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Subject - Organic Chemistry
Cyclic organic compounds in which at least one carbon atom is substituted by another element, the heteroatom (frequently , and and less frequently , , , and ), are collectively referred to as heterocyclic compounds. These compounds also include all condensed systems in which one or several heterocyclic rings are annulated with each other or with carbocyclic compounds. Besides saturated (heterocycloalkanes) and partially unsaturated systems (heterocycloalkenes), aromatic heterocyclic compounds represent the largest and most important group within the compound class of heterocycles. The same rules regarding aromaticity in carbocycles also apply for heterocycles.
About 50% of the ca. 20 millions compounds presently described are heterocycles. Accordingly, the number of many variants for the syntheses of heterocyclic compounds also is very large.
Heterocycles are widely distributed in nature and to some extent form their own class of natural products, e.g. carbohydrates, nucleic acids, alkaloids, porphyrines etc.
The nomenclature of heterocyclic compounds is very complicated. Very often, mainly in the case of older and familiar heterocycles, trivial names are being used. Replacement nomenclature and the Hantzsch-Widman-Patterson nomenclature, in which trivial names are being given high priority, also exist.