Isolation and characterization of three pairs of indolediketopiperazine enantiomers containing infrequent N-methoxy substitution from the marine algal-derived endophytic fungus Acrostalagmus luteoalbus TK-43

Three pairs of new N-methoxy-containing indolediketopiperazine enantiomers, acrozines A-C (1-3), were isolated from the culture extract of Acrostalagmus luteoalbus TK-43, an endophytic fungus obtained from the marine green alga Codium fragile. The optical resolution of compounds 1-3 by chiral HPLC successfully afforded individual enantiomers (+)-1/(-)-1, (+)-2/(-)-2, and (+)-3/(-)-3, respectively. The structures of all these compounds were established on the basis of detailed interpretation of their NMR and mass spectroscopic data. X-ray crystallographic analysis confirmed the structures of compounds 1-3, while the absolute configurations were determined by TDDFT-ECD calculations. All these compounds containing a N-methoxy group which is uncommon in indolediketopiperazines. The enantiomers, (+)-2/(-)- 2, showed different antimicrobial activities against several plant-pathogenic fungi, while (+)-1 displayed better inhibitory activity against acetylcholinesterase than that of (-)-1.