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.
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.