Abstract The reaction of 1‐aryl‐3‐(dimethylamino)‐1‐propanones 1 with one equivalent of 4,5‐diamino‐1 H ‐pyrimidin‐6‐ones 2 , in acidic medium, leads to the formation of 4‐aryl‐2,3,6,7‐tetrahydro‐1 H ‐pyrimido[4,5‐ b ]‐[1,4]diazepin‐6‐ones 3 . The structure elucidation of the products is based on detail nmr analysis of experiments such as 13 C, 1 H and DEPT including selective 13 C{ 1 H} decoupling experiments.