The esters have acidic hydrogens with pKa=25 in their position, which can be subtracted using bases. The conjugate base is an enolate ester, a highly nucleophilic species that attacks a varied number of electrophiles.

 
enolatos-ester
 
Reaction of ester enolates with primary haloalkanes
The ester enolate acts as a nucleophile, reacting with primary haloalkanes via the S N 2 mechanism. Secondary and tertiary haloalkanes give mostly eliminations, due to the significant basicity of the enolate.
 
enolatos-ester
 
Reaction of ester enolates with oxacyclopropanes (epoxides)
The ester enolates attack and open oxacyclopropane. Oxacyclopropane contributes to the a-carbon of the enolate a chain with two carbons, which contains an -OH on the second carbon.
 
enolatos-ester
 
Reaction of ester enolates with aldehydes and ketones
The final product under suitable conditions can cyclize to form a lactone.
 
enolatos-ester