SolutionsAcylation reactions: Claisen, Dieckmann and others

With ethoxide, all the steps of the condensation are reversible and unfavourable; the reaction proceeds only if the product is deprotonated. Here the β-keto ester that would form, ethyl 2,2,4-trimethyl-3-oxopentanoate, has no hydrogens between the two carbonyls and cannot form its enolate: the equilibrium reverts. Triphenylmethylsodium is a much stronger base that forms the ester enolate irreversibly and completely, which reacts with another ester molecule; the product is isolated on acidification.

Pimelate: ethyl 2-oxocyclohexanecarboxylate (six-membered ring). 3-Methyladipate: two five-membered β-keto esters can form, depending on which ester provides the enolate. Since the reaction is under thermodynamic control, the product able to form a stable enolate between the two carbonyls predominates: ethyl 4-methyl-2-oxocyclopentanecarboxylate. The alternative, the 5-methyl-2-oxo isomer, also has an acidic hydrogen, but its enolate is more strained by the adjacent methyl, and in practice mixtures are obtained in which the first one predominates.

Acetone (which provides the enolate) and ethyl benzoate (non-enolizable, electrophile only), with a strong base such as NaH or excess NaOEt. Benzoate cannot self-condense, and deprotonation of the 1,3-diketone formed (pK_a ≈ 9) drives the equilibrium. Acetophenone and ethyl acetate can also be used, but then the acetate may self-condense.