have two iffere gomcecolngersasencondenatonareautn eoac 8,y一 好 es undergo mixed claisen reactions. sare more aci nsations in syntheti tePpgaoaytepotnnes,p-ketogaehydsor 1.aasencndensatonsalnasfoml,3dlcarbon r the synth of 2-benzoylycohexanone onene8bea8asb传omweghgmgPeaiy aas8eo8 n this case,there are two possibilities: 22 Claisen condensations can have two different esters as reactants. Mixed Claisen condensations start with two different esters, are typically unselective and furnish product mixtures unless one of the esters has no α-hydrogens. Intramolecular Claisen condensations result in cyclic compounds. The Dieckmann condensation is an intramolecular version of the Claisen condensation which produces cyclic 3-keto esters. The method works best for the formation of five- and six-membered rings. Ketones undergo mixed Claisen reactions. Ketones are more acidic than esters and as a result are deprotonated and react before the ester has a chance to undergo self-condensation. The products may be β-diketones, β-ketoaldehydes or other β-dicarbonyl compounds. Retrosynthetic analysis clarifies the synthetic utility of the Claisen condensation. Three facts govern the use of Claisen condensations in synthetic design: 1. Claisen condensations always form 1,3-dicarbonyl compounds. 2. One of the reaction partners must be an ester. 3. The other reaction partner must contain at least two acidic hydrogens on an α-carbon. In addition, if a mixed condensation is being considered, one reaction partner should be incapable of self-reaction (no α- hydrogens). Consider the synthesis of 2-benzoylcyclohexanone: A Claisen condensation always forms a new bond connecting one of the carbonyl groups of the 1,3-dicarbonyl moiety to the carbon atom between them. Prior to bond formation, one of the carbonyls will be in the form of an ester. In this case, there are two possibilities: Both possibilities are feasible, however, the second possibility is preferred because it starts from two smaller fragments