Isolation, characterization, and anti-MRSA activity of formyl-phloroglucinol-derivatives from Eucalyptus robusta fruits

Six new derivatives of formyl-phloroglucinol, namely eucalyprobusals G-L (1-6), were isolated from the lowpolarity extract of Eucalyptus robusta fruits. Among them, eucalyprobusal G (1) features an unprecedented 6/ 6/6/6/6-fused ring meroterpenoid structure, consisting of two phellandrene units coupled with a formylphloroglucinol. Eucalyprobusal H (2) is a dimeric acylphloroglucinol, while eucalyprobusals I-L (3-6) are meroterpenoids of formyl phloroglucinol coupled with a monoterpenoid. Their structures were elucidated using NMR, HRESIMS, electron circular dichroism (ECD), and single-crystal X-ray diffraction. Compound 3 demonstrated broad-spectrum antibacterial activity against both of Gram-positive and Gram-negative bacteria, exhibiting a notable efficacy against Methicillin-resistant Staphylococcus aureus (MRSA) with an MIC value of 12.5 mu g/ mL. The growth and time-kill curves showed that bacterial growth was completely inhibited and killed in a dosedependent mode. At the same time, SEM and TEM assays showed that compound 3 caused irregular wrinkling of the bacterial surface and uneven aggregation of the cytoplasm, potentially disrupting the structure of the bacterial cell wall, cell membrane, and intracellular components.