2-Ketose

D-fructose, C HijOo. Crystallizes in large needles m.p. 102-104 C. The most eommon ketose sugar. Combined with glucose it occurs as sucrose and rafftnose mixed with glucose it is present in fruit juices, honey and other products inulin and levan are built of fructose residues only. In natural products it is always in the furanose form, but it crystallizes in the pyranose form. It is very soluble in... 

Carbohydrates may be divided into monosaccharides, disaccharides and polysaccharides. The monosaccharides under certain conditions react as polyhydroxy-aldehydes or polyhydroxy-ketones two important representatives are glucose CjHjjO (an aldose) and fructose (laevulose) CgHuO, (a ketose). Upon hydrolysis di- and polysaccharides 3deld ultimately monosaccharides. Common disaccharides are sucrose, lactose and maltose (all of molecular formula C,2H2. 0,), whilst starch, dextrin and cellulose, (CjHjoOj), in which n > 4, are typical polysaccharides. 

Methylphenylosazones. cw-Methylphenylhydrazine does not form osazones with aldoses presumably because the base or more probably the methylphenylhydrazonium ion [CgH5NCH3NH3]+ will oxidise a—CHjOH but not a >CHOH group it readily forms osazones with ketoses, thus providing an excellent reagent for fructose. 

The simple sugars or monosaccharides are polyhydroxy aldehydes or ketones, and belong to Solubility Group II. They are termed tetroses, pentoses, hexoses. etc. according to the number of carbon atoms in the long chain constituting the molecule, and aldoses or ketoses if they are aldehydes or ketones. Most of the monosaccharides that occur in nature are pentoses and hexoses. 

Both aldoses and ketoses reduce Fehling s solution (for details, see under 4). This fact may appear surprising when it is remembered that Fehling s solution is one of the reagents for distinguishing between aldehydes and ketones (see 4). The explanation lies in the fact that a-hydroxyketones are much more readily oxidised than simple ketones, perhaps because the hydroxy ketone allows its isomerisation, in the presence of alkali, into an aldehyde. For example, fructose, a keto-hexose, might Isomerlse thus ... 

Over 200 different monosaccharides are known They can be grouped according to the number of carbon atoms they contain and whether they are polyhydroxy aide hydes or polyhydroxy ketones Monosaccharides that are polyhydroxy aldehydes are called aldoses, those that are polyhydroxy ketones are ketoses Aldoses and ketoses are further classified according to the number of carbon atoms m the mam chain Table 25 1 lists the terms applied to monosaccharides having four to eight carbon atoms... 

Up to this point all our attention has been directed toward aldoses carbohydrates hav ing an aldehyde function in their open chain form Aldoses are more common than ketoses and their role m biological processes has been more thoroughly studied Nev ertheless a large number of ketoses are known and several of them are pivotal inter mediates m carbohydrate biosynthesis and metabolism Examples of some ketoses include d nbulose l xylulose and d fructose... 

In these three examples the carbonyl group is located at C 2 which is the most com mon location for the carbonyl function m naturally occurring ketoses... 

Ketoses like aldoses exist mainly as cyclic hemiacetals In the case of d ribulose furanose forms result from addition of the C 5 hydroxyl to the carbonyl group... 

The anomeric carbon of a furanose or pyranose form of a ketose bears both a hydroxyl group and a carbon substituent In the case of 2 ketoses this substituent is a CH2OH group As with aldoses the anomeric carbon of a cyclic hemiacetal is readily identifi able because it is bonded to two oxygens... 

Aldoses are reducing sugars because they possess an aldehyde function m then-open chain form Ketoses are also reducing sugars Under the conditions of the test ketoses equilibrate with aldoses by way of enediol intermediates and the aldoses are oxidized by the reagent... 

There is another reaction available to the enediol intermediate Proton transfer from water to C 1 converts the enediol not to an aldose but to the ketose d fructose... 

Carbohydrates are polyhydroxy aldehydes and ketones Those derived from aldehydes are classified as aldoses, those derived from ketones are ketoses... 

Ketoses are characterized by the ending ulose m their name Most nat urally occurring ketoses have their carbonyl group located at C 2 Like aldoses ketoses cyclize to hemiacetals and exist as furanose or pyranose forms... 

Enolization of an aldose or a ketose gives an enediol Enediols can revert to aldoses or ketoses with loss of stereochemical integrity at the a carbon atom... 

Ketose (Section 25 1) A carbohydrate that contains a ketone carbonyl group in its open chain form Kiliam-Fischer synthesis (Section 25 20) A synthetic method for carbohydrate chain extension The new carbon-carbon bond IS formed by converting an aldose to its cyanohydnn Reduction of the cyano group to an aldehyde function com pletes the synthesis... 

Isomerization. Both the carbonyl group and the adjacent hydroxyl group are involved in isomerization of monosaccharides. This reaction can be catalyzed by either a base or an enzyme. By this reaction, an aldose is converted into another aldose and a ketose, and a ketose is converted into two... 

The parallel synthesis of furans from a-hydroxycarbonyl compounds is frequently conducted using aldoses or ketoses as readily available sources of this functional grouping, especially as the resulting polyhydroxyalkyl side-chain can be removed easily by oxidative degradation (Schemes 67d and 67e) 56MI30300). 

A M A D 0 R I Glucosamine Rearrangement Conversion of N-glucosxfes of aldoses to N-glucosides of ketoses... 

In order to characterize them and more readily separate them from interfering accompanying substances carbonyl compounds (aldehydes, ketones) can be converted to hydrazones at the start. The reagent mainly employed is 2,4-dinitro-phenylhydrazine in acidic solution [70], This yields osazones with aldoses and ketoses. Some examples are listed in Table 15. 

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