Medicinal Chemistry

Five-membered aromatic heterocycles containing more than one heteroatom

Pyrazoles, oxazoles, thiazoles

Six-membered non-aromatic heterocycles containing one heteroatom

Piperidine

Six-membered non-aromatic heterocycles containing more than one heteroatom

Morpholines, 1,4-dioxanes, 1,4-oxathianes

Six-membered aromatic heterocycles containing one heteroatom

Pyridine

Six-membered aromatic heterocycles containing more than one heteroatom

Pyrimidines, pyrazines

Two-ring aromatic heterocycles containing one heteroatom

Indole, benzofuran, benzothiophene

Quinoline, isoquinoline

Two-ring heterocycles containing more than one heteroatom

Purines

Complex heterocyclic compounds

Porphyrins and bile pigments

Hemoglobins, myoglobins, cytochromes

Alkaloids

Tropane alkaloids, isoquinoline alkaloids, indole alkaloids

Many of the simple heterocycles occur naturally within human biochemistry. For example,
the amino acids proline, histidine, and tryptophan contain, respectively, a pyrrolidine,
an imidazole, and an indole ring. The nucleic acids contain purine and pyrimidine rings.
Vitamins are heterocyclic compounds: vitamin B 6 (8.8) is a substituted pyridine; vitamin
B 1 (8.9) contains a pyrimidine ring. Simple heterocycles are therefore important to
human biochemistry and thus to drug design.

The complex heterocycles are likewise important to drug design. Porphyrinsare cyclic
compounds formed by the linkage of four pyrrole rings through methylene bridges. The
general name of porphyrin is used to denote a compound constructed from substituted
porphins.Porphin, with the four nitrogen atoms of the pyrrole rings pointing toward the
center of its large ring system, complexes efficiently with metal ions; in heme, this ion
is Fe(II), and in chlorophyll it is magnesium (II). The porphyrin ring system is very stable
and has aromatic character. In nature, the metalloporphyrins are conjugated to proteins
(globins) to form a number of important biological macromolecules. Hemoglobin serves
as a transport mechanism for oxygen within the blood; myoglobin is a respiratory
protein found in the muscle cells of vertebrates and invertebrates; cytochromes act as

ENDOGENOUS MACROMOLECULES 479