Rubiadin
Rubiadin is a bioactive anthraquinone dye that occurs naturally in several plant species, including Morinda citrifolia.
History
Although rubiadin was first isolated in 1853 by Edward Schunck from the Rubia tinctorum, it was not until 1893 that Schunck and Leon Marchlewski succeeded in isolating rubiadin glucoside from Dutch madder root. The first synthesis of the compound was achieved in 1927.
Occurrence
Rubiadin occurs in plants of the genus dyer's madder and others. It is one of the pigments of dyer's madders, where it is present as a glucoside. Its monomethyl ether has been isolated from Morinda longiflora and Morinda citrifolia.
Extraction and presentation
Several methods for synthesizing rubiadin have been described. One approach is the synthesis of rubiadin from phthalic anhydride and 2,6-dihydroxytoluene in two reaction steps. However, the yields of both steps were low, and the reproducibility of this method was very poor. Another synthesis route proceeds via 2-methylpurpurin, but this method requires five steps and each reaction step affords a low yield. A further described synthesis of rubiadin derivatives involves Diels-Alder reactions with halogenated naphthoquinones and dienes. In addition, rubiadin can be obtained by the condensation of 3-bromo-2,6-dimethoxytoluene with a phthalide.
Properties
Rubiadin is a yellow to orange solid, and is soluble in organic solvents.
Use
Rubiadin is a constituent of natural colorants.