heterocyclic compounds

Heterocyclic compound

Heterocyclic compounds are organic compounds containing at least one atom of carbon, and at least one element other than carbon, such as sulfur, oxygen or nitrogen within a ring structure. These structures may comprise 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 prefix '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.

3-Membered rings

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)

4-Membered rings

Heteroatom Saturated Unsaturated
Nitrogen Azetidine
Oxygen Oxetane
Sulfur Thietane

Compound with two sulfur heteroatoms include dithietane and dithiete.

5-Membered rings

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

Heteroatom Saturated Unsaturated
Nitrogen Tetrahydropyrrole (azolidine, pyrrolidine) Dihydropyrrole (pyrroline), Pyrrole
Oxygen Tetrahydrofuran (oxolane) Dihydrofuran, Furan
Sulfur Tetrahydrothiophene (thiolane) Dihydrothiophene, Thiophene
Arsenic Arsole

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

6-Membered rings

Six membered rings with a single heteroatom:

Heteroatom Saturated Unsaturated
Nitrogen Piperidine Pyridine
Oxygen Tetrahydropyran Pyran
Sulfur Thiane Thiine (thiapyrane)

With two heteroatoms:

Heteroatom Saturated Unsaturated
Nitrogen Piperazine Diazines
Nitrogen / oxygen Oxazine
Nitrogen / sulfur Thiazine
Sulfur Dithiane
Oxygen Dioxane Dioxin

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.


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