Ketoses properties

The taste properties of various other ketoses, including 2-pentuloses, 2-hexuloses, and 2-octuloses, have also been studied (see Table XV) to determine the significance of the 2-hydroxyl-2-hydroxymethyl arrangement of 2-ketopyranoses. Some of these, for example, a-D-tagatose (25), -d-g/uco-2-heptulose (26), a-D-mnMno-2-heptulose (27), a-D-fa/o-2-heptulose (28), a-D-a/tro-2-heptulose (29), and D-g/ycero-a-D-g/ co-2-octulose (30),... 

Ketone synthesis, 72 173-174 73 667-668 Ketone-water azeotropes, 74 563 Ketone-water mixture properties, 74 568-569t Ketose, 4 696... 

All carbohydrates can exist in either of these two forms and the prefix of D or L only refers to the configuration around the highest numbered asymmetric carbon atom. Enantiomers have the same name (e.g. D-glucose and L-glucose) and are chemically similar compounds but have different optical properties. The majority of naturally occurring monosaccharides, whether they be aldoses or ketoses, are of the D configuration. 

Unfortunately, this procedure did not permit preparation of the corresponding ketonucleosides postulated as key intermediates in many biosynthetic routes. In fact, -elimination reactions precluded the coupling of ketoses with nitrogen heterocycles. These difficulties may explain the scarcity of information concerning their biological properties or functions. 

Very many properties of oligosaccharides are determined by a unique group — the acetal fragment at Cl in aldoses or C2 in ketoses. This fragment shows unique effects the anomeric effect and exo anomeric effect which represent two important facts ruling the conformations of oligosaccharides [9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. 

Most compounds of this class are derived from 2-ketoses and exhibit properties related to the anhydroaldoses previously discussed. Among them,... 

Ordinary glucose yields in addition to fructose and mannose a new ketose, pseudofructose, which has not yet been obtained in a pure state, and another substance, which is reducing but not fermentable, having the properties of a ketose and for whieh we suggest the name glutose. This substance can easily be prepared by the action of lead hydroxide on commercial fructose.. .. It can also be prepared by using invert sugar and lime. 

By virtue of the aldehyde at the reducing end, sugars are susceptible to deprotonation and isomerization. The rearrangement from an aldose sugar to a ketose sugar, shown in Scheme 6.80, is a direct result of this property [123]. Based on the initial enolization step, this chemistry is easily applied to the direct C-2 epimerization of 2-deoxy-2-aminosugars. Scheme 6.81 illustrates this reaction in the conversion of A-acetyl-D-glucosamine to A-acetyl-D-mannosamine [124,125]. 

The aldehyde group of aldoses can either be oxidized or reduced and ketoses can be reduced. The best laboratory reagent for reduction is sodium boro-hydride which acts rapidly in neutral aqueous solutions (see Eq. 4-2). Since both NaB H4 and NaB H4 are available, radioactive or heavy isotope labels can be introduced in this way. The aldehyde groups can be oxidized by a variety of agents to the corresponding aldonic acids, a fact that accounts for the reducing properties of these sugars. In alkaline solution aldoses reduce Cu + ions to cuprous oxide (Eq. 4-3), silver ions... 

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