Abstract The reaction of 4,5‐diamino‐1,6‐dihydropyrimidin‐6‐ones 1 with two equivalents of the chalcones 2 leads in an acidic medium to the formation of the 2,3,6,7‐tetrahydro‐1 H ‐pyrimido[4,5‐ b ][1,4]diazepin‐6‐one derivatives 3a‐d . The structure elucidation of the products is based on nmr measurements and an X‐ray diffraction.