The reaction of 2,4-dichloro-5-(chloromethyl)-6-methylpyrimidine with 4,5-dihydrothiazol-2-amine in chloroform led to the formation of 2-chloro-4-methyl-7,8-dihydro-5H-pyrimido[4,5-d]thiazolo[3,2-a]pyrimidine (3). The true structure of the target compound was confirmed by density functional theory calculations of condensed Fukui indices. The substitution of the 2-Cl function in compound 3 with different secondary amines yielded 2-substituted-4-methyl-7,8-dihydro-5H-pyrimido[4,5-d]thiazolo[3,2-a]pyrimidines in good yields. The assigned products were tested for their potential antibacterial activity and the results indicated that a few derivatives are valuable compounds with great potential to be used as antibacterial agents.

Several 3H-[1,2,3]triazolo[4,5-d][1,2,4]triazolo[4,3-a]pyrimidine derivatives of a novel ring system have been synthesised. The initial substitution of the 4-Cl function of 2,4-dichloro-6-methylpyrimidin-5-amine with benzylamine followed by treatment with sodium nitrite generated 3-benzyl-5-chloro-7-methyl-3H-[1,2,3]triazolo[4,5-d]pyrimidine. Nucleophilic substitution of the 5-Cl moiety of the latter compound with hydrazine hydrate and subsequent treatment with carbon disulfide in boiling pyridine gave, quantitatively, the corresponding [1,2,3]triazolo[4,5-d][1,2,4]triazolo[4,3-a]pyrimidine derivatives. Density functional theory (DFT) studies were also performed to reveal regioselectivity of the ring closure via Gauge-Independent Atomic Orbital (GIAO) 1H NMR calculations. The WP04 method was used as the DFT code to yield accurate chemical shift values. Theoretical results correlated well with the expected regioisomer and aided assignment of the final structure.