Toonaolides A−X, Limonoids from Toona ciliata: Isolation, Structural Elucidation, and Bioactivity against NLRP3 Inflammasome

Abstract Twenty-four new limonoids, toonaolides A−X (1−24), characterized with an α,β-unsaturated-γ-lactone A-ring were isolated from the twigs of Toona ciliata. Their structures and absolute configurations were elucidated by spectroscopic data, X-ray diffraction crystallography, and quantum chemistry calculations. Most of the isolated compounds (except 9, 18, and 24 which possessed the maleimide ring) featured the rare 21-hydroxybutenolide or 23-hydroxybutenolide moieties. In particular, compound 1 has an unprecedented limonoid architecture with 6/6 cis-fused A/B ring system and 2 has an unusual tetrahydrofuran ring B skeleton, featuring a 7/5/6/5 ring system. The biological evaluation showed that compounds 9, 11, 12, 14, and 18 exhibited significantly anti-NLRP3 inflammasome activity with IC50 values ranging from 3.2 to 9.7 μM. Analysis of IL-1β and caspase-1 expression revealed that compounds 11 and 12 are selective inhibitors of NLRP3 inflammasome, which could ameliorate cell pyroptosis by blocking NLRP3 inflammasome activation.