Oxygenated Acyclic Diterpenes with Anticancer Activity from the Irish Brown Seaweed Bifurcaria bifurcata

Brown alga Bifurcaria bifurcata is a prolific source of bioactive acyclic (linear) diterpenes with high structural diversity. In the continuation of our investigations on Irish brown algae, we undertook an in-depth chemical study on the n-hexanes and chloroform subextracts of B. bifurcata that led to isolation of six new (1-6) and two known (7-8) acyclic diterpenes. Chemical structures of the compounds were elucidated by a combination of 1D and 2D NMR, HRMS, FT-IR, [α]_D and vibrational circular dichroism (VCD) spectroscopy. Compounds 1-8, as well as three additional linear diterpenes (9-11), which we isolated from the same seaweed before, were tested against the human breast cancer cell line (MDA-MB-231). Several compounds moderately inhibited the growth of the MDA-MB-231 cell line with IC₅₀ values ranging from 11.6 to 32.0 μg/mL. The present study carried out on the lipophilic extracts of the Irish B. bifurcata shows the enormous capacity of this seaweed to produce a large palette of acyclic diterpenes with diverse oxygenation and substitution patterns and promising bioactivities.