Alcimaterpenoids A and B, Two Enantiomeric Pairs of Bis-Denudaquinol-Monoterpenoid Meroterpenoids from the Endangered Plant Alcimandra cathcartii

Alcimaterpenoids A (1) and B (2), two pairs of racemic bis-denudaquinol-monoterpenoid meroterpenoids, were isolated and identified from Alcimandra cathcartii. Electronic circular dichroism (ECD) computations, GIAO 13C{1H} NMR (in conjunction with DP4+ probability analysis), and extensive 2D-NMR spectroscopic analysis techniques were used to determine their structures and absolute configurations. Structurally, compounds 1 and 2 are the first enantiomeric bis-denudaquinol-monoterpenoid meroterpenoids. Plausible biosynthetic pathways were briefly proposed. Compound (±)–1 exhibited moderate cytotoxic activity against the human PLC/PRF/5 cell line (IC50: 20.6 μM).