thus, cyclic monosaccharides (hemiacetals and hemiketals) readily react with alcohols in the presence of acid to form acetals and ketals. The general name for these carbohydrate products is glycosides. The reaction of a-D-glucose with methanol is shown in Reaction 7.6. 

In the same way, treatment of a monosaccharide hemiacetal with an alcohol and an acid catalyst yields an acetal called a glycoside, in which the... 

CYCLIC MONOSACCHARIDES HEMIACETALS AND HEMIKETALS Haworth Projection Formulas... 

The stmcture of monosaccharides is often written in the acycHc form although only very minor amounts of it ever occur in that form. Because the interconversions are rapid, the carbonyl groups of sugars can and do react both as if they are free and as if they are in a hemiacetal ring form. 

Some monosaccharides also exist in a five-mem be red cyclic hemiacetal form called a furanose form. D-Fructose, for instance, exists in water solution as 70% /Tpvranose, 2% a-pyranose, 0.7% open-chain, 23% /3-furanose, and 5% a-furanose. The pyranose form results from addition of the -OH at C6 to the carbonyl group, while the furanose form results from addition of the —OH at C5 to the carbonyl group (Figure 25.5). 

Monosaccharides normally exist as cyclic hemiacetals rather than as open-chain aldehydes or ketones. The hemiacetal linkage results from reaction of the carbonyl group with an —OH group three or four carbon atoms away. A... 

Carb-29. Hemiacetals and hemithioacetals 2-Carb-30. Acetals, ketals and their thio analogues 2-Carb-31. Names for monosaccharide residues... 

Monosaccharides in which an alcoholic hydroxy group has been replaced by an amino group are called amino sugars (see 2-Carb-14). When the hemiacetal hydroxy group is replaced, the compounds are called glycosylamines. 

This is a modified form of the 1980 recommendations [4]. Priority is now given to naming cyclic forms, since in most cases branched-chain monosaccharides will form cyclic hemiacetals or hemiketals. 

If the branched monosaccharide forms a cyclic hemiacetal or hemiketal, the chain which includes the ring atoms rather than any alternative open chain must be the basis of the name. Otherwise the parent is chosen according to the principles given in 2-Carb-2.1. 

Haworth representation, cyclic monosaccharides, 61-63 Hemiacetal groups disaccharides with, 149-150 without, 148-149 nomenclature, 122-123 oligosaccharides with, 153-154 without, 151-153... 

Glycosides are formed by condensation between the hydroxyl group of the anomeric carbon of a monosaccharide, or monosaccharide residue, and a second compound that may—or may not (in the case of an aglycone)—be another monosaccharide. If the second group is a hydroxyl, the O-glycosidic bond is an acetal link because it results from a reaction between a hemiacetal group (formed from an aldehyde and an —OH group) and an-... 

The c.d. spectra of these 21 monosaccharides that were studied contain a wealth of information, although proper analysis of the data is not always obvious. However, c.d.-difference spectra between pairs of sugars that differ at only one carbon atom can be used to simplify the analysis. Each of the chromophores in a monosaccharide (hydroxyl, methoxyl, hydroxymethyl, hemiacetal, and acetal) are symmetric and obtain their c.d. by interaction... 

The aldehyde or ketone group of monosaccharides can undergo an intramolecular reaction with one of its own hydroxyl groups to form a cyclic, hemiacetal, or hemiketal structure, respectively (Figure 1.26). In aqueous solutions, this cyclic structure actually predominates. The open-chain aldehyde or ketone form of monosaccharides is in equilibrium with the cyclic form, but the open structure exists less than 0.5 percent of the time in aqueous environments. It is the... 

Although aliphatic alcohols are typically poor acceptors in the Mitsunobu-type glycosylation, Szarek and coworkers have highlighted one advance to this end [95]. For the triphenylphosphine and diethylazodicarboxylate promoted glycosylation of a monosaccharide acceptor, the addition of mercuric bromide is necessary to promote the reaction. For example, the (1,6)-disaccharide 44 was obtained in 80% yield using this modified Mitsunobu protocol. Unlike previous examples with phenol or N-acceptors, preactivation of the hemiacetal donor was performed for 10 min at room temperature prior to addition of the aliphatic alcohol nucleophile. 

A typical procedure calls for reaction of the hemiacetal donor with dicydohexyl carbodiimide and copper(I) chloride (0.1 equiv) at 80 °C, followed by an addition of the acceptor and continued heating. As an early demonstration of this protocol, oc-riboside 86 was prepared in moderate yield but with exclusive stereoselectivity [141]. Further measures were required for the glycosylation of monosaccharide acceptors, such as addition of p-toluenesulfonic add (0.1 equiv) to promote the formation of disaccharide 87 [144]. The method was more suitably applied to the synthesis of O-acyl glycopeptides, as evidenced by the formation of 88 in 60% yield [143,144]. Various peptides with non-nudeophilic side chains were found to be amenable to this stereoselective reaction. The [3-selectivity was suggested to arise from a preponderance of the a-isourea intermediate 85 in the activation step. 

Monosaccharide structures may be depicted in open-chain forms showing their carbonyl character, or in cyclic hemiacetal or hemiketal forms. Alongside the Fischer projections of glucose, ribose, and fructose shown earlier, we included an alternative... 

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