A novel protocol for the Castagnoli–Cushman reaction of dicarboxylic acids and imines comprises the use of 1,1′-carbonyldiimidazole as the cyclodehydrating agent to in situ produce the intermediate anhydrides. In contrast to previously developed procedure involving the use of acetic anhydride, the current protocol allows one to utilize substrates prone to acylation or acid-promoted transformations, which significantly broadens the reaction scope of lactams to be obtained.