Angularly Prenylated Indole Alkaloids with Antimicrobial and Insecticidal Activities from an Endophytic Fungus <i>Fusarium sambucinum</i> TE-6L

Abstract

Bioactivity-guided isolation of the endophytic fungus <i>Fusarium sambucinum</i> TE-6L residing in <i>Nicotiana tabacum</i> L. led to the discovery of two new angularly prenylated indole alkaloids (PIAs) with pyrano[2,3-<i>g</i>]indole moieties, amoenamide C (<b>1</b>) and sclerotiamide B (<b>2</b>), and four known biosynthetic congeners (<b>3</b>-<b>6</b>). Their structures were determined by comprehensive spectroscopic techniques, electronic circular dichroism (ECD), and X-ray diffraction. Compound <b>1</b> containing the bicyclo[2.2.2]diazaoctane core and indoxyl unit is rarely reported. All the compounds were evaluated for their antimicrobial and insecticidal activities. Notably, compounds <b>1</b>-<b>3</b> showed potent inhibitory effects against three human- and one plant-pathogenic bacterium, and seven plant-pathogenic fungi. Compounds <b>2</b>-<b>4</b> also exhibited remarkable larvicidal activity against first instar larvae of the cotton bollworm <i>Helicoverpa armigera</i> with mortality rates of 70.2%, 83.2%, and 70.5%, respectively. Further toxicity tests on zebrafish embryos were performed to evaluate the potential toxicity of PIAs. Of significance was that compound <b>3</b> in particular exhibited the highest activities but the lowest effects on the hatching of embryos among all the compounds. This study provides a basis for understanding developmental toxicity of PIAs exposure to zebrafish embryos, and also indicates the potential environmental risks of other natural compounds exposure in the aquatic ecosystem.