In general, if the atoms that make up the ring contain heteroatoms, such rings become heterocycles, and organic compounds containing heterocycles are called heterocyclic compounds. An article called Fluorine in medicinal chemistry: β-fluorination of peripheral pyrrolidines attached to acridine ligands affects their interactions with G-quadruplex DNA, published in 2011-03-07, which mentions a compound: 136725-55-8, Name is (R)-(-)-3-Fluoropyrrolidine Hydrochloride, Molecular C4H9ClFN, Related Products of 136725-55-8.
Comparative X-ray structure studies reveal that C-F bond incorporation into the peripheral pyrrolidine moieties of the G-quadruplex DNA binding ligand BSU6039 leads to a distinct pyrrolidine ring conformation, relative to the non-fluorinated analog, and with a different binding mode involving reversal of the pyrrolidinium N+-H orientation.
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Reference:
Pyridazine – Wikipedia,
Pyridazine | C4H4N2 – PubChem