Lobry-de-Bruyn-Alberda-van-Ekenstein rearrangement

The Lobry-de-Bruyn-Alberda-van-Ekenstein rearrangement , usually also known as the Lobry-de-Bruyn-van-Ekenstein rearrangement (a special case of the ketol-enediol tautomerism ), is the acid or base-catalyzed one in carbohydrate chemistry aldose - ketose - isomerization . It takes place in a rearrangement , for which the equilibrium between glyceraldehyde and dihydroxyacetone is exemplary, but can also be transferred to other aldoses and ketoses.

The reaction, which is important 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, with the remainder R - depending on the respective carbohydrate - being variable:

The position of the reaction equilibrium with regard to the reactant-product ratio depends on the reaction conditions ( concentration , solvent , pH , temperature, etc.). When equilibrium is established, there is usually an aldose-ketose mixture. In the specific case of the glyceraldehyde-dihydroxyacetone rearrangement (R = CH ₂ OH), this mixture is called glycerose .

Stereochemical aspect

Since the carbon atom on which the initial deprotonation takes place is a stereocenter (for R ≠ H in the above scheme), epimerization occurs due to the equilibrium character of all mechanistic steps . If, for example, D - glucose (an aldose ) is used as a starting material, the rearrangement results in D - fructose (a ketose ). The stereochemical configuration at the carbon atom in question is initially lost because this position is no longer a stereocenter in the enediol. In the reverse reaction, which also takes place permanently as part of the establishment of equilibrium , the (planar) C = C double bond of the enediol can optionally be protonated from both sides with the formation of the stereocenter . This leads to the fact that two products arise with a different configuration of the re-established stereocenter. In the example mentioned, these would be D- glucose and D - mannose . The reaction equilibrium thus provides a mixture of D -glucose, D- fructose and D- mannose:

literature

• SJ Angyal: The Lobry de Bruyn – Alberda van Ekenstein transformation and related reactions. In: Arnold E. Stütz (Ed.): Glycoscience: epimerisation, isomerisation and rearrangement reactions of carbohydrates. (= Topics in Current Chemistry. Vol. 215) Springer, Berlin 2001, pp. 1-14, doi : 10.1007 / 3-540-44422-X_1 .