Introduction to Heterocyclic Compounds

James Richard Fromm


Incorporation of an oxygen, a nitrogen, a sulfur, or an atom of a related element into an organic ring structure in place of a carbon atom gives rise to a heterocyclic compound. Since the heterocyclic atom must form more than one bond in order to be incorporated into a ring structure, halogens do not form heterocyclic compounds although they may be substituents on a heterocyclic ring structure. Heterocyclic compounds, like polycyclic ring compounds, are usually known by non-systematic names.

They may be either simple aromatic rings or non-aromatic rings.  Some examples are pyridine (C5H5N), pyrimidine (C4H4N2) and dioxane (C4H8O2).

pyridine.jpg (3147 bytes)

Pyridine (C5H5N)

101px-Cytosine.jpg (3522 bytes) 127px-Thymine.jpg (3583 bytes) 102px-Uracil.jpg (3396 bytes)
Three Pyrimidines (C4H4N2) - Cytosine, Thymine, and Uracil
Dioxane_isomers.jpg (10722 bytes)

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.

cy-3ane.jpg (1098 bytes) cycloh-c.jpg (1430 bytes)
Cyclopropane Cyclohexane

Heterocyclic chemistry is the chemistry branch dealing exclusively with synthesis, properties and applications of heterocyclics 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 heterocyclics are:

aziridine.jpg (2095 bytes) oxirane.jpg (1975 bytes) thiirane.jpg (1922 bytes)
Aziridine Ethylene Oxide, Oxirane Ethylene Sulfide, Thiirane

4-Membered Rings

Azetidine, oxetane, and thietane—the four-membered rings containing, respectively, nitrogen, oxygen, and sulfur atoms—are prepared by nucleophilic displacement reactions such as those used to prepare the corresponding three-membered rings

azetidine.jpg (1779 bytes) 1,3-epoxypropane.jpg (2569 bytes) trimethylenesulfide.jpg (1521 bytes)
Trimethylene Imine, Azetidine 1,3-Epoxypropane, Oxetane Trimethylene Sulfide, Thietane

5-Membered Rings

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

pyrrole.jpg (3742 bytes) furan.jpg (3171 bytes)
Pyrrole Furan
thiophene.jpg (3238 bytes) 100px-Arsole.jpg (3437 bytes)
Thiophene Arsole
1,2-diazole_pyrazole.jpg (3491 bytes) 1,3-dithiolane.jpg (2654 bytes)
1,2-Diazole, Pyrazole 1,3-Dithiolane

6-Membered Rings

pyridine.jpg (3147 bytes) piperidine.jpg (3180 bytes)
Pyridine Piperidine
pyran.jpg (2990 bytes) thiane.jpg (2937 bytes)
Pyran Tetrahydrothiopyran, Thiane
pyridazine.jpg (3573 bytes) pyrimidine.jpg (3415 bytes) pyrazine.jpg (3590 bytes)
1,2-Diazine, Pyridazine 1,3-Diazine, Pyrimidine 1,4-Diazine, Pyrazine
piperazine.jpg (3294 bytes) 100px-Thiazine.jpg (3777 bytes) morpholine.jpg (2887 bytes)
Piperazine Thiazine Morpholine, Oxazine

300px-Dithiane.jpg (5615 bytes)

1,2-dithiane       1,3-dithiane        1,4-dithiane

Dioxane_isomers.jpg (10722 bytes)


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Copyright 1997 James R. Fromm