Darzens reaction

The oxygen anion in this aldol-like product then SN2 attacks on the formerly-nucleophilic halide-bearing position, displacing the halide to form an epoxide.

Depending on the specific structures involved, the epoxide may exist in cis and trans forms.

The specific stereochemical outcome of the reaction is affected by several aspects of the intermediate steps in the sequence.

The initial stereochemistry of the reaction sequence is established in the step where the carbanion attacks the carbonyl.

[8] Glycidic esters can also be obtained via nucleophilic epoxidation of an α,β-unsaturated ester, but that approach requires synthesis of the alkene substrate first whereas the Darzens condensation allows formation of the carbon–carbon connectivity and epoxide ring in a single reaction.