The cyclization of 2-aminopyridine (I) with ethyl 4-bromoacetoacetate (II) by means of NaHCO3 in hot dioxane gives the imidazopyridine (III), which is hydrolyzed at the ester function with KOH in ethanol yielding the free acid (IV) . Finally, the treatment of acid (IV) with phosphorous acid and PCl3 in chlorobenzene at 115 C, followed by hydrolysis in boiling 6N HCl yielded the target bisphosphonate.
This compound can be obtained by two related ways: 1) By cyclization of aminomalonamide (I) with ethyl orthoformate (II) at 150 C catalyzed with acetic acid. 2) By cyclization of (I) with ethyl formimidate (III) at 150 C catalyzed with acetic acid.