Heterocyclic compound

Overview
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, while '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.

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

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

With two heteroatoms:
 * The azoles:
 * Two N: pyrazole & imidazole
 * Three N: triazole
 * Four N: tetrazole
 * One N and one O: oxazole & isoxazole
 * One N and one S: thiazole & isothiazole
 * Two S: Dithiolane

6-membered rings
With two heteroatoms:
 * Two N: Pyridazine, Pyrimidine, and Pyrazine are the 1,2-, 1,3-, and 1,4-isomers, respectively.
 * Two N: Piperazine
 * One N and one O: Oxazines
 * One N and one S: Thiazine
 * Two S: Dithiane
 * Two O: Dioxane

Heterocyclic amines and cancer
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. 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.