Hexaricins, Pradimicin-like Polyketides from a Marine Sediment-Derived Streptosporangium sp. and Their Antioxidant Effects

Seven pradimicin-like polyketides were isolated from the dichloromethane extract of the marine sediment-derived Streptosporangium sp. CGMCC 4.7309, including five new hexaricins, D-H (1-5), and known hexaricins A (6) and C (7). Their structures were determined by HRESIMS, 1D and 2D NMR, and other spectroscopic analyses. The absolute configurations of compounds 1-5 were determined on the basis of circular dichroism and specific rotation data. All isolated compounds 1-7 were tested for their antioxidant capacities by DPPH• scavenging, •OH scavenging, and •O2̅ scavenging assays. Compounds 3 and 4 displayed stronger antioxidant activities than the positive control ( tert-butylhydroquinone). The relationship between structure and antioxidant activity is discussed. These compounds could be effective natural antioxidants with considerable pharmaceutical value.