Isolation and extraction of ruberythric acid from Rubia tinctorum L. and crystal structure elucidation

Madder (Rubia tinctorum L.) has been exploited as a dye throughout history. The roots of the plant are very rich in the highly coloured glycosidic compounds ruberythric acid and lucidin primeveroside, alongside the corresponding aglycons which can be readily formed by deglycosylation, particularly during extraction. Supported by 1H and 13C NMR data, the conclusive X-ray crystal structure of the natural dye ruberythric acid is presented for the first time. The solid state structure revealed extensive intermolecular hydrogen bonding interactions between the sugar moieties in the unit cell, but only intramolecular hydrogen bonding through the hydroxyquinone groups. There is also some additional π–π stacking from the anthraquinone moiety.