Uncommon N‐Methoxyindolediketopiperazines from Acrostalagmus luteoalbus, a Marine Algal Isolate of Endophytic Fungus

Main observation and conclusion Four new N‐methoxyindolediketopiperazines (acrozines D—G, 1—4) and six known congeners (5—10) were purified from the organic extract of Acrostalagmus luteoalbus TK‐43, which was a marine algal‐derived fungus obtained from Codium fragile. Their structures were determined by interpretation of NMR and mass spectroscopic data. The structures of compounds 1, 2, and 4 including their absolute configurations were confirmed by single‐crystal X‐ray diffraction, while the absolute configuration of compound 3 was elucidated by comparative analysis of ECD and TDDFT‐ECD calculations. Compounds 1—4, with a unique methoxy substitution at N‐2, were rarely discovered among indolediketopiperazine alkaloids. All these compounds were evaluated for antimicrobial activities against human‐ and aquatic‐pathogenic bacteria and plant‐related pathogenic fungi, with compounds 5 and 7 exhibiting potent activity against Edwardsiella icataluri (MIC = 3 and 5 μmol/L, respectively), while compound 9 displayed a broad spectrum of antibacterial activities. The four new compounds were further tested for anti‐acetylcholinesterase (AChE) properties, and compound 3 exhibited inhibitory activity with IC50 value of 8.4 μmol/L.