15.39: Cyclohexenones via Michael Addition and Aldol Condensation: The Robinson Annulation
Robinson annulation is a base-catalyzed reaction for the synthesis of 2-cyclohexenone derivatives from 1,3-dicarbonyl donors (such as cyclic diketones, β-ketoesters, or β-diketones) and α,β-unsaturated carbonyl acceptors. Named after Sir Robert Robinson, who discovered it, this reaction yields a six-membered ring with three new C–C bonds (two σ bonds and one π bond).
The ring-forming reaction occurs in two stages: Michael addition and the subsequent intramolecular aldol condensation. The reaction commences with the deprotonation of the acidic hydrogen in the donor, generating an enolate ion.
The α,β-unsaturated compound undergoes nucleophilic attack by the enolate via Michael addition, forming an anionic species that gives the Michael adduct upon protonation.
Consequently, the base abstracts an appropriate α proton from the adduct, forming an enolate ion. It undergoes an intramolecular aldol condensation via attack at the carbonyl carbon, forming a cyclic alkoxy intermediate. Finally, protonation of the alkoxy ion and subsequent dehydration gives the annulated product.