Aldoses cyclic forms

Anomeric configuration in cyclic forms of higher aldoses 2-Carb-9. Dialdases... 

Replacement of the ring oxygen atom of the cyclic form of an aldose or ketose by sulfur is indicated in the same way, the number of the non-anomeric adjacent carbon atom of the ring being used as locant. 

The generic term glycosides defines all mixed acetals formed by the acetalisation of the cyclic forms of aldoses and ketoses. Glycosyl groups are monosaccharides that have lost their anomeric -OH group the suffix -yl is used to indicate the change that has occurred at C-1. 

Figure 20-1 Structure and configuration of the D-aldoses from C3 to C6l showing the configurational relationship to D-glyceraldehyde. Open-chain and cyclic forms are shown. The oxacyclohexane (pyranose) form is more stable than the oxacyclopentane (furanose) form for the free sugar. The oxacyclopentane structure is shown for ribose because this is the form in which it occurs in many important substances, such as the nucleic acids. Only the a anomers are shown (see Section 20-2B).

Most, if not all, of the stable forms of crystalline aldose and ketose monosaccharides exist in the pyranose structure. Each in solution, as with D-glucose, exists as an equilibrium mixture of open chain and of a- and / -anomers of the cyclic forms. The cyclic five- and six-membered structures formulated below are an illustrative selection of monosaccharides. 

Kinetics of oxidation of four pentoses by bromamide-T were conducted in alkaline medium at different temperatures and the overall activation parameters have been calculated.52 Aldonic acids were the oxidation products, and a mechanism was suggested in which formation of the enediol anion of the sugar is the rate-limiting step. As aldoses may undergo epimerization in alkaline solutions, the oxidation of monosaccharides with bromamide-T was also performed in hydrochloric acid solution.53 Kinetic parameters revealed a low reactivity of ketoses relative to aldoses, and indicated that the cyclic forms of the latter are involved in the oxidations. 

At The American University, Isbell s major interest in research turned to the study of the oxidation of saccharides with hydrogen peroxide. In collaboration with Dr. Frush, he published some forty papers on the subject. A number of major discoveries were made, including that of a stepwise degradative peroxidation, which is catalyzed by base or by such metals as iron(II). It starts at the anomeric carbon of an aldose, either in the acyclic or the cyclic form, and affords the lower aldose and formic acid (see Fig. 8). Two mechanisms were recognized an ionic one prevalent in strong alkali, and a free-radical process catalyzed by Fe(II) (see Fig. 9). 

In general, a six-membered pyranose form is preferred over a five-membered furanose form because of the lower ring strain, and these cyclic forms are very much favoured over the acyclic aldehyde or ketofte forms. As can be seen in Table 1.3, at equilibrium, the anomeric ratios of pyranoses differ considerably between aldoses. These observations are a direct consequence of differences in anomeric and steric effects between monosaccharides. The amount of the pyranose and furanose present in aqueous solution varies considerably for the different monosaccharides. Some sugars, such as D-glucose, have undetectable amounts of furanose according H-NMR spectroscopic measurements whereas others, such as D-altrose, have 30% furanose content under identical conditions. 

Fig. 2-6. Cyclic forms of some a-D-aldoses (Haworth formulas). 1, Erythrose 2, threose 3, xylofuranose 4, xylopyranose 5, arabinofuranose 6, arabinopyranose 7, glucofuranose 8, glucopyranose 9, mannofuranose 10, mannopyranose 11, galactofuranose 12, galac-topyranose.

Most imidazoline, imidazolone, and imidazole derivatives are stable ring structures and usually exist only in the cyclic forms. For example, in the reaction of methyl 2-deoxy-2-isothiocyanato-a-D-glucopyranoside 149 with D-glucosamine 148, a-deoxy-2-(3-substituted thioureido)-D-aldose, 150, is presumably the reaction intermediate. However, the ring-chain tautomeric equilibrium of 150 shifts towards the cyclic form to yield 151 which is the only detectable form by NMR analysis (Scheme 41) <1999TA3011>. 

Aldoses are reducing sugars because in their non-cyclic form they contain A. A ketone group B. An ester group... 

Modern concepts of organic chemistry have provided interpretation and explanation for a variety of supposedly anomalous and unusual reactions of carbohydrate compounds. These concepts have often been derived from a study of relatively more simple substances in which there has been little complication resulting from interfering and conflicting factors. Consequently, direct application of these concepts to the more complex carbohydrate compounds without due consideration of alternative possibilities may result in oversimplification and a false picture. Despite such inherent hazards, it is believed that a useful purpose can be served by the correlation and discussion of the carbohydrate reactions with direct reference to analogous properties of simpler organic compounds. For this objective, we can define the acyclic forms of the aldoses as polyhydroxy-aldehydes, and their cyclic forms as polyhydroxy-cyclohemiacetals. In these compounds, besides the additive, inductive effect of the hydroxyl... 

Monosaccharides, polyhydroxy aldehydes or ketones, are either aldoses or ketoses. Sugars that contain four or more carbons primarily have cyclic forms. Cyclic aldoses or ketoses are hemiacetals and hemiketals, respectively. 

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