From Wikipedia, the free encyclopedia

Heterocyclic compounds are organic compounds that contain a ring structure containing atoms in addition to carbon, such as sulfur, oxygen or nitrogen, as part of the ring. They may be either simple aromatic rings or non-aromatic rings. Some examples are pyridine (C₅H₅N), pyrimidine (C₄H₄N₂) and dioxane (C₄H₈O₂).

Note that compounds such as cyclopropane, an anaesthetic with explosive properties, and cyclohexane, a solvent, are not heterocyclic; they are merely cycloalkanes. The suffix 'cyclic' implies a ring structure, whereas 'hetero' refers to an atom other than carbon, as above. Many heterocyclic compounds, including some amines, are carcinogenic.

Heterocyclic chemistry is the chemistry branch dealing exclusively with synthesis, properties, and applications of heterocycles especially vital to drug design.

Contents 1 3-membered rings 2 4-membered rings 3 5-membered rings 4 6-membered rings 5 Heterocyclic amines and cancer 6 References 7 External links

Heterocycles with three atoms in the ring are more reactive because of ring strain. Those containing one heteroatom are, in general, stable. Those with two heteroatoms are more likely to occur as reactive intermediates. Common 3-membered heterocycles are:

heteroatom	saturated	unsaturated
Nitrogen	aziridine
Oxygen	ethylene oxide (epoxides, oxiranes)	oxirene
Sulfur	thiirane (episulfides)

heteroatom	saturated	unsaturated
Nitrogen	azetidine
Oxygen	oxetane
Sulfur	thietane, dithietane	dithiete

With heterocycles containing five atoms, the unsaturated compounds are frequently more stable because of aromaticity.

heteroatom	saturated	unsaturated
Nitrogen	dihydropyrrole (pyrroline) & tetrahydropyrrole (pyrrolidine)	pyrrole
Oxygen	dihydrofuran & tetrahydrofuran	furan
Sulfur	dihydrothiophene & tetrahydrothiophene	thiophene (thiole)
Arsenic		arsole

A large group of 5-membered ring compounds with two heteroatoms are collectively called the azoles. Dithiolanes have two sulfur atoms.

Six membered rings with a single heteroatom:

heteroatom	saturated	unsaturated
Nitrogen	piperidine	pyridine
Oxygen	tetrahydropyran	pyran
Sulfur	thiane	thiine aka thiapyrane

With two heteroatoms:

heteroatom	saturated	unsaturated
Nitrogen	piperazine	the diazines
Nitrogen / oxygen	oxazine
Nitrogen / sulfur	thiazine
Sulfur	dithiane
Oxygen	dioxane

Some heterocyclic amines (HCAs) found in cooked meat are known carcinogens. Research has shown that cooking certain meats at high temperatures creates chemicals that are not present in uncooked meats. For example, heterocyclic amines are the carcinogenic chemicals formed from the cooking of muscle meats such as beef, pork, fowl, and fish. HCAs form when amino acids and creatine (a chemical found in muscles) react at high cooking temperatures. Researchers have identified 17 different HCAs resulting from the cooking of muscle meats that may pose human cancer risk.^[1] NCI's Division of Cancer Epidemiology and Genetics found a link between individuals with stomach cancer and the consumption of cooked meat, and other studies for colorectal, pancreatic, and breast cancer is associated with high intakes of well-done, fried, or barbecued meats. Other sources of protein (milk, eggs, tofu, and organ meats such as liver) have very little or no HCA content naturally or when cooked.

1. ^ Heterocyclic Amines in Cooked Meats. National Cancer Institute (15 Sep 2004). Retrieved on 2007-08-09.

• Heterocyclic amines in cooked meat, US CDC
• List of known and probable carcinogens, American Cancer Society
• List of known carcinogens by the State of California, Proposition 65 (more comprehensive)