Structure elucidation of new brominated sesquiterpenes from the sea hare Aplysia dactylomela by experimental and DFT computational methods

•Two new and two known compounds were isolated from the sea hare Aplysia dactylomela.•The structures were elucidated by spectroscopic methods and theoretical calculation.•All isolated compounds were evaluated for their cytotoxicity against 3 cancer cells. Two new brominated sesquiterpenes (1 and 2) and two previously reported compounds (3 and 4) were isolated from the Vietnamese sea hare Aplysia dactylomela. The new structures were elucidated by spectroscopic analysis combined with density functional theory (DFT) calculations of NMR chemical shifts and 3JH,H coupling constants, and by DP4+ analysis. Compound 2 is the first chamigrane sesquiterpenoid containing the B ring with two epoxide moieties. The absolute configurations of compounds 1 and 3 were determined by quantum chemical electronic circular dichroism (ECD) calculation. Compounds 1, 2, and 4 exhibited moderate or weak cytotoxicity against HepG2, DU145 and A549 cancer cells. [Display omitted]