Isolation of Three New Diketopiperazine Alkaloids from Penicillium sp. SCH3-Sd2

Intensive analysis of the culture broth of the marine-derived Penicillium sp. strain SCH3-Sd2 led to the isolation of one spirocyclic diketopiperazine alkaloid, spirotryprostatin H (1), as well as two indolyl diketopiperazines, fumitremorgins O and P (2 and 3). The planar structures of 1-3 were elucidated by comprehensive spectroscopic analyses, while their absolute configurations were determined by combined NOESY and ECD analyses, and the configuration of the proline moiety was further confirmed using the advanced Marfey's method. The evaluation of the antibacterial activity of compounds 1-3 against six bacterial strains revealed weak and selective activity only against Kocuria rhizophila KCTC 1915, with minimum inhibitory concentration (MIC) values of 64, 128, and 64 μg/mL, respectively.