Sesquiterpenoids From the Red Sea Soft Coral <i>Lemnalia</i> sp.: Isolation, Structural Elucidation, and Cytotoxic Evaluation

Phytochemical investigation of the CH₂Cl₂/MeOH (1:1, v/v) extract of the Red Sea coral Lemnalia sp. led to the identification and purification of a previously undescribed ylangene-type sesquiterpenoid, designated as (1S,2S,3S,6R,7R,8R)-6,7-dihydroxy-13-methoxy-3,4-dihydro-α-ylangene (1), together with a known nardosinane-type sesquiterpene, 6,7-seco-13-nornardosinane (2), and three known steroids: 24-methylcholesterol (3), 24-methylenecholesterol (4), and 4α,24-dimethyl-5α-cholest-24(28)-en-3β,8β-diol (5). The structures of all isolated metabolites were elucidated through comprehensive spectroscopic analyses, including one- and two-dimensional nuclear magnetic resonance experiments, high-resolution electrospray ionization mass spectrometry, and electronic circular dichroism calculations. The antitumor activities of the purified sesquiterpenoid compounds were assessed against two cancer cell lines, MCF-7 (breast adenocarcinoma) and HT-29 (colorectal adenocarcinoma), using doxorubicin as the reference drug. Metabolite 1 exhibited cytotoxic effects against MCF-7 (half-maximal inhibitory concentration [IC₅₀] = 15.7 ± 1.21 µM) and HT-29 (IC₅₀ = 12.9 ± 1.22 µM), whereas compound 2 showed cytotoxicity toward MCF-7 (IC₅₀ = 22.5 ± 1.54 µM) and HT-29 (IC₅₀ = 17.8 ± 1.35 µM). Although these compounds display limited potency, their distinctive structural features broaden the chemical diversity of Lemnalia-derived metabolites and may offer promising scaffolds for future chemical or biosynthetic optimization.