# Introduction to Heterocycles

## Heterocyclic Compounds

In organic chemistry, cyclic compounds that contain at least one ring atom that is not a carbon are called heterocyclic compounds or heterocycles. However, the rings of most heterocycles that pertain to organic chemistry contain more carbons than heteroatoms. However, "heterocycles" that do not contain any carbon, such as the $S8$ ring, are also known in inorganic chemistry, but that is exactly why they are classified under inorganic chemistry.

Definition
In organic chemistry, carbocyclic compounds that contain at least one ring atom that is not a carbon are called heterocycles.

The most frequently occuring heteroatoms in heterocycles are nitrogen, oxygen, and sulfur. However, heterocycles with other heteroatoms, such as phosphorus and selenium, also appear. In nature, heterocycles are of great importance. Roughly more than one half of all natural products contains heterocyclic components. Many of them have important functions in the human organism. In addition, many natural compounds are pharmacologically active. Nitrogen-containing heterocycles are particularly widespread in nature. The , for instance, are a special class of nitrogen-containing, naturally occuring heterocycles. Furthermore, deoxyribonucleic acid (), which is the carrier of genetic information in all living beings, contains the nitrogen-containing heterocycles adenine and guanine (purine bases), as well as cytosine and thymine (pyrimidine bases). Ribonucleic acid (RNA) additionally contains the pyrimidine base uracil. Genetic information is saved in the sequence of these purine and pyrimidine bases in the DNA and RNA chains.

Tab.1
Examples of important naturally occuring heterocycles.
 Fig.1 Fig.2 Fig.3 Fig.4Morphine Fig.5Uracil: A pyrimidine base in RNA Fig.6Section of a DNA molecule

The ring bonds of heterocycles are more or less polarized, due to the electronegativity differences between the carbons and the heteroatoms. Thus, heterocycles may participate in polar interactions, such as hydrogen bridge bonds. Such polar interactions are decisive in molecular recognition, such as in the base pairing involved in DNA and the translation of genetic information into protein structures.

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