New Cytotoxic Naphthohydroquinone Dimers from <i>Rubia alata</i>

Two novel naphthohydroquinone dimers with unprecedented skeletons, rubialatins A (1) and B (2), were isolated from the herbal plant Rubia alata together with their precursor, mollugin (3). The structures were elucidated on the basis of NMR spectra and crystal X-ray diffraction. Compound 1, a racemate, was separated by chiral column chromatography, and the absolute configurations of the enantiomers were determined by the computational methods. Cytotoxicity of 1–3 was evaluated as well as the effect on the NF-κB pathway. Compound (+)-1 showed cytotoxicity and could inhibit NF-κB pathway. Meanwhile, 2 showed cytotoxicity and a synergistic effect with TNF-α on NF-κB activation.