Briarane-type diterpenoids from the Vietnamese gorgonian Junceella fragilis

Abstract Ten briarane-type diterpenoids (1–10), including one new stereoisomer 17-epi-junceellolide B (1), were isolated from the MeOH extract of the Vietnamese gorgonian Junceella fragilis. Their structures were elucidated by spectroscopic experiments including 1D and 2D NMR, and HR-QTOF-MS. In addition, the in vitro cytotoxic activity against eight human cancer cell lines (LNCaP, HepG2, KB, MCF-7, SK-Mel2, HL-60, LU-1 and SW480) of all isolated compounds was evaluated by SRB assays.


Introduction
Briarane-type diterpenoids are 3,8-cyclized cembranoid compounds, all containing a bicyclo[8.4.0] system, and most possess a c-lactone in their structures. This type of compounds was abundantly found in soft coral subclass Octocorallia, including Gorgonacea, Pennatulacea, Alcyonacea and Stolonifera (Su et al. 2017, Sung et al. 2002. Among Gorgonacea octocorals, Junceella species are reach sources of briaranetype diterpenoids (Sung et al. 2004, Wu et al. 2011. Previous investigations on the Junceella fragilis species reported series of briarane-type diterpenoids with novel structures and interesting bioactivities such as cytotoxic, antifouling and anti-inflammatory effects (Cheng et al. 2017, Su et al. 2017, Sung et al. 2004, Wu et al. 2011, Zheng et al. 2018. In continuation of our ongoing investigations on chemical constituents from Vietnamese gorgonians , Nam et al. 2017, Nam et al. 2018, this paper deals with the isolation, structure elucidation and in vitro cytotoxic evaluation of ten briarane-type diterpenoids (1-10) from the gorgonian J. fragilis.
17-epi-Junceellolide B (1) was obtained as a white solid with a molecular formula of C 26 H 33 ClO 9 , determined by HR-QTOF-MS exhibiting quasi-molecular ion peak at m/z 523.1745 [M-H] -. The NMR features indicated an acylated briarane-type diterpenoid, one main constituent isolated from Junceella gorgonians (Sung et al. 2004, Wu et al. 2011. The 1 H and 13 C NMR spectra exhibited typical signals of three acetyl groups  Figure S13). This evidence, and HMBC cross-peaks of 8-OH with C-8 and C-9; H-15 with C-1, C-2, C-10, and C-14; H-16 with C-4, C-5, and C-6; H-18 with C-8, C-17, and C-19; as well as H-20 with C-10, C-11, and C-12 clearly confirmed the planar structure of 1 as shown in Figure S13. The 1 H and 13 C NMR data of 1 (Table S1) were similar to those of junceellolide B (7) (Shin et al. 1989). However, the 13 C NMR signal of C-18 of 1 was strongly shifted downfield at d C 9.1 relative to that of junceellolide B (7) at d C 6.7, suggesting for a b-orientation of the methyl group at C-17. This was further confirmed by a NOESY correlation of H-7 with H-18 in 1 versus that of H-7 with H-17 in 7. The other NOESY peaks of 1 were identical to those of 7 ( Figure S13) indicating that these two compounds have the same configurations at the remaining positions. In addition, the ECD curves of 1 and 7 showed the similar cotton effects ( Figure S15) meaning that the isomerism has occurred in a chiral center that does not influence the ECD spectra or far from the chromophore that results in ECD curves of these two compounds.
The in vitro cytotoxic activity of all the isolated compounds against eight cancer cell lines, as LNCaP, HepG2, KB, MCF-7, SK-Mel2, HL-60, LU-1 and SW480, was evaluated by SRB assay (Monks et al. 1991) following the previously described protocols (Nam et al. 2018). However, among isolates, only junceellolide B (7) showed weak cytotoxicity against the LNCaP cell line with an IC 50 of 85.34 ± 4.96 lM, relative to that of the positive control ellipticine (IC 50 ¼ 1.42 ± 0.08 lM). The other compounds did not showed significant cytotoxic activity against all eight tested cancer cell lines (IC 50 > 100 lM).

Supplementary material
General experimental procedures, HR-QTOF mass spectrum of the new compound 1 as well as 1D and 2D NMR, CD spectra, HPLC chromatograms, and 1 H and 13 C NMR data of compounds 1 and 7.
Bui Huu Tai, Institute of Marine Biochemistry, VAST for measurement of the CD and HR-QTOF mass spectra.

Disclosure statement
No potential conflict of interest was reported by the authors.