Abstract Several new 6‐amino‐ and 6,8‐diamino‐4‐aryl‐2,3‐dihydropyrimido[4,5‐ b ][1,4]diazepines were obtained from the reaction of 4,5,6‐triaminopyrimidine 1a and 2,4,5,6‐tetraaminopyrimidine 1b with one equivalent of 3‐dimethylaminopropiophenones 2 in absolute ethanol. Structure analysis of 6‐amino‐ and 6,8‐diamino‐4‐aryl‐2,3‐dihydropyrimido[4,5‐ b ][1,4]diazepines 3a‐i , determined by detailed nmr measurements, reveals a high regioselectivity of this reaction.