Four new diterpenes from the mangrove Ceriops tagal and structure revision of four dolabranes with a 4,18-epoxy group.

Four new diterpenes named tagalons A-D (1-4), comprising an isopimarane (1), two 16-nor-pimaranes (2-3), and a dolabrane (4), were isolated from the Chinese mangrove, Ceriops tagal, together with four known dolabranes containing a 4,18-epoxy group, viz. tagalene I (5), 4-epitagalene I (6), tagalsin A (7), and tagalsin B (8). The structures of these compounds were unambiguously established by HR-ESIMS and NMR spectroscopic data. Based on the new criteria of NOE interactions between H-18/H-19 and H-18/H-20, previously reported relative configurations of the above four known dolabranes were correctly revised as their C-4 epimers, respectively. The relative configuration of tagalon B (2) and the absolute configuration of tagalene I (5) were established by single-crystal X-ray diffraction analyses, conducted with Cu Kα radiation. Most notably, tagalons C (3) and D (4) exhibited selective cytotoxicities against the human breast cancer cell line MT-1 with IC values of 3.75 and 8.07μM, respectively; whereas tagalene I (5) displayed potent cytotoxic effects against four human breast cancer cell lines MDA-MB-453, MDA-MB-231, SK-BR-3, and MT-1 with IC values of 8.97, 8.97, 4.62, and 3.93μM, respectively.