In organic chemistry, an
active ester is an
ester functional group that is highly susceptible toward nucleophilic attack. Activation can be imparted by modifications of the acyl or the alkoxy components of a normal ester, say
ethyl acetate. Typical modifications call for electronegative substituents. Active esters are employed in both synthetic and biological chemistry.
Reactivity
Active esters are mainly used as
acylation. They undergo the same reactions as their unactivated analogues but do so more rapidly. They are prone to
hydrolysis, for example. Of great interest is the enhanced reactivity of active esters toward
to give
.
Examples
Biochemistry
Active esters are prominent in biochemistry. Glutamine synthetase is an enzyme that forms an active ester from the terminal carboxylate of glutamic acid. This activation, imparted by phosphorylation, facilitates the conversion of the carboxylate to an amide called
glutamine.
are prominent active esters, as illustrated by the esters of coenzyme A.
and terpenoids are generated from active esters. Some biosynthetically significant active esters include isopentenyl pyrophosphate, dimethylallyl pyrophosphate, and geranyl pyrophosphate.
Synthetic chemistry
Hydroxybenzotriazole is used in peptide synthesis by forming an active ester from acyl isoureas.
Classically, activated esters are derivatives of and pentafluorophenol. These esters react with nucleophiles much more rapdily than the related aryl and especially alkyl esters.
Active esters of acrylic acid are precursors to polymers with reactive side chains.
The concept of active esters extends to esters of phosphoric and . One such case is dimethylsulfate, a strong methylating agent.
