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These are cyclic compounds containing one or more heteroatoms in the ring. The most commonly used heterocycles in organic reactions are pyridine, pyrimidine, and furans. These heterocycles may also contain selenium, tellurium, and phosphorous in the ring in addition to the usual oxygen, nitrogen, and sulfur atoms. Selenium containing heterocycles are less stable compared to sulfur containing heterocycles but they have considerable biological significance. Selenophene has a structure similar to thiophene. Carbon-selenium bonds are weaker than carbon-sulfur bonds. Tellurium heterocycles are very rare and even less stable then selinium heterocycles. Dibenzotellurophene was the first tellurium heterocycle to be prepared. Phosphorous heterocycles are used in pharmaceutical. Cyclophosphamide has anticancer properties. In addition to the above, various five-membered heterocycles of arsenic and bismuth are known. Due to their decreased stability they have less practical applications.
Some heterocyclic compounds are bridged and having interlocking rings. For example, methenamine is a heterocyclic compound containing four nitrogen atoms having a cage like structure similar to adamantane and is used in the treatment of urinary track infections. Heterocyclic compounds play a vital role in the metabolism of living cells. Proline, histidine and tryptophan are the most common heterocylcles used in medicines. Vitamin and co enzymes also contain some heterocycles like thiamine, riboflavin, pyridoxine, and pyridimindine derivatives. Sulfanilamidopyrimidines, sulfadiazines, sulfamethoxydiazines and sulfa-diazines also have antibacterial properties.