Reaction of 2-mercaptobenzimidazole with ethyl phenylpropiolate

The reaction of 2-mercaptobenzimidazole (I) with phenylpropiolic ester results in the formation of 2-phenyl-thiazino[3, 2-a]benzimidazoI-4-one (II). The structure 4-phenyl-2H-l-thiapyrimido[l,2-a]benzimidazol-2-one (III) assigned by earlier workers