Saliniquinones A–F, New Members of the Highly Cytotoxic Anthraquinone-γ-Pyrones from the Marine Actinomycete Salinispora arenicola

Six new anthraquinone-γ-pyrones, saliniquinones A–F (1–6), which are related to metabolites of the pluramycin/altromycin class, were isolated from a fermentation broth of the marine actinomycete Salinispora arenicola (strain CNS-325). Their structures were determined by analysis of one- and two-dimensional NMR spectroscopic and high-resolution mass spectrometric data. The relative and absolute configurations of compounds 1–6 were determined by analysis of NOESY NMR spectroscopic data and by comparison of circular dichroism and optical rotation data with model compounds found in the literature. Saliniquinone A (1) exhibited potent inhibition of the human colon adenocarcinoma cell line (HCT-116) with an IC50 of 9.9 × 10–9 M. In the context of the biosynthetic diversity of S. arenicola, compounds 1–6 represent secondary metabolites that appear to be strain specific and thus occur outside of the core group of compounds commonly observed from this species.