Atromentin B, Obtained by Microbial Fermentation and Total Synthesis, Potentiates the Activity of β-Lactams Against Methicillin-Resistant <i>Staphylococcus aureus</i>

Methicillin-resistant <i>Staphylococcus aureus</i> (MRSA) poses a major clinical threat due to resistance to β-lactam antibiotics, such as cefazolin. Although several conventional antibiotics can be used for the treatment of MRSA, the rapid emergence of resistant strains limits their utility. To address this challenge, we envisioned that a combination therapy using β-lactam potentiators would be a promising approach to expand the application of cefazolin to MRSA. In this study, with our aim to explore natural products that potentiate the β-lactams against clinical MRSA strains, we identified a new atromentin A congener, named atromentin B (<b>1</b>), from the culture broth of <i>Aspergillus</i> sp. FKI-9941. Although our structural analysis revealed the presence of a 2,5-dihydroxycyclohexa-2,5-diene-1,4-dione motif in <b>1</b>, the inherent ¹³C NMR propensity of this class of compounds made definitive structural confirmation difficult. In addition, its low productivity through fermentation limited detailed biological evaluation. The total synthesis of <b>1</b> enabled unambiguous structural determination and provided sufficient information for biological studies. The synthetic sample of <b>1</b> exhibited cefazolin potentiation activity comparable to the naturally occurring product. Combination assays demonstrated that <b>1</b> enhanced cefazolin activity, even against clinical MRSA isolates. These findings show that atromentin B (<b>1</b>) is a promising β-lactam potentiator.