Indoles are a kind of aromatic heterocyclic compounds, which possess a bicyclic structure, containing a benzene moiety fused to a pyrrole ring. Indoles are ubiquitously distributed in natural world and can be produced by various bacteria. The synthesis and functionalization of indoles has also been a major area of focus for synthetic organic chemists, and numerous methods for the preparation of indoles have been well developed.
Fig. 1 indole and its vital derivatives
Indole and the simple alkylindoles are colorless crystalline solids with a range of odors from naphthalene-like in the case of indole itself to faecal in the case of skatole (3-methyl-1H-indole). Like pyrrole, indole is a very weak base and hence indole itself and its simple derivatives are merely reactive towards strong acids. As an electron-rich heteroaromatic, indole is subject to oxidative processes, including photosensitized electron transfer. Many indoles are readily oxidized by exposure to atmospheric oxygen. The indole ring is also reactive toward electrophilic substitution, the 3-position being the most reactive site for substitution, which has been extensively studied to synthesize a large number of indole derivatives.
The indole nucleus is an important element of many natural and synthetic molecules with crucial biological activity, which have been widely applied in many pharmaceutical fields to cure different diseases. There are several drugs, such as sumatriptan (1, used for the treatment of migraine), ondansetron (2, used for the suppression of the nausea and vomiting caused by cancer chemotherapy and radiotherapy), indomethacin (3, used for the treatment of rheumatoid arthritis), delavirdine (4, used for anti-HIV drug), apaziquone and mitraphylline (5 and 6, both work for anticancer drug) shown in fig. 2.
Fig. 2 indole-based drugs
Indoles can be used as plant hormones that play a significant role in growth and development of plants. Indole-3-ylacetic acid or IAA, the first discovered plant hormone, participates in regulations of many physiological processes of a plant such as the pattern of cell division and differentiation, apical dominance and the directional growth. 3-dimethylamino indole is a botanical pesticide, which not only greatly inhibit the growth of undesired weeds but also make insects resistant to feed.
Fig. 3 indoles applied in agriculture
Indole derivatives have strong odors and can be modulated artificially for perfumes manufacturing. 3-methyl-1H-indole, also called skatole, emits flowers fragrant flavor when it is diluted appropriately and extensively applied in preparations of manmade flower oils.
Indoles have wide use in dyestuffs industries such as phthalocyanine, azo dyes and cationic dyes. In this case, many indole-based dyes are also invented to meet different demands. For instance, indocyanine green is a diagnostic dye for angiogram in medical field.
Fig. 4 indole-based dyes