Cytotoxic bafilomycin analogues 6/5/5 with tricyclic ring system from a marine-derived Streptomyces sp

Abstract Two bafilomycin derivatives with an unprecedented 6/5/5 tricyclic ring system (1 and 2), along with five known analogues (3–7), were identified from a marine Streptomyces sp. The structures of 1 and 2 were elucidated by analysis of HRESIMS, extensive NMR spectroscopic methods, GIAO DET 13C NMR calculations and comparison of the spectroscopic data with those of known related compounds. Compound 2 exhibited significant cytotoxicity against PC3 human prostate carcinoma cells with IC50 value of 4.99 ± 0.15 μM.