Stereochemical assignments of rubiaquinones A-C, naphthoquinone derivatives from &ITRubia yunnanensis&IT

Three new naphthoquinone derivatives, rubiaquinones A-C (1-3), were isolated from the roots of Rubia yunnanensis. Rubiaquinone A (1) was a racemic naphthoquinone dimer consisting of a 1,4-dihydroxy-naphthalene and a 4-hydroxy-1,2-naphthoquinone moieties with a 2-oxo-propyl group. Rubiaquinones B (2) and C (3) were structurally unique trimeric naphthoquinones with a racemic nature possessing one chiral axis and one chiral carbon in common. The planar structures of 1-3 were assigned by detailed spectroscopic analyses, and enantiomers of 1-3 were obtained by optical resolutions. The absolute configurations of (+)-1 and (-)-1 were elucidated by interpretation of the ECD spectra with the aid of TDDFT ECD calculation, while those of enantiomers obtained from 2 and 3 were assigned by analyses of the composite ECD spectra generated by summing appropriate ECD spectra of enantiomers. Rubiaquinone A (1) exhibited antimicrobial activity against Bacillus subtilis. (C) 2017 Elsevier Ltd. All rights reserved.