New isofuranonaphthoquinones and isoindolequinones from Streptomyces sp. CB01883

Isofuranonaphthoquinones (IFQs) and Isoindolequinones (IIQs) comprise a small family of natural products, with the latter ones are especially uncommon in nature. Here we report the discovery of seven new IFQs, IFQ A-G (1-7), and three new IIQs, IIQ A-C (8-10), along with the known anthraquinone desoxyerythrolaccin (11), from Streptomyces sp. CB01883, expanding the chemical diversity of this family of natural products. The structures of these natural products were established on the basis of their HR-ESI-MS and nuclear magnetic resonance (NMR) spectroscopic data. All compounds were assessed for antibacterial activity, with 11 and 1, 5-7 exhibiting moderate and weak activities, respectively, against several Gram-positive bacteria tested. Bioinformatics analysis of the Streptomyces sp. CB01883 genome revealed the ifq gene cluster that showed identical genetic organization, with high-sequence identity, to the ifn gene cluster recently cloned from Streptomyces sp. RI-77 and confirmed to encode the biosynthesis of two IFQs, JBIR-76 and JBIR-77. Co-isolation of IFQs with IIQs from Streptomyces sp. CB01883 and facile chemical transformation of selected IFQs to IIQs, as exemplified by 1 to 9, together with the finding of the ifq cluster that most likely only encodes IFQ biosynthesis, support the proposal that IIQs may be derived nonenzymatically from IFQs in the presence of an amine.