The reaction between the phenolic Mannich bases 1,3-bis[2′-hydroxybenzyl]imidazolidines and the Mannich intermediary macrocyclic aminal 1,3,6,8-tetraazatricyclo[4.4.1.13,8]dodecane (TATD) was studied under solvent-free conditions. It was established that the major product is a heterocalixarene-type Mannich base, and three linear benzylimidazolidine oligomers were identified as minor products. The formation