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

ABSTRACT Phytochemical investigation of the CH 2 Cl 2 /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 (1 S ,2 S ,3 S ,6 R ,7 R ,8 R )‐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‐3 β ,8 β ‐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 [IC 50 ] = 15.7 ± 1.21 µM) and HT‐29 (IC 50 = 12.9 ± 1.22 µM), whereas compound 2 showed cytotoxicity toward MCF‐7 (IC 50 = 22.5 ± 1.54 µM) and HT‐29 (IC 50 = 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.