Two New 2,5-Diketopiperazines Produced by Streptomyces Sp. SC0581

Two new diketopiperazines, cyclo(l-Phe-l-NMe-DOPA) (2) and cyclo[l-Phe-l-(NMe-3-(NMe-3-O-α-l-rhamnopyranosyl)-DOPA] (3), along with a known diketopiperazine (1), were isolated from the cultures of Streptomyces sp. SC0581. Their structures were elucidated by extensive spectroscopic analysis, single-crystal X-ray crystallographic analysis, and chemical correlation. Compounds 1 − 3 exhibited more potent ABTS radical cation scavenging activity (IC50 values: 3.7 − 14.6 μM) than l-ascorbic acid (IC50: 17.7 μM). Compounds 2 and 3 also showed remarkable DPPH radical scavenging activity.