Glucose Wohl degradation

The Wohl degradation consists of three steps, illustrated here beginning with D-glucose. 

The reagent reacts with aldohexoses to form sparingly soluble and nicely crystalline diphenylhydrazones. In the preparation of D-arabinose by Wohl degradation of D-glucose, 1,1-diphenylhydrazine is useful for isolation of additional D-arabinose from the mother liquor... 

Scheme 11.15. An example of the Wohl degradation. The anomers of D-glucose (i.e., the isomers that are epimeric at the anomeric carbon) can be considered as the aldehyde rather than the hemiacetals. Formation of the oximes (presumably both ( )- and (Z)-oxime isomers form) followed by acetylation results in elimination to the nitrile. Hydrolysis of the remaining acetates is accompanied by elimination of hydrogen cyanide and formation of an aldehyde with one carbon less than the aldehyde with which the sequence began. Thus, both mannose and glucose yield arabinose.

D-Gluconic nitrile, which can be obtained in 55 % yield by the action of hydroxylamine acetate on n-glucose in acetic acid solution, is degraded quantitatively to n-arabinose simply by the action of hot water 196), However, the usual Wohl degradation via the acetylated nitriles 197), even in the most favorable instances, gives over-all (aldose aldose) yields of only 30 to 35%. 

When D-glucose undergoes a Wohl degradation followed by a Kiliani-Fischer chain-lengthening process, a mixture of two epimeric products are obtained. Identify both epimers. 

A Wohl degradation will remove a carbon atom from D-glucose. This carbon atom is then restored with a BCUiani-Fischer synthesis, giving D-glucose and its C2 epimer, D-mannose. 

The method proposed by Zempl4n has given in some cases, as in the degradation of n-glucose to n-arabinose, higher yields than Wohl s method, but the difficulty of isolation of the new sugars has hindered its application to preparative work. It has been extended to the propiony-lated nitriles by Gim4nez and to benzoylated ones by Restelli de Labriola and Deulofeu. Usually yields lower than with the acetyl derivatives have been obtained. 

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