Heterocyclic Fused Rings with Bridgehead Nitrogen Atoms: Single-Step Synthesis of Azolo[1",2":1',2']pyrido[5',6':5,4]pyrimido[1,6-a]benzimidazole; Pyrido[1,2-a]benzimidazole; Pyrido[4",5":2',3']pyrido[6',5':4,5]pyrimido-[1,6-a]benzimidazole and Polysubstituted Pyridine Derivatives