Benzimidazole condensed ring systems. 5. Studies on the Synthesis of Pyrimido[1,6-α]benzimidazole-1,3(2H,5H)-diones

The reaction of ethyl 1H-benzimidazole-2-acetate (1) with methyl or ethyl isocyantes 2a,b resulted in excellent yields of the respective 2-methyl- or 2-ethylpyrimido[1,6-a]benzimidazole-1,3(2H,5H)-diones 3a,b , while the reaction of 1 with phenyl isocyanate (2c) gave, unexpectedly, ethyl 2-(1-phenylcarbamoyl-1H,3H-benzimidazol-2-ylidene)-2-phenylcarbamoylacetate (4). Alkylation of 3 with trimethyl or triethyl phosphates 5a,b led to the 5-methyl or 5-ethyl derivatives 6a-d . Chlorination of 6 with sulfuryl chloride afforded the 4-chloro derivatives 7a-d.