Pauciflorins A–E, Unexpected Chromone–Monoterpene-Derived Meroterpenoids from Centrapalus pauciflorus

Five unusual meroterpenoids based on new carbon skeletons, pauciflorins A–E (1–5), were isolated by multistep chromatographic separations of a methanol extract of the aerial parts of Centrapalus pauciflorus. Compounds 1–3 are derived by the connection of a 2-nor-chromone and a monoterpene unit, whereas 4 and 5 are dihydrochromone–monoterpene adducts with a rarely occurring orthoester functionality. The structures were solved using 1D and 2D NMR, HRESIMS, and single-crystal X-ray diffraction. Pauciflorins A–E were evaluated for antiproliferative activity against human gynecological cancer cell lines, but were inactive (IC₅₀ < 10 μM) in each case.