Mutarotation, monosaccharides

Mechanism 23-1 Formation of a Cyclic Hemiacetal 1108 23-7 Anomers of Monosaccharides Mutarotation 1112 23-8 Reactions of Monosaccharides Side Reactions in Base 1114... 

Glycosides are named by first citing the alkyl group and then replacing the -ose ending of the sugar with -oside. Like all acetals, glycosides are stable to neutral water. They aren t in equilibrium with an open-chain form, and they don t show mutarotation. They can, however, be converted back to the free monosaccharide by hydrolysis with aqueous acid (Section 19.10). 

The reversible reactions are initiated by an equilibrium between neutral and ionized forms of the monosaccharides (see Fig. 6). The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization via an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar, the three reducing sugars are in equilibrium via the enediol intermediate. The mechanism of isomerization, known as the Lobry de Bruyn-... 

When the anomeric hydroxyl group of one monosaccharide is bound glycosidically with one of the OH groups of another, a disaccharide is formed. As in all glycosides, the glyco-sidic bond does not allow mutarotation. Since this type of bond is formed stereospecifically by enzymes in natural disaccharides, they are only found in one of the possible configurations (a or P). 

In analyzing polysaccharide hydrolyzates, some workers have assumed that, because mutarotation in pyridine is slow and the tri-methylsilylation reaction is fast, no change in the composition occurs during trimethylsilylation. Others have equilibrated the monosaccharides in pyridine, before trimethylsilylation.172,255 In nonaqueous solvents, the mutarotation may be catalyzed by lithium perchlorate172 or, more conveniently, by 2-pyridinol,256,257 which is volatile, but does not interfere with subsequent analyses. 

The original publication by Sweeley and coworkers5 was concerned with the separation of a wide range of carbohydrates, from mono- to tetra-saccharides. Most of the subsequent publications have considered the quantitative analysis of mixtures of varied complexity, although two studies have demonstrated the separation of the protium from the deuterium fonns of monosaccharides.200,201 The study of mutarotational equilibria by gas-liquid chromatography has been discussed in Section IV (see p. 38). 

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. 

Define, by reference to glucose, the meaning of each of the following terms (a) a monosaccharide, (b) mutarotation, (c) a pyranose ring system, (d) anomers and (e) a- and (3-glucosides. 

For some monosaccharides, the rate of mutarotation (K = k 3-kf) is found to obey a simple first-order rate law in which — d[a]/dt = A [a] — A2[P] (Scheme 1.4). Glucose, mannose, lyxose and xylose exhibit... 

Answer Lactose (Gal061— 4)Glc) has a free anomeric carbon (on the glucose residue). In sucrose (Glc(al<- 2 8)Fru), the anomeric carbons of both monosaccharide units are involved in the glycosidic bond, and the disaccharide has no free anomeric carbon to undergo mutarotation. 

Integrated, 13C-n.m.r. spectra have been used extensively to study mutarotational equilibria of monosaccharides, especially of ketoses, which do not have well-resolved, H-n.m.r. spectra,26-33 and also have been used to determine the composition of crude reaction-mixtures.34... 

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