New clerodane diterpenoids from Callicarpa pseudorubella and their antitumor proliferative activity

Six previously undescribed clerodane diterpenes, cardorubellas A-F (1-6), along with seven known ones (7-13), were isolated from the aerial parts of Callicarpa pseudorubella. Their chemical structures were established by analysis of 1D and 2D NMR, HR-ESI-MS, X-ray diffraction, and electronic circular dichroism (ECD) data. Notably, cardorubella B (2) represented the first examples of naturally occurring succinic anhydride-containing clerodane diterpenes derivatives. The anti-proliferative activities of these compounds were assessed. Remarkably, compound 2 exhibited comparable inhibitory activity against HEL cell lines, surpassing the positive control with an IC₅₀ value of 14.01 ± 0.77 μM, compared to 17.02 ± 4.70 μM for 5-fluorouracil.