6.8.6 Knoevenagel Reaction

In Knoevenagel reaction, the carbanion generated from an ester may attack an aldehyde or a ketone to undergo a condensation reaction to form a α,β-unsaturated acid or ester (Scheme 15). Usually a secondary amine like piperidine used as base. Knoevenagel reactions may be extended to other compounds containing an active methylene groups such as nitroalkanes. Some examples are given below.

Scheme 15

When the active methylene group is sandwiched between carboxylic acid groups, it often undergoes decarboxylation as shown in Scheme 16.

Scheme 16

6.8.7 Stobbe Condensation

It involves the condensation dialkyl succinate with carbonyl compounds (Scheme 17). The reaction is usually carried out using bases such as sodium ethoxide and sodium hydride.

Scheme 17

6.8.8 Claisen Condensation

When the carbanion generated from an ester reacts with another molecule of ester, the reaction is known as Claisen ester condensation (Scheme 18) The reaction is usually carried out using sodium ethoxide as a base in ethanol. This reaction is reversible in nature and can be induced irreversible by using extremely strong base like triphenylmethanide. Furthermore. crossed Claisen condensations between two esters are useful only when one of the esters does not possess an α-H.

Scheme 18

An intramolecular version of this reaction known as Dieckmann cyclization is also possible where medium sized rings are formed in this reaction.