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

The reaction of ethyl 1 H ‐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(2 H ,5 H )‐diones 3a,b , while the reaction of 1 with phenyl isocyanate (2c) gave, unexpectedly, ethyl 2‐(1‐phenylcarbamoyl‐1 H ,3 H ‐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.