Add time:08/25/2019         Source:sciencedirect.com

In 1887, Claisen and Lowman1 reported that the condensation of 2 mol of an ester, such as ethyl acetate, in the presence of base gave the β-keto ester, ethyl acetoacetate (ethyl 3-oxobutanoate; equation 1). The intramolecular equivalent was recognized by Dieckmann in 1894.2 He found that heating an adipic acid ester with sodium and a trace of alcohol led to cyclization, with the formation of a cyclopentanone (equation 2). The reaction was, at an early stage, extended to the acylation of ketones. Claisen himself reported the base-catalyzed reaction of acetophenone and ethyl benzoate to give dibenzoylmethane in 1887.3 This reaction, too, has an intramolecular parallel. The acylation of ketones with esters and other acid derivatives is sometimes called a Claisen condensation, although this usage is criticized by some writers and avoided by others. A widely used example of ketone acylation is the synthesis of α-formyl (hydroxymethylene) ketones (equation 3). Intramolecular variants of this reaction include the classical synthesis of dimedone

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