Enolate Anion Reactions
A carbonyl compound with an alpha-hydrogen readily undergoes abstraction by a nucleophile to produce the enolate anion/carbanion. As is seen below, the intermediate exists as two resonance forms, the carbanion and the enolate form. The reactions we discussed involve the carbanion form as the reactive form, considering the reactions involve this alpha-carbon attacking the carbonyl of another molecule.
The enolate anion, being a good nucleophile, can then attack an electrophilic reagent. In the following reactions, the electrophile is the carbonyl carbon of a similar compound.
All of these reactions are seen to produce, in their first step, very similar species, namely beta-alkoxy carbonyl compounds. The major difference in the condensation reactions arises from the fate of the alkoxide. In the case of the "Aldol" type reactions, the beta-hydroxy carbonyl is produced.
The two "Claisen" condensations and the Dieckmann condensation form the beta-keto ester. The carbonyl is reformed as the leaving group is identical to the initial attacking nucleophile, the alkoxide anion..