Okaramines S–U, three new indole diketopiperazine alkaloids from Aspergillus taichungensis ZHN-7-07

Three new indole diketopiperazine alkaloids, okaramines S–U (1–3), with two (1 and 2) of them prenylated, were obtained from the Aspergillus taichungensis ZHN-7-07. Their structures including the absolute configurations were determined by combination of NMR, MS, TDDFT ECD calculation, and Marfey's analysis. Okaramine S (1) showed the best cytotoxic activity against HL-60 cell with IC50 value of 0.78 μM.