BF3·Et2O promoted selective synthesis of benzimidazoles

Benzimidazoles 3, 6 and 9 have been synthesized selectively in excellent yields by cyclocondensation of β-(3-methyl-5-styryl-4-isoxazolyl amido) benzoic acids, acrylic acids and propionic acids with 1,2-phenylene diamines by employing BF3·Et2O as the catalyst. When the same reaction was carried out in pyridine it resulted in mixture of products in each case (3 & 4, 6 & 7 and 9 & 10). Other methods tried by using polyphosphoric acid, HCl, TFA also led to mixtures of 3 & 4, 6 & 7 and 9 & 10 in each case, similar to that of pyridine reaction.