Ketoses 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. 

In 1937, the true structure of the rearranged product was reported by Kuhn and Weygand. Amadori s stable isomers were shown to be unbranched A-substituted l-amino-l-deoxy-2-ketoses. Therefore, in a strict sense, the reaction named by Kuhn and Weygand the Amadori rearrangement is the complete conversion of a Af-substituted aldosylamine to a iV-substituted l-amino-l-deoxy-2-ketose. On this basis, simple enolization of an aldosylamine or ketosylamine to the intermediate 1,2-enolic form common to both should not be construed as the Amadori rearrangement. Also, the recently demonstrated conversion of a ketosylamine to a 2-amino-2-deoxyaldose does not come under the definition in spite of an obvious similarity to the Amadori rearrangement. 

Besides D-fructose, there are three D-2-ketohexoses D psicose, D-sorbose and t>-tagatose, (a) Draw the possible configurations for these three ketoses, (b) Given the configurations of all aldohexoses, tell how you could assign definite configurations to the ketoses. 

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. 

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 ... 

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