New Formyl Phloroglucinol Meroterpenoids from the Leaves of Eucalyptus robusta

Seven new formyl phloroglucinol meroterpenoids (FPMs), namely eucalrobusones J-P (1-7), as well as three known ones (8-10) were isolated from the leaves of Eucalyptus robusta. Their structures were elucidated by spectroscopic data analysis, and their absolute configurations were determined by applications of the Snatzke's helicity rule and the electron circular dichroism (ECD) calculation. These FPMs are diverse in coupling patterns between phloroglucinol and sesquiterpenoid units, forming novel polycyclic ring systems. Compound 1 possesses a new carbon skeleton that a 1-oxaspiro[5.6]dodecane core is formed through C-14 rather than C-4 of the aromadendrane moiety. Compound 2 features a novel 6/7/5 ring-fused 6-oxabicyclo[3.2.2]nonane skeleton. Compounds 3-5 are rare aristolane-based FPMs. By forming different oxo bridges, compound 3 is the first sample of FPM with benzo-dihydrofuran structure, and compound 4 possesses a novel 6/6/6/6/3-fused pentacyclic skeleton. Compounds 1, 6, and 8 exhibited significant antifungal activities against Candida glabrata with MIC₅₀ values of 2.57, 1.95, and 2.49 μg/mL, respectively.