Ketoses structure

S) Ketose Structures.—The equilibria for ketoses differ markedly from those of aldoses, because the hydroxymethyl group attached to the anomeric carbon atom tends to assume an equatorial position. Maintenance of the anomeric hydroxymethyl group in the equatorial position keeps the anomeric hydroxyl group in the axial position. With a-D-sorbose (a-D-xylo-hexulose) and a-D-gluco-heptulose in the CA conformation, all large groups except the anomeric hydroxyl... 

In addition, its proton magnetic resonance spectrum unambiguously supported the 5-ketose structure (69). Rearrangements of epoxides to ketones when dicobalt octacarbonyl is used as the catalyst at temperatures above 100 , or when cobalt hydrocarbonyl is used at lower temperatures, are well known. By applying the technique of double irradiation to a sample of (68), the main component was shown to possess structure (68). Presumably, the free aldehyde group of the hydroformylation product immediately cyclized with the free hydroxyl group on C-3 to give the tricyclic structure (68). A third component (68a) (isolated in less than 5 % yield) was undoubtedly formed by subsequent reduction of the dialdose derivative (68). 

Ketoses are the functional isomers of the aldoses. A family of ketose structures can be generated in the same manner as aldoses. 

On the basis of these results, Hakomori and coworkers53,54 proposed a l-deoxy-l-(N-peptidyl)-D-ketose structure for the linkage of the peptide and carbohydrate components of UGP I, II, and III this kind of structure is identical with that of the product of the Amadori rearrangement of glycosylamines. 

O-nucleophiles." The synthesis of a new aminopolysaccharide (288) having an amino-ketose structure, has been achieved" utilizing the thermal polymerization of... 

The Amadori rearrangement of aldosyl amines to aminodeoxyketoses (Chapter VIII), which is an intermediate reaction during osazone formation, also provides a means of passing from aldose to ketose structures. 

FIGURE 7.1 Structure of a simple aldose (glyceraldehyde) aud a simple ketose (dihy-droxyacetoue). 

FIGURE 7.3 The Structure and stereochemical relationships of D-ketoses having three to six carbons. The configuration in each case is determined by the highest numbered asymmetric carbon (shown in gray). In each row, the new asymmetric carbon is shown in red. 

In a similar manner, ketones can react with alcohols to form hemiketals. The analogous intramolecular reaction of a ketose sugar such as fructose yields a cyclic hemiketal (Figure 7.6). The five-membered ring thus formed is reminiscent of furan and is referred to as a furanose. The cyclic pyranose and fura-nose forms are the preferred structures for monosaccharides in aqueous solution. At equilibrium, the linear aldehyde or ketone structure is only a minor component of the mixture (generally much less than 1%). 

In addition to the common carbohydrates mentioned in previous sections, there are a variety of important carbohydrate-derived materials. Their structural resemblance to sugars is clear, but they aren t simple aldoses or ketoses. 

The basis for the name is the structure of the parent monosaccharide in the acyclic form. Charts I and IV (2-Carb-10) give trivial names for parent aldoses and ketoses with up to six carbon atoms. 2-Carb-8.2 and 2-Carb-10.3 describe systematic naming procedures. 

Chart IV. Structures, with systematic and trivial names, of the 2-ketoses with three to six carbon atoms... 

A more general access to biologically important and structurally more diverse aldose isomers makes use of ketol isomerases for the enzymatic interconversion of ketoses to aldoses. For a full realization of the concept of enzymatic stereodivergent carbohydrate synthesis, the stereochemically complementary i-rhamnose (Rhal EC 5.3.1.14) and i-fucose isomerases (Fuel EC 5.3.1.3) from E. coli have been shown to display a relaxed substrate tolerance [16,99,113,131]. Both enzymes convert sugars and their derivatives that have a common (3 J )-OH configuration, but may deviate in... 

Aldose-ketose isomerism Fructose has the same molecular formula as glucose but differs in its structural formula, since there is a potential keto group in position 2, the anomeric carbon of fmctose (Figures 13 and 13-7), whereas there is a potential aldehyde group in position 1, the anomeric carbon of glucose (Figures 13-2 and 13-6). 

Reducing sugars can be detected by reaction with phenylhydrazine to yield a hydrazone product, except the result of the reaction is not what one might imagine giving the structure of aldoses and ketoses. Glucose, for example, can react with phenylhydrazine to yield the anticipated... 

The reaction of ethyl acetoacetate with simple hydroxy ketones has been compared with the corresponding reactions of the ketoses. The results obtained with l-hydroxy-2-propanone and 3-hydroxy-2-butanone, under the same experimental conditions as with D-fructose, establish a parallel between these reactions. However, as in the case of the aldoses, the yield is greater for these simpler hydroxy ketones than for the ketoses.9 The resultant esters, (XV and XVI), were obtained in the form of sirups, but the free acids, (XVII and XVIII), and their phenacyl esters are crystalline. The acids were shown to be identical with those of known structure described in the literature.9... 

Ketoses should react under a similar scheme. Indeed they do but an important problem in the chemistry of ketoses consists on the lack of selectivity due to (1) the complexity of their tautomeric equilibria and (2) their tendency to form tertiary oxocarbenium ions under acidic conditions. Thus, mixtures of open-chain, cyclic and dehydrated products are frequently obtained.7 The discussion about OZT structures obtained from D-fructose as proposed by Zemplen, Wickstrom and more recently by Grouiller et al. continues today. In fact, the first authors claimed the fusion of OZT on a pyran form of D-fructose, while Grouiller suggested the formation of a mixture of fused OZTs with /1-pyran (major) and p-furan (minor) forms (Scheme 22).18... 

Over thirty publications resulted from Tipson s work in Levene s laboratory. Along with the work on nucleic acid components, he also studied the structures of gum arabic and other plant gums, and conducted a range of synthetic investigations on sugars, with particular emphasis on uronic acids and 5-carbon ketoses. His 1939 observation that acetylated glycosyl halides... 

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