Abstract
Three new polyoxygenated diterpenoids with a rare 4-isopropyl-1,5,8a-trimethylperhydrophenanthrane structure of the klysimplexane skeleton, briarols A‒C (1‒3), and one eunicellin-based diterpenoid, briarol D (4), were isolated from Briareum violaceum, a gorgonian inhabiting Taiwanese waters. The chemical structures of these compounds were determined by employing extensive analyses of NMR and high-resolution electrospray ionization mass spectrometry (HRESIMS) data. Metabolites 1‒3 were found to possess the rarely found skeleton of the diterpenoid klysimplexin T. All isolated compounds showed very weak cytotoxic activity against the growth of three cancer cell lines. A plausible biosynthetic pathway for briarols A‒C from the coexisting eunicellin diterpenoid briarol D (4) was postulated.
Highlights
Gorgonian corals belonging to genus Briareum (Cnidaria, Octocorallia, Briareidae) inhabiting the western Pacific Ocean and Caribbean waters have been found to be a rich source of diterpenoids [1,2] possessing fused bicarbocyclic structures of briarane [3,4,5,6,7], eunicellin [8,9,10,11], and asbestinane [12,13,14] types, in addition to cembranoids [15,16]
Our previous study on the chemical constituents of Briareum violaceum afforded the isolation of briarellins (2,9:3,16diepoxyeunicellins), which were shown to possess interesting structures generated from intramolecular cyclization of corresponding cembranoids [10]
Briarols A-C (1-3), were identified as compounds of a rare (4-isopropyl-1,5,8a-trimethylperhydrophenanthrane) skeleton, which was discovered for only one time as klysimplexin T in 2011 [25] and is denominated as the klysimplexane skeleton (Figure 1)
Summary
Gorgonian corals belonging to genus Briareum (Cnidaria, Octocorallia, Briareidae) inhabiting the western Pacific Ocean and Caribbean waters have been found to be a rich source of diterpenoids [1,2] possessing fused bicarbocyclic structures of briarane [3,4,5,6,7], eunicellin [8,9,10,11], and asbestinane [12,13,14] types, in addition to cembranoids [15,16] Many of these metabolites exhibit a wide range of bioactivities, including anti-inflammatory [3,11,17,18,19,20], cytotoxic [14,21,22,23], antiviral [14,21,24], antimalarial [8], antimicrobial [14], and analgesic [20] activities. The structure elucidation of the new metabolites was performed by extensive spectroscopic analyses, including two-dimensional (2D) NMR correlation and high-resolution electrospray ionization mass spectroscopy (HRESIMS)
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