The Kiliani–Fischer synthesis lengthens the carbon chain of carbohydrates by adding one carbon to the aldehyde group of an aldose.

For example, using this method, D-arabinose can be converted into a mixture of D-glucose and D-mannose:

Notice that two epimers are formed as a product since they only differ at the new stereogenic center at C2.

So, let’s now understand the mechanism behind the formation of these epimers. In the first part of the Kiliani–Fischer synthesis, we have an addition reaction of the cyanide to the aldehyde, forming a cyanohydrin. This is when a new carbon is added to the carbohydrate chain:

Remember, we have seen this quite often in the synthesis of carboxylic acids and their derivatives. The nitrile group is a good electrophile that can further be reacted with a variety of nucleophiles such as Grignard or Organocuprate nucleophiles.

Now, in this particular synthesis, the nitrile group is reduced to an imine using H₂ and a Pd-BaSO₄ catalyst, and in the last step, we only need to convert the imine to an aldehyde by acidic hydrolysis:

Let’s put these steps into a little summary of the Kiliani–Fischer synthesis, which increases the carbohydrate chain of an aldose by one carbon atom:

How to Shorten Carbohydrates

It is possible to perform the reverse reaction of the Kiliani-Fischer synthesis; that is, we can also shorten the length of the carbon chain of carbohydrates. There are two main approaches for this that you may need to know: 1) The Ruff degradation and 2) The Wohl degradation.

In the Ruff degradation, the carbon chain of the carbohydrate is shortened by oxidation of the aldehyde group to a carboxylic acid, followed by decarboxylation, which removes one carbon atom as CO₂: