1. Introduction
The natural product tryptanthrin (indolo[2,1-b]quinazoline-6,12-dione, Figure 1) is a weakly basic alkaloid. This bright yellow compound consists of a quinazoline ring fused to an indole moiety with carbonyl groups in the 6- and 12-positions. The name tryptanthrin is derived from the observation that this compound is produced by the yeast Candida lipolytica when grown in L-tryptophan-containing medium.1 Occasionally the alternative spelling tryptanthrine is used to describe this compound. Chemical Abstracts appear to index both spellings.
Figure 1
This review describes the synthesis of compounds with a tryptanthrin core and selected reactions of this class of compounds.2 Tryptanthrin and its derivatives are of particular interest due to their biological activities. Tryptanthrin compounds have been shown to possess antibacterial3, antiparasitic4, and antineoplastic5,6 properties. Extensive work has been put forth to elucidate the usefulness of tryptanthrin derivatives as dyes and pigments and as photoelectric materials.7,8 The physicochemical properties of tryptanthrin compounds deposited onto surfaces have also been investigated.9
In 1892 O'Neil described the formation of “silky golden-yellow crystals” that form upon oxidation of indigo with potassium permanganate.10,11 In 1915 Friedländer and Roschdestwensky were able to elucidate the structure of tryptanthrin,12 which was verified 60 years later by X-ray crystallography.13
Tryptanthrin has been isolated from numerous natural sources. In particular, tryptanthrin is found in plant material traditionally used as colorants including Chinese woad (Isatis tinctoria),14,15 Japanese indigo (Polygonum tinctorium),14,16 Assam indigo (Strobilanthes cusia),17 Indigo naturalis (Strobilanthes formosanus),18 and dyer’s oleander (Wrightia tinctoria).19 Furthermore tryptanthrin has also been isolated from the fruits of the cannonball tree (Couroupita guaianensis),20, 21 the orchids Phaius mishmensis22 and Calanthe discolor,23 and the fungi Schizophyllum commune and Leucopaxillus cerealis,24,25 and a strain of the bacterial Cytophaga genus.26 Tryptanthrin has also been found in mammals, specifically in the urine of the Asian elephant (Elephas maximus)27 and the wing sac liquids of the bat Saccopteryx bilineata.28
Several compounds have been observed in nature with an indolo[2,1-b]quinazoline core in which the keto group in 6-position has been replaced with a different functional group (Figure 2).
