Lobry de Bruyn–van Ekenstein transformation

The Lobry de Bruyn - Alberda -van- Ekenstein rearrangement, usually referred to as Lobry -de Bruyn -van- Ekenstein rearrangement ( a special case of keto -enol tautomerism ), is in carbohydrate chemistry, the base-catalyzed aldose - ketose isomerization. It takes place in a rearrangement of the balance between glyceraldehyde and dihydroxyacetone is exemplary, but can also be transferred to other aldoses and ketoses.

The reaction, which is important, among other things for the industrial synthesis of ketoses, was first described in 1885 by Cornelis Adriaan Lobry van Troostenburg de Bruyn and Willem Alberda van Ekenstein.

Mechanism

The following scheme illustrates the reaction mechanism, where the radical R is dependent on the particular carbohydrate:

The position of the reaction equilibrium, starting material with respect to the product ratio is dependent on the reaction conditions (concentration, solvent, pH, temperature, etc.). In equilibrium id R there is an aldose - ketose mixture. In the specific case of glyceraldehyde, dihydroxyacetone rearrangement ( R = CH2OH ) called this mixture glycerose.

Stereochemical aspect

Since the carbon atom to which the initial deprotonation occurs, a stereocenter is ( R ≠ H in the above scheme ), it is a result of the equilibrium character of all mechanistic steps to epimerization. So if, for example, D- glucose ( an aldose ) is used as starting material, is formed by the rearrangement of D- fructose ( a ketose ). The stereochemical configuration at the carbon atom in question is initially lost, since this position is no longer a stereocenter already in the enediol. When in the context of equilibrium also permanently held back reaction, the (planar ) carbon-carbon double bond of the enediol can be protonated by either party formation of the stereocenter optional. This means that two products are formed with different configuration of the revived stereocenter. In the example, this would be the D-glucose and D- mannose. The reaction equilibrium is thus provides a mixture of D -glucose, D -fructose and D-mannose: