Sarubicinols A-C, Cytotoxic Benzoxazoles from a Streptomyces

A chemical investigation of Streptomyces sp. Hu186 afforded two known quinone Antibiotics, sarubicin A (1) and sarubicin B (2), together with three unusual variants, sarubicinols A-C (3-5), and two new 1,4-naphthoquinone metabolites, sarubicin B₁ (6) and sarubicin B₂ (7). Compounds 3-5 possess a rare 2-oxabicyclo [2.2.2] substructure and a benzoxazole ring system. Their structures were elucidated using 1D and 2D nuclear magnetic resonance and high-resolution electrospray ionization mass spectrometry data. The absolute configurations of the side-chain moieties in 4 and 5 were solved by electronic circular dichroism calculations. Compounds 1-7 showed moderate cytotoxic activity against four tumor cell lines.