Abstract
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.
Highlights
In recent years, phloroglucinol derivatives have been a hot research topic of natural products, because of their numerous bioactivities, such as anti-HIV1, antimicrobial2, and antitumor3 effects, and due to their diversified skeletons with highly complex polycyclic systems and multiple chiral centers4–6
According to the Snatzke’s helicity rule, the absolute configurations of C-1, C-10, and C-7′in 4 were assigned as 1R, 10S, and 7′S by electron circular dichroism (ECD) spectrum, in which positive Cotton effect at 295 nm (Δε =+ 6.67, Fig. 6B) was observed10
ECD spectra were measured on a Jasco J-810 spectropolarimeter (Jasco, Tokyo, Japan) using spectroscopic grade solvent (MeOH)
Summary
Meroterpenoids from the Leaves of Eucalyptus robusta received: 06 October 2016 accepted: 28 November 2016 Published: 22 December 2016. Seven new formyl phloroglucinol meroterpenoids (FPMs), namely eucalrobusones J-P [1,2,3,4,5,6,7], as well as three known ones [8,9,10] 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. Compounds 3–5 are rare aristolane-based FPMs that only two examples were reported before12 These natural products showed different antifungal activities against Candida albicans and C. glabrata. We report the isolation and structure elucidation of these FPMs, as well as their antifungal activities
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