Membered Rings-6
With one heteroatom:
With N: Pyridine, Piperidine
With O: Pyran
With S: Thiane
With two heteroatoms:
Two N: Pyridazine, Pyrimidine, Pyrazine are the 1,2-, 1,3-, and 1,4- isomers, respectively.
Two N: Piperazine
One N and one O: Oxazine, Morpholine
One N and one S...