Pyrrolidine, also known as tetrahydropyrrole, is a saturated five-membered heterocyclic ring, which is miscible with water. Pyrrolidine exists in many alkaloids and drug molecules, such as kappa opioids, antagonists of dopamine D4 receptors, and HIV reverse transcriptase inhibitors.
Compared to quinolines, isoquinolines are also prominent structural motifs present in many biologically significant natural and synthetic compounds. Some well-known isoquinoline alkaloids include the anticancer and anticonvulsant berberine, the vasodilator and antispasmodic drugs papaverine and emetine. In addition to naturally occurring isoquinolines, synthetic analogs have also shown significant biological activity.