A practical and expedient synthesis of the title compounds is described. They were prepared by Stille reaction of nitro halopyridines 4 or nitro fluro-halobenzenes 10, followed by Michael addition of tert-butyl 4-aminopiperidine-1-carboxylate to the resulting activated vinyl compounds 5 and 11, hydrogenation (–NO2→–NH2), cyclic urea formation, Boc removal, and HCl salt formation. However, N3 and F1 analogs could not be made by this general ...