Oxalierpenes A and B ( 1 and 2 ), two unusual indole-diterpenoid derivatives, were obtained from the marine-derived fungus Penicillium oxalicum . The absolute configurations of 1 and 2 were elucidated by calculated TDDFT ECD and DP4plus methods. Oxalierpene A ( 1 ) represents the first indole-diterpenoid derivative with a five-membered ring of 4-hydroxy-5,5-dimethyldihydrofuran-3-one as a side chain. Oxalierpene B ( 2 ) has a unique 6/5/6/5/5/6/6/5/5 ring system. Compounds 1 and 2 showed antiviral activity against the H1N1 virus and respiratory syncytial virus (RSV), with IC_{50} values ranging from 2.8 to 9.4 μM.
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