Aldoses definition

Aldoses generally undergo benzilic acid-type rearrangements to produce saccharinic acids, as well as reverse aldol (retro-aldol) reactions with j3-elimination, to afford a-dicarbonyl compounds. The products of these reactions are in considerable evidence at elevated temperatures. The conversions of ketoses and alduronic acids, however, are also of definite interest and will be emphasized as well. Furthermore, aldoses undergo anomerization and aldose-ketose isomerization (the Lobry de Bruyn-Alberda van Ekenstein transformation ) in aqueous base. However, both of these isomerizations are more appropriately studied at room temperature, and will be considered only in the context of other mechanisms. 

A definite method for the determination of D-fructose in the presence of aldoses was worked out by Kruisheer 98.3% of theory was found. The aldoses were oxidized with sodium hypoiodite, after which the excess iodine was titrated with sodium sulfite. Thiosulfate could not be used here, since the subsequent determination of D-fructose was carried out with an alkaline copper solution by the Luff-Schoorl method. 

The question of the nature of the oxidant in an aqueous buffered bromine solution at pH 5-6 appears to be settled for the aldoses. But the same cannot be said for oxidation with bromine in a strongly acid solution in which hydrobromic acid is allowed to accumulate. The data of Isbell and Pigman show that an increase in bromide concentration has little effect on the velocity of the reaction. Yet Bunzel and Matthews have shown that the addition of hydrobromic acid has a very definite inhibiting effect on the oxidation. Apparently, the combination of hydrogen ion and bromide ion has a pronounced effect, whereas either one alone is of minor importance. 

The definition is not, perhaps, entirely satisfactory. It is arbitrary and formal. It does not include those acids of formula C H2 0 which contain a quaternary carbon atom, because the hydroxy-aldehyde of like general formula required by the definition is structurally impossible. In other words, such acids could not be formed by the imaginary reaction which is the basis for the defintion and they would, therefore, not be called saccharinic acids. On the other hand, the definition does have the advantage of perfect clarity. Furthermore, it establishes a connection between the saccharinic acids and the aldoses. Finally, it escapes the drawback of a functional definition, because it depends, not on an actual, but on a theoretically possible, number of isomers. The term saccharinic acid will be used in this article to denote only those compounds encompassed by the Glattfeld-Sherman definition. 

Since Fehling s test gives a positive result for any aldose or 2-ketose, it is evidence that there could be a sugar in urine but would not be a definitive test for glucose per se. 

To answer this problem, one must know the structures of the molecules in question and a couple of definitions. By definition, epimers are a pair of molecules that differ from each other only in their configuration at a single asymmetric center. Anomers are special epimers that differ only in their configuration at a carbonyl carbon hence, they are usually acetals or hemiacetals. An aldose-ketose pair is obvious. Inspection of Fischer representations of the molecular pairs leads to the conclusion that (a), (c), and (e) are aldose-ketose pairs (b) and (f) are epimers and (e) are anomers. 

WORKED PROBLEM 22.1 Generalize from the definition of an aldotriose to generate the structures of an aldose with four carbons and an aldose with five carbons. 

In an extensive series of studies, Yoshikawa,Yano and co-workers have confirmed that various kinds of aldose can be complexed with nickel and amino compounds such as diamines and amino acid in a methanolic solution [43]. They studied the nickel(ll) complexes coordinating N-glycosides derived from diamine and monosaccharide. The structure of the complex was determined from three-dimensional X-ray data [43 d]. Their further studies elucidated that aldoses can be epimerized by reaction systems containing a nickel/diamine complex. This novel epimerization proceeds through an alternative mechanism that is definitely distinct from the epimerization mechanism by reversible enolization that was introduced in Sect. 2.1. It results in the formation of an equilibrium mixture of the two epimers. According to their study [ 39], three significant pecu-harities of this epimerization are ... 

The osazones are yellow crystalline solids, sparingly soluble, and with definite melting-points. On heating with hydrochloric acid they lose phenylhydrazine and form an oscme, which is not the original sugar. By forming and then reducing an osone, an aldose is converted into a ketose ... 

---