Synthesis of the Imidazopyridines

Imidazopyridines (IPs) may be synthesized from pyridine or imidazole derivatives by building up the second ring, and also by ring transformations from the other heterocyclic compounds. However, these methods are not of equal efficiency, and the most important preparative procedures utilize amino derivatives of pyridine as initial compounds for IP s synthesis. The main IP s precursors are o-diaminopyridines (o-DAP). 

Syntheses from Pyridines 1. Unsubstituted and Substituted Imidazopyridines 

The syntheses of 1 and 2 commonly involve aminopyridine or o-diaminopyridine (o-DAP) derivatives, the latter being the most important. 

The simplest but uncommon procedure for imidazo[4,5-b]pyridine (IbP) derivative 11 from monoaminopyridines involved oxidation of amidine 9 by sodium hypochlorite to give A-chloroamidine 10 followed by boiling its methanol-aqueous solution with sodium carbonate. 

Amidine of a similar structure obtained from 2-aminopyridine under the same conditions gave triazolopyridine (65JOC259) instead of IP. 

---