Four Hybrid Flavan–Chalcones, Caesalpinnone A Possessing a 10,11-Dioxatricyclic [5.3.3.01,6]Tridecane-Bridged System and Caesalpinflavans A–C from Caesalpinia enneaphylla

Caesalpinnone A (1), an unprecedented hybrid of flavan and chalcone, possessing a 10,11-dioxatricyclic [5.3.3.01,6]­tridecane-bridged system, and caesalpinflavans A–C (2–4), three new hybrid flavan–chalcones, were isolated from the twigs and leaves of Caesalpinia enneaphylla. Their structures were elucidated by a combination of spectroscopic analyses and single-crystal X-ray diffraction. Caesalpinnone A showed the highest cytotoxicity against the HL-60, SMMC-7721, A-549, MCF-7, and SW-480 human tumor cell lines with an IC50 in the range of 0.54–0.87 μM.