Unexpected One‐pot Synthesis of Novel 2‐Aminopyrimidine‐4‐ones under Microwave Irradiation

One‐pot, three‐component condensation of guanidine, ethylbenzoylacetate and various aromatic aldehydes in the presence of NaHCO₃ have been investigated by microwave irradiation. The aromatic aldehydes bearing electron‐withdrawing groups undergo condensation with guanidine and ethylbenzoyl‐acetate to afford ethyl‐2‐amino‐4‐aryl‐1,4‐dihydro‐6‐phenylpyrimidine‐5‐carboxylate derivatives via Biginelli reaction. However, reaction of the aromatic aldehydes having electron‐releasing groups with guanidine and ethylbenzoylacetate did not give the corresponding dihydropyrimidines. Instead, novel 2‐amino‐5‐benzoyl‐5,6‐dihydro‐6‐arylpyrimidine‐4(3H)‐ones were obtained via an unexpected mechanism.