Wohl Degradation

The Wohl degradation cleaves the C1–C2 bond of an aldose chain, shortening it by one carbon. Like in the Kiliani–Fischer synthesis, the transformation relies on first converting the aldehyde group to a cyanohydrin.

Notice, however, the key difference: the (red) carbon atom of the cyanohydrin group, which is cleaved in the next step, comes from the aldehyde rather than the -CN ion, as in the Kiliani–Fischer synthesis. Recall that the Kiliani–Fischer synthesis was used to increase the carbon chain by a nucleophilic addition of a cyanide:

So, these two reactions are reverse processes if we compare the net transformations, even though the mechanisms are different.

So, let’s discuss the mechanism of Wolf degradation in more detail. The aldehyde is converted into a cyanohydrin via oxime formation, as we have seen in the reaction of aldehydes.

Dehydration of the oxime by acetic anhydride produces a cyanohydrin, which is kicked out by a strong base, forming the new carbohydrate with one less carbon atom:

Another strategy for shortening the length of the carbohydrate chain is the Ruff degradation, which uses Fe²⁺ ion and peroxide to achieve an oxidation followed by decarboxylation: