Indoles

Abstract

The indole unit is widely distributed in nature. Well over a thousand indole alkaloids have been described, many of major pharmacological importance. A majority of these alkaloids are of complex structure but have their biosynthetic origin in the simple indole amino acid tryptophan (1) which, itself, is an important cellular constituent. The vat dye indigo (2) is also an indole and has been known and widely used since antiquity. It was prepared by crushing the leaves of Isatis tinctora (woad) or Indigofera tinctora and allowing them to ferment. The resulting hydrolysis of plant indican’ (indol-3-yl β-glucoside; (3) liberates the aglycone which is then enzymically oxidised by atmospheric oxygen to yield indigo (2). Today, indigo is still an important dyestuff but is of synthetic origin. Indole (4) the parent compound of the series was first isolated, from indigo, and its structure elucidated by von Bayer. In highly dilute solutions, indole has a pleasant odour and is used in perfumery but in marked contrast, skatole (3-methylindole; 5) is responsible for the characteristically unpleasant smell of faeces.