(−)- and (+)-Asperginulin A, a Pair of Indole Diketopiperazine Alkaloid Dimers with a 6/5/4/5/6 Pentacyclic Skeleton from the Mangrove Endophytic Fungus Aspergillus sp. SK-28

A pair of enantiomeric indole diketopiperazine alkaloid dimers [(-)- and (+)-asperginulin A (1a and 1b)] with an unprecedented 6/5/4/5/6 pentacyclic skeleton were isolated from the mangrove endophytic fungus Aspergillus sp. SK-28. The enantiomeric dimers were separated by chiral-phase HPLC. Their structures including absolute configurations were elucidated by spectroscopic analysis, X-ray diffraction, and quantum chemical calculation. (+)-Asperginulin A (1b) exhibited antifouling activity against the barnacle Balanus reticulatus.