Indole Alkaloids

Indole Alkaloids

Indole (1-H-indole) is a benzopyrrole in which the benzene and pyrrole rings are through the 2, 3-positions of the pyrrole. The indole nucleus is found in a large number of naturally occurring compounds. It is of commercial importance as a component of perfumes. Isoindole (I-H-isoindole), the isomer in which the benzene and pyrrole rings are fused through the 3- and 4-positions of the pyrrole, is not stable. A few of its derivatives are known, the simplest being N-methylisoindole.

Indole was first obtained (and its structure elucidated) in 1866 by Adolf von Baeyer. Interest in indole chemistry revived about 1930 when it was discovered that the essential amino acid, tryptophan, the plant growth hormone, heteroauxin, and several groups of important alkaloids are indole derivatives. It was shown that 3-methylindole (skatole) is produced with indole during pancreatic digestion or putrefactive decomposition of proteins and, hence, both are found in the intestines and feces. Interest has centered on medicinal and biochemical aspects of indole chemistry. Serotonin, which has been identified as a metabolite in brain chemistry; the psychotomimetic indoles, psilocin and psilocybin from mushrooms; the tranquilizer reserpine; and the melanin pigments are a few of the compounds that have been studied.

Indole is a colourless crystalline solid (mp 52–54°C, bp 254°C). The heat of combustion at constant volume is 4,268 MJ/mol (10–20 kcal/mol). The molecule is planar and has only moderate polarity. Indole has good solubility in a wide range of solvents including petroleum ether, benzene, chloroform and hot water. The solubility in cold water is only 1:540 at 25°C; thus, water is a good solvent for puri-fication by recrystallization. Indole forms salts with high concentrations of both strong bases and strong acids.

The various plants containing indole alkaloids are vinca, ergot, Rauwolfia, nux vomica, physostigma, etc.