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.
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