Claisen

Claisen refers to reactions named after the German chemist Ludwig Claisen. In organic synthesis, two well-known reactions bear his name: the Claisen condensation and the Claisen rearrangement.

Claisen condensation is a base-catalyzed reaction in which esters containing an active α-hydrogen undergo self- or cross-condensation to give β-keto esters after expulsion of an alkoxide. The mechanism proceeds via formation of an enolate from one ester, nucleophilic acyl substitution on a second ester, and elimination of an alkoxide. The process often serves to build carbon skeletons and to prepare acetoacetic-type derivatives; the reaction can also occur intramolecularly in a Dieckmann condensation, forming cyclic β-keto esters.

Claisen rearrangement is a thermally induced [3,3]-sigmatropic rearrangement of allyl vinyl ethers (and related substrates) into γ,δ-unsaturated carbonyl compounds. It proceeds through a chair-like suprafacial transition state and typically requires heating, proceeding without a catalyst. The rearrangement is valued for reorganizing carbon frameworks and installing new C–C bonds in natural product synthesis and complex molecular construction.

Both reactions illustrate Claisen’s impact on carbonyl chemistry and pericyclic processes. They remain foundational tools in organic synthesis, offering routes to acyl and carbon skeleton transformations that are widely applied in research and industry.