Zelkovamycins B–E, Cyclic Octapeptides Containing Rare Amino Acid Residues from an Endophytic Kitasatospora sp

sp. Zelkovamycin B was featured by an unprecedented 3-methyl-5-hydroxypyrrolidine-2,4-dione ring system linked to the cyclopeptide skeleton. Their structures and full configurations were established by spectroscopic analysis, Marfey's method, and NMR calculations. A plausible biosynthetic pathway for zelkovamycins was proposed based on gene cluster analysis. Zelkovamycin E displayed potent inhibitory activity against H1N1 influenza A virus.