Wohl degradation

Just as the Kiliani-Fischer synthesis lengthens an aldose chain by one carbon, the Wohl degradation shortens an aldose chain by one carbon. The Wohl degradation is almost the exact opposite of the Kiliani-Fischer sequence. That is, the aldose aldehyde carbonyl group is first converted into a nitrile, and the resulting cyanohydrin loses HCN under basic conditions—the reverse of a nucleophilic addition reaction. 

Conversion of the aldehyde into a nitrile is accomplished by treatment of an aldose with hydroxvlamine to give an oxime (Section 19.8), followed by dehydration of the oxJme with acetic anhydride. The Wohl degradation does not give particularly high yields of chain-shortened aldoses, but the reaction is general for all aldopentoses and aldohexoses. For example, D-galactose is converted by Wohl degradation into n-lyxose. 

Much of the chemistry of monosaccharides is the familiar chemistry of alcohols and aldehydes/ketones. Thus, the hydroxyl groups of carbohydrates form esters and ethers. The carbonyl group of a monosaccharide can be reduced with NaBH4 to form an alditol, oxidized with aqueous Br2 to form an aldonic acid, oxidized with HNO3 to form an aldaric acid, oxidized enzymatically to form a uronic acid, or treated with an alcohol in the presence of acid to form a glycoside. Monosaccharides can also be chain-lengthened by the multistep Kiliani-Fischer synthesis and can be chain-shortened by the Wohl degradation. 

Wohl degradation (Section 25.6) A method for shortening the chain of an aldose sugar. 

III. Degradation of the Nitriles op the Aldonic Acids 1. The Wohl Degradation... 

The Wohl degradation can be considered to consist of two steps (a) the elimination of the acetyl and nitrile groups by the action of ammonia or ammoniacal silver oxide, and (b) the formation of the diacetamide compound. The formation of the latter was explained by Wohl on the basis of the intermediate formation of oIde%do-D-arabinose... 

When a Wohl degradation is carried out in the laboratory, hydrolysis of the acyl and nitrile groups begins immediately upon addition of ammonia. The rate of hydrolysis of each of the groups present in the molecule is probably different and at the present time we have no knowledge of these rates of hydrolysis. As Hockett and Chandler have pointed out, the diacetamides are obviously formed as a result of a delicate balance among the rates of several concurrent reactions. ... 

An indication of the sensitivity of the reaction to changes in the structure of the acylated nitrile is furnished by the work of Neuberg and Wolff, who obtained hydrocyanic acid from the Wohl degradation of pentaacetyl-D-glucosaminonitrile, but could not obtain a diacetamide compound. 

Problem 22.10 Outline the steps in the Wohl degradation, which employs a dehydration of an aldose oxime and is thus a reversal of the Kiliani-Fischer step-up method. 

WOHL DEGRADATION. Method for the conversion of an aldose into an aldose with one less carbon atom by the reversal of the cyanohydrin synthesis. In the Wohl method, the nitrile group is eliminated by treatment with ammoniacal silver oxide. 

As applied to 4, 5, 6, and 7, the Wohl degradation forms enantiomers of glyceraldehyde ... 

Selective debenzoylation has been shown to occur by Restelli de Labriola and Deulofeu160 in the cleavage of penta-O-benzoylhexonic acid nitriles (LXXXIII) to form O-benzoyl-pentose dibenzamides (LXXXIV) with ammonia. Since tetra-O-benzoyl-L-rhamnonic acid nitrile (LXXXV) yields 5-deoxy-L-arabinose dibenzamide (LXXXVI), the benzoyl group in the Wohl degradation products is assumed to be in the 5 positions of the resulting pentoses. 

Which two aldoses yield D-lyxose after Wohl degradation ... 

Reacts with aldoses to yield oximes as the first step in the Wohl degradation of aldoses (Section 25.6). 

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