Hemiacetal groups monosaccharide

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-... 

Alternately, the importance of the hemiacetal group can be studied by using a compound that is an analog for a monosaccharide without the rest of the hydroxyl groups. Tetrahydro-2H-pyran-2-ol (THP) is such an analog for xylose. THP was reacted with an equimolar amount of DMU, in the absence of phenol, to follow the course of the hemiacetal-urea reactions. The acid-catalyzed mixture was heated between 81 and 116 °C over a 2-hour period. Samples were frozen to prevent further reaction prior to analysis. 

The reactions of carbohydrates, with few exceptions, are the reactions of functional groups that we have studied in earlier chapters, especially those of aldehydes, ketones, and alcohols. The most central reactions of carbohydrates are those of hemiacetal and acetal formation and hydrolysis. Hemiacetal groups form the pyranose and furanose rings in carbohydrates, and acetal groups form glycoside derivatives and join monosaccharides together to form di-, tri-, oligo-, and polysaccharides. 

Diene. A hydrocarbon that contains two carbon-carbon double bonds. Glycoside. A monosaccharide in which the hemiacetal group has been converted to an acetal with an alkoxy group bonded to an anomeric carbon. 

The aldehydic or hemiacetal group is the most easily oxidised portion of the monosaccharide molecule. All aldopyranoses (CLVIII) are oxidised with aqueous buffered bromine solution (pH 5.6). The initial product is the 1,5-lactone (CLIX) which then hydrolyses to the aldonic acid (CLX). Evaporation of the aqueous solution yields the y-lactone (CLXI), but from certain solvents such as p uidine which inhibit lactone formation the... 

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... 

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. 

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... 

Negative evidence for the importance of the free aldehyde group is found in the failure of all efForts - to condense amides with free monosaccharides, which have a hemiacetal structure. 

Monosaccharides commonly form internal hemiacetals or hemiketals, in which the aldehyde or ketone group joins with a hydroxyl group of the same molecule, creating a cyclic structure this can be represented as a Haworth perspective formula. The carbon atom originally found in the aldehyde or ketone group (the anomeric carbon) can assume either of two configurations, a and /3, which are interconvertible by mutarotation. In the linear form, which is in equilibrium with the cyclized forms, the anomeric carbon is easily oxidized. 

In order to synthesize monosaccharides having phosphorus in the hemiacetal ring, it is obvious that two fundamental problems have to be solved (I) how to introduce a suitable phosphine group, having the desired orientation, into an appropriate position on the carbon skeleton of the precursor, and (2) how to accomplish efficient ring-closure in order to yield the desired monosaccharide possessing a ring-phosphorus atom. 

Monosaccharides Cyclize to Form Hemiacetals Aldehydes can add hydroxyl compounds to the carbonyl group. If a molecule of water is added, the product is an aldehyde hydrate, as shown in figure 12.4. If a molecule of alcohol is added, the product is a hemiacetal the addition of a second alcohol results in an acetal. Sugars readily form intramolecular hemiacetals in cases in which the resulting compound has a five- or six-member ring. 

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