Knorr Cyclizations and Distonic Superelectrophiles

The acid-catalyzed Knorr cyclization has been studied by experimental and theoretical methods. The results of these studies indicate that beta-ketoamides such as acetoacetanilide undergo diprotonation at the two carbonyl oxygen atoms to form distonic superelectrophiles. Direct observation of a dicationic superelectrophile was achieved by low-temperature 1H, 15N, and 13C NMR from FSO3H-SbF5-SO2ClF solutions. In synthetic studies, the Brønsted superacid CF3SO3H is found to be an effective acid catalyst for the Knorr cyclization.