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A Knoevenagel condensation is a nucleophilic addition of an active hydrogen compound to a carbonyl group followed by a dehydration reaction in which a molecule of water is eliminated (hence condensation). The product is often an alpha, beta conjugated enone.
where Z is an electron withdrawing functional group. Z must be powerful enough to facilitate hydrogen abstraction to the enolate ion even with a mild base. Using a strong base in this reaction would induce self-condensation of the aldehyde or ketone.
With malonic compounds the reaction product can lose a molecule of carbon dioxide in a subsequent step. In the so-called Doebner modification  the base required is pyridine. For example the reaction product of acrolein and malonic acid in pyridine is trans-2,4-Pentadienoic acid with one carboxylic acid group and not two 
A Knoevenagel condensation is demonstrated in the reaction of 2-methoxybenzaldehyde 1 with the barbituric acid 2 in ethanol using piperidine as a base . The resulting enone 3 is a charge transfer complex molecule.
The Knoevelagel condensation is a key step in the commercial production of the antimalarial drug lumefantrine (a component of Coartem) :
The initial reaction product is a 50:50 mixture of E and Z isomers but because both isomers equilibrate rapidly around their common hydroxyl precursor, the more stable Z-isomer can eventually be obtained.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Knoevenagel_condensation". A list of authors is available in Wikipedia.|