Marinoterpins A–C: Rare Linear Merosesterterpenoids from Marine-Derived Actinomycete Bacteria of the Family Streptomycetaceae

The chemical examination of two undescribed marine actinobacteria has yielded three rare merosesterterpenoids, marinoterpins A–C (1–3, respectively). These compounds were isolated from the culture broth extracts of two marine-derived actinomycetes associated with the family Streptomycetaceae, (our strains were CNQ-253 and AJS-327). The structures of the new compounds were determined by extensive interpretation of 1D and 2D NMR, MS, and combined spectroscopic data. These compounds represent new chemical motifs, combining quinoline-N-oxides with a linear sesterterpenoid side chain. Additionally, consistent in all three metabolites is the rare occurrence of two five-ring ethers, which were derived from an apparent cyclization of methyl group carbons to adjacent hydroxy-bearing methylene groups in the sesterterpenoid side chain. Genome scanning of AJS-327 allowed for the identification of the marinoterpin (mrt) biosynthetic cluster, which consists of 16 open-reading frames that code for a sesterterpene pyrophosphate synthase, prenyltransferase, type II polyketide synthase, anthranilate:CoA-ligase, and several tailoring enzymes apparently responsible for installing the N-oxide and bis-tetrahydrofuran ring motifs.