Aldoses groups

Wolfrom, Sowden, and Lassettre measured the rate of hydrolysis of methylated cellulose in fuming hydrochloric acid at 0°C by continuous mere apt alation of the aldose group formed for each bond ruptured. 

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. 

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... 

Aldoses incorporate two functional groups C=0 and OH which are capable of react mg with each other We saw m Section 17 8 that nucleophilic addition of an alcohol function to a carbonyl group gives a hemiacetal When the hydroxyl and carbonyl groups are part of the same molecule a cyclic hemiacetal results as illustrated m Figure 25 3 Cyclic hemiacetal formation is most common when the ring that results is five or SIX membered Five membered cyclic hemiacetals of carbohydrates are called furanose forms SIX membered ones are called pyranose forms The nng carbon that is derived... 

Generating Haworth formulas to show stereochemistry m furanose forms of higher aldoses is slightly more complicated and requires an additional operation Furanose forms of D ribose are frequently encountered building blocks m biologically important organic molecules They result from hemiacetal formation between the aldehyde group and the C 4 hydroxyl... 

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... 

The reaction of aldoses with nitric acid leads to the formation of aldaric acids by oxidation of both the aldehyde and the terminal primary alcohol function to carboxylic acid groups Aldaric acids are also known as saccharic acids and are named by substi tutmg aric acid for the ose ending of the corresponding carbohydrate... 

The reaction is used for the chain extension of aldoses in the synthesis of new or unusual sugars In this case the starting material l arabinose is an abundant natural product and possesses the correct configurations at its three chirality centers for elaboration to the relatively rare l enantiomers of glucose and mannose After cyanohydrin formation the cyano groups are converted to aldehyde functions by hydrogenation m aqueous solution Under these conditions —C=N is reduced to —CH=NH and hydrolyzes rapidly to —CH=0 Use of a poisoned palladium on barium sulfate catalyst prevents further reduction to the alditols... 

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... 

The Kiliani-Fischer synthesis pro ceeds by nucleophilic addition of HCN to an aldose followed by con version of the cyano group to an al dehyde A mixture of stereoisomers results the two aldoses are epi meric at C 2 Section 25 20 de scribes the modern version of the Kiliani-Fischer synthesis The example at the right illus trates the classical version... 

Aldonic acid (Section 25 19) Carboxylic acid obtained by oxi dation of the aldehyde function of an aldose Aldose (Section 25 1) Carbohydrate that contains an aldehyde carbonyl group in its open chain form Alicyclic (Section 2 15) Term describing an a/iphatic cyclic structural unit... 

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... 

Oxidation to Sugar Acids and Lactones. When the aldehyde group of an aldose is oxidized, the resulting compound is an aldonic acid (salt form = aldonate) (11)4. Some aldonic acids are products of carbohydrate metaboHsm. 

Oxidation of the aldehyde group of an aldose to form a carboxyUc acid or carboxyUc acid anion is often used analytically to determine the amount of reducing sugar. The Benedict and Fehling methods measure the amount of reducing sugar present in a fluid. In these reactions, the oxidant, Cu ", is reduced to Cu". Cu" precipitates as CU2O, which can be measured in a variety of ways. In the ToUens test, Ag" is reduced to Ag. ... 

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). 

Free aldehyde and keto groups [1 — 3] e.g. in aldoses and ketoses... 

Dinitrophenylhydrazine reacts with carbonyl groups with the elimination of water to yield hydrazones (I) and with aldoses or ketoses to yield colored osazones (II). 

Note The reagent can be employed on sihca gel, kieselguhr. Si 50 000 and cellulose layers. At room temperature sugars and sugar derivatives react at different rates depending on the functional groups present [1], e.g. ketoses react more rapidly than aldoses. It is possible to differentiate substance types on this basis [1, 3]. 

Reduction (Section 25.18) The carbonyl group of aldoses and ketoses is reduced by sodium borohydride or by catalytic hydrogenation. The products are called alditols. 

Aldose (Section 25.1) Carbohydrate that contains an aldehyde carbonyl group in its open-chain form. 

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