| Abstract: | The tautomerism of 1,4‐diazepines fused with pyrimidine rings was studied by means of nmr spec‐troscopy, X‐ray analysis and quantum chemical calculations. It was found that in the case of 6,8‐diphenyl‐pyrimido4,5‐b]1,4]diazepin‐4‐ols (7a ‐ e) the enamine form is more stable than the diimine form. This result is rationalized with the electron‐withdrawing effect of the 4‐hydroxypyrimidine ring and with the formation of intermolecular hydrogen bonds. In contrast to 7a ‐ e , the 6,8‐diaryl‐2,3,4,7‐tetrahydro‐1,3‐dimethyl‐1H‐pyrimido4,5‐b]1,4]diazepine‐2,4‐diones (9a, c, f) exist in the diimine form. |
