A new phenylamine-incorporated angucyclinone (1) featuring a unique 1-phenylbenzo­[cd]­indol-3­(1H)-one moiety was discovered from marine Streptomyces sp. PKU-MA00218. A series of experimental investigations identified that 1 was produced from the nonenzymatic conversion of a C-ring-cleaved angucyclinone (2) with phenylamine. Utilizing the nonenzymatic conversion, 18 phenylamine-incorporated angucyclinone derivatives with halogen, methyl, methoxy, and carboxy substitutions were efficiently generated under mild conditions. These results highlighted the impressive roles of nonenzymatic reactions in expanding the structural diversity of angucyclinones.

Seven new 4-acyl-2-aminoimidazoles, designated strepimidazoles A–G (1–7), were discovered from the endophytic Streptomyces sp. PKU-EA00015 isolated from Salvia miltiorrhiza Bunge, whose dry root “Danshen” is one of the most widely used traditional Chinese medicines. The resonance signals of the 2-aminoimidazole moiety in 1–7 were absent in the NMR spectra due to tautomerization, and the structures of 1–7 were identified after preparation of their acetylation products 1a–7a, respectively. Compounds 1–7 represent a new family of 2-aminoimidazole-containing natural products, enriching the structural diversity of natural products from endophytic origin. Compounds 1–7 showed different degrees of inhibitory activities against the plant pathogenic fungus Verticillium dahliae V991, revealing structure–activity relationships on the acyl moieties. The plant pathogenic fungus V. dahliae has been confirmed to cause serious chlorosis of cultivated S. miltiorrhiza Bunge in China. This study opens the door for further investigation of mutualistic relationships between S. miltiorrhiza Bunge and their endophytic actinomycetes and for possible antifungal agent development for biological control of V. dahliae in the future.