Abstract
Six previously undescribed ophiobolin-type sesterterpenes, namely, bipolatoxins A–F (1–6); and one previously undescribed pimarane-type diterpene, namely, 1β-hydroxy momilactone A (7); together with three known compounds, namely, 25-hydroxyophiobolin I (8), ophiobolin I (9), and ophiobolin A lactone (10); were isolated and identified from the endophytic fungus Bipolaris species TJ403-B1. Their structures with absolute configurations were elucidated on the basis of extensive spectroscopic analyses (including 1D and 2D nuclear magnetic resonance (NMR) and high-resolution electrospray ionization mass spectroscopy data), single-crystal X-ray diffraction analyses, and comparison of experimental circular dichroism data. All compounds (except for 5) were evaluated for antimicrobial potential, which indicated that bipolatoxin D (4) showed significant inhibitory activity against Enterococcus faecalis with a minimum inhibitory concentration (MIC) value of 8 μg/mL, and ophiobolin A lactone (10) showed significant inhibitory activity against Acinetobacter baumannii and E. faecalis with MIC values of 8 and 8 μg/mL, respectively.
Highlights
Microbial natural products and their derivatives have been an important source of new antibiotics required for the treatment of infectious diseases (Wright, 2017)
Circular dichroism (CD) data were collected from a JASCO J-810 spectrometer (JASCO Co., Ltd., Tokyo, Japan). 1D and 2D nuclear magnetic resonance (NMR) spectra were acquired on a Bruker AM-400, a DRX600, and a Bruker AM-800 instrument, and the 1H and 13C NMR chemical shifts were referenced to the solvent impurity peaks for methanol-d4 and acetone-d6
The EtOAc extract of the solid medium of Bipolaris species TJ403-B1 was subjected to extensive chromatographic separations over silica gel column, RP-C18 gel column, Sephadex LH-20, and semipreparative high-performance liquid chromatography (HPLC) to afford six previously undescribed ophiobolin-type sesterterpenes, namely, bipolatoxins A–F (1–6); and one previously undescribed pimarane-type diterpene, namely, 1β-hydroxy momilactone A (7); along with three known compounds (8–10)
Summary
Microbial natural products and their derivatives have been an important source of new antibiotics required for the treatment of infectious diseases (Wright, 2017). Penicillin, was discovered from the fungus Penicillium notatum in 1928 (Wright, 2017), multiple classes of anti-infectives have been isolated from a variety of fungi, such as gliotoxin, beauvericin, and roquefortine C (Jakubczyk and Dussart, 2020). The rapid evolution of antimicrobial resistance in both hospital and community settings is decreasing the efficacy of our current therapies and causing a serious global public health crisis (Baker, 2015; Brown and Wright, 2016). Fungi-derived natural products hold great promise in the search for new therapies.
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