Mechanism for annelation ([2+4] cyclocondensation) of Schiff bases by β-dicarbonyl and β,β′-tricarbonyl compounds in amphiprotic media (review)
The mechanism was established for annelation ([2+4] cyclocondensation) of Schiff bases or azomethines by β-dicarbonyl and β,β′-tricarbonyl compounds in amphiprotic media. The condensation was found to be a self-catalyzed pericyclic process achieved through six-membered transition states. The key reaction intermediates are dipoles of quaternized azomethine ions and enol-anions of the β-dicarbonyl and β,β′-tricarbonyl compounds, which display 1,4-dipolarophilicity.