Antiplasmodial β‑Triketone–Flavanone Hybrids from the Flowers of the Australian Tree Corymbia torelliana

The methanol extract of the flowers of the Australian eucalypt tree Corymbia torelliana yielded six new β-triketone–flavanone hybrids, torellianones A–F (1–6), the tetrahydroxycyclohexane torellianol A (7), and known β-triketones (4S)-ficifolidione (8) and (4R)-ficifolidione (9), and β-triketone–flavanones kunzeanone A (10) and kunzeanone B (11). Torellianones A and B, C and D, and E and F were each isolated as inseparable diastereomeric mixtures. Exchange correlations observed in a ROESY spectrum indicated that 5 and 6 also interconverted between stable conformers. The structures of 1–7 were elucidated from the analysis of 1D/2D NMR and MS data. Relative configurations of torellianones C–F and torrellianol A were determined from analysis of ROESY data. Compounds 1–10 were tested for antiplasmodial activity against a drug-sensitive (3D7) strain of Plasmodium falciparum, with 3–6 and 8–10 showing limited antiplasmodial activity, with IC₅₀ values ranging from 3.2 to 16.6 μM.