Definition
The Kiliani-Fischer synthesis is a method by which we can lengthen the carbon chain on a sugar molecule by adding an additional carbon atom to that sugar.
Explanation
Larger sugars can be made from smaller ones and vice versa, by chain lengthening and chain shortening. The Kiliani-Fischer synthesis lengthens a carbohydrate chain by adding one carbon to the aldehyde end of an aldose, thus forming a new stereogenic center at C2 of the product. The product consists of epimers that differ only in their configuration about the new stereogenic center.
This process occurs via a nucleophilic acyl addition. An aldose is first treated with HCN to give the corresponding cyanohydrin. Because this transformation forms a new stereocenter, two diastereomers appear.
Partial reduction of the nitrile group by catalytic hydrogenation in an aqueous acid gives the aldehyde groups of the chain-extended sugars. In this hydrogenation, a modified palladium catalyst is necessary to prevent the hydrogenation from proceeding all the way to the amine.
