Aldoses ring structure

Acetylated aldose oximes with a ring structure have also been obtained during the preparation of the nitriles, both with sodium acetate-acetic anhydride and with pyridine-acetic anhydride. Thus it appears that under these conditions at least a certain amount of the oxime is present in the ring form. 

For simplicity, we have thus far represented the structures of aldoses and ketoses as straight-chain molecules (Figs 7-3, 7-4). In fact, in aqueous solution, aldotet-roses and all monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl... 

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>. 

The various types of carbohydrates considered in this chapter are shown in formulas I-XVIII. The two ring forms (II and III) of the aldoses are shown because careful oxidation can produce the lactones X and XI directly without breaking the oxygen bridge. In most cases, however, the products of oxidation have been isolated as salts of aldonic acids (IX) thus the nature of the initial oxidation product, either a lactone having the same ring structure as the sugar, or possibly the free... 

This method was of value in that it gave an insight into the actual reaction of an aldose during bromine oxidation, and was also indicative of its ring structure. The method should be used with caution, however, especially in respect to the relative velocity of the anomeric forms of the various sugars. Formation of 5-lactones can be considered excellent... 

The glucose-6-phosphate formed in the first reaction is rearranged to produce the structural isomer fructose-6-phosphate. The enzyme phosphoglucose isomerase catalyzes this isomerization. The result is that the C-1 carbon of the six-carbon sugar is exposed it is no longer part of the ring structure. Examination of the open-chain structures reveals that this isomerization converts an aldose into a ketose. 

There are fewer ketoses than there are aldoses because ketoses have one less chiral carbon. The most prevalent of the ketoses are dihydroxyacetone, ribu-lose, xylulose, and fructose (see Figure 4). All four of these sugars are important intermediates in metabolism. Fructose is, along with glucose, part of sucrose or table sugar. Fructose is a ketohexose, and the only one of the four ketoses that can assume a ring structure. Like ribose, fructose forms a five-membered (or furanose) ring and has a and j8 anomers. 

Less ambiguous results have been obtained by examination of the electrophoretic behavior of cyclitols. With these, the possibility of complex-formation with open-chain and with five-membered ring structures, as in the case of aldoses and ketoses, does not arise. The complex-forming compounds of this group all possess a m-m-1,2,3-triol system. Their rates of migration have been related to the instability factors of their conformations possessing this triol system as in (13). 

No. The reaction catalyzed by phosphoglucose isomerase is a simple isomerization between an aldose and a ketose and involves the open-chain structures of both sugars. Since glucose 6-phosphate and fructose 6-phosphate are both reducing sugars, their Haworth ring structures are in equilibrium with their open-chain forms. This equilibration is very rapid and does not require an additional enzyme. Note that this isomerization reaction is of the same type as that catalyzed by triosephosphate isomerase. 

The reactivity of the 2,5-anhydrides of aldoses is determined by two essential structural features that do not exist in the sugars, namely, the presence of an oxolane ring and of a carbonyl group (most frequently, free) a to the ring-oxygen atom. These two characteristics make the 2,5-anhydroaldoses closer to tetrahydro-2-furaldehyde than to the aldoses, where only in exceptional cases is the carbonyl group not masked by the formation of an intramolecular, five- or six-membered, hemiacetal ring. 

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