15.27 : 酯到 β-酮酯：克莱森缩合机理

Recall regular Claisen condensation as a method to obtain β-ketoesters from identical ester molecules containing two α protons in the presence of an alkoxide base.

The reaction begins with the base abstracting the acidic α hydrogen from the ester to produce a nucleophilic enolate ion stabilized by resonance.

The nucleophile attacks the carbonyl carbon of another ester molecule to produce a tetrahedral alkoxide intermediate.

The intermediate undergoes expulsion of the alkoxide group to give an acyl-substituted ester and an alkoxide by-product.

The by-product abstracts the second α hydrogen from the β-dicarbonyl compound to form a doubly-stabilized enolate, which is the driving force of the Claisen condensation.

This step emphasizes the essential requirement of two α protons in the starting esters. 

Finally, the enolate is protonated by an acid to give the desired product, the β-ketoester.