Aldol Qaisen condensation

Qaisen condensations, like the aldol reaction, require a base. Aqueous bases, such as NaOH, however, carmot be used in Claisen condensations because they would bring about hydrolysis of the ester (saponification. Section 14.3C) instead. Rather, the bases most commonly used in Claisen condensations are nonaqueous bases, such as sodium ethoxide in ethanol and sodium methoxide in methanol. Furthermore, to prevent transesterification (Section 14.4C), the alkyl group (—R) of the base should match the R group in the alkoxyl portion (— OR) of the ester. 

Apart from the thoroughly studied aqueous Diels-Alder reaction, a limited number of other transformations have been reported to benefit considerably from the use of water. These include the aldol condensation , the benzoin condensation , the Baylis-Hillman reaction (tertiary-amine catalysed coupling of aldehydes with acrylic acid derivatives) and pericyclic reactions like the 1,3-dipolar cycloaddition and the Qaisen rearrangement (see below). These reactions have one thing in common a negative volume of activation. This observation has tempted many authors to propose hydrophobic effects as primary cause of ftie observed rate enhancements. 

Enolate ion formation allows coenzyme A-bound acyl groups to serve as nucleophiles and to react at electrophilic centres. This permits thioesters to participate in the formation or degradation of carbon—carbon linkages by mechanisms analogous to the aldol condensation or more specifically the Qaisen type ester condensation. There are few available mechanisms for carbon—carbon bond formation or deavage which can be employed under biological reaction conditions, and pathways which depend on coenzyme A thioesters for this purpose are widespread. 

The aldol reaction (aldol condensation) is one of the fundamental reactions of organic chemistry because it leads to the formation of a new carbon-carbon bond (see Experiment [20] for a very similar example of the Qaisen-Schmidt type of aldol reaction). In this version, the condensation of 4-nitrobenzaldehyde (an aldehyde without an a-hydrogen atom) with acetophenone (a ketone) gives frans-4-nitrochalcone.The aldol condensation of the unsubstituted aromatic aldehyde, benzaldehyde with acetophenone, yields frans-l,3-diphenyl-2-propenone (PhCH CHCOPh), which has the common name, chalcone.Thus, the substituted derivatives of this system are known collectively as chalcones. 

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