New chromane and chromene meroterpenoids from flowers of Rhododendron rubiginosum Franch. var. rubiginosum

Abstract

Six new chromane and chromene meroterpenoids rubiginosins A–F (1–2, and 4–7), together with three known ones, rubiginosin G (3) and anthopogochromenes A and B (8–9),were isolated from the flowers of Rhododendron rubiginosum Franch. var. rubiginosum. Among them, 1–4 were the chromane ones derived from an intramolecular [2 + 2] cycloaddition of their respective chromene precursors, making a 6/6/6/4- or 6/6/5/4-ring fused scaffold. The absolute configuration of the chiral center at C-2 of 1–9 was determined as S by chromane/chromene helicity rule and X-ray crystallograph. Notably, more attention should be paid to 6-carboxyl derivatives, since the 6-carboxyl derivatives showed an abnormal diagnostic Cotton effect (CE) with respect to their normal diagnostic CE. Compounds 1–9 were tested for cytotoxicity against four cell lines (A549, HCT116, SK-HEP-1, and HL-60), and only 1, 3, 5, and 9 showed moderate cytotoxicity, while others were inactive, discovering the 6-carboxyl is crucial for their low cytotoxicity.