Aldol Reactions and Aldol-like Reactions
Intramolecular Aldol Reaction
Intramolecular aldol reaction is an important pathway for the synthesis of cyclic compounds. However, only five- and six-membered rings may be easily achieved by intramolecular aldol reactions. Furthermore, the two carbonyl components of the starting product can principally react as electrophilic carbonyl component, as well as nucleophilic enolate. In addition, each carbonyl component in ketones can form two different enolates. Thus, in principle, a variety of products is possible. However, in the illustrated case below, the starting product is symmetric, that is, the two carbonyl compounds are indistinguishable. In addition, if the enolate with the terminal double bond, which is the less stable enolate, attacks the carbonyl component, an energetically unfavorable seven-membered ring would be formed. Thus, only one product is typically obtainable - the five-membered ring.
If diesters are applied in the intramolecular aldol reaction, the reaction is called Dieckmann condensation (after Walter Dieckmann). Dieckmann condensations yield cyclic β-keto esters. Saponification of the β-keto ester yields a β-keto carboxylic acid, which easily decarboxylates. Thus, by Dieckmann condensation, diesters are converted into cyclic ketones.