Azines and azoles: CXXVI. First example of a novel heterocyclic system: Pyrimido[1,6-<Emphasis Type="Italic">a</Emphasis>][1,5]benzodiazepines

4-Hydroxy-5-(2-R-2,3-dihydro-1H-1,5-benzodiazepin-4-yl)-2H-1,3-thiazine-2,6-diones readily undergo acylation at the N¹ atom of the benzodiazepine system by the action of acetic anhydride. Heating of the acetylated products in boiling dimethylformamide leads to the formation of 75–93% of the corresponding 7-acetyl-6-R-6,7-dihydropyrimido1,6-a]1,5]benzodiazepine-1,3-diones that are derivatives of hitherto unknown fused heterocyclic system, pyrimido1,6-a]1,5]benzodiazepine. 4-Hydroxy-5-(2-R-2,3-dihydro-1H-1,5-benzodiazepin-4-yl)-2H-1,3-thiazine-2,6-diones are converted into 1-(2-aminophenyl)-6-(2-R-vinyl)uracils on heating in boiling DMF.