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

Abstract

A pair of enantiomeric indole diketopiperazine alkaloid dimers [(-)- and (+)-asperginulin A (<b>1a</b> and <b>1b</b>)] with an unprecedented 6/5/4/5/6 pentacyclic skeleton were isolated from the mangrove endophytic fungus <i>Aspergillus</i> 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 (<b>1b</b>) exhibited antifouling activity against the barnacle <i>Balanus reticulatus</i>.