Kiliani Fischer Synthesis
Kiliani-Fischer Synthesis 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.
Kiliani-Fischer Synthesis Explained:
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.