Abstract The reaction of 4,5‐diamino‐1,6‐dihydropyrimidin‐6‐ones 1 with one equivalent of the chalcones 2 leads in an acidic medium to the formation of the 2,4‐diaryl‐2,3,6,7‐tetrahydro‐1 H ‐pyrimido[4,5‐ b ][1,4]diazepin‐6‐ones 3a‐m . The structure elucidation of the products is based on detailed nmr investigations including selective 13 C[ 1 H] decoupling experiments.