New Briarane Diterpenoids from the Gorgonian Coral Junceella juncea

Chemical investigation of Junceella juncea has resulted in the isolation of three new briaranes designated juncenolides M–O (1–3). The structures of these compounds were determined by spectroscopic analysis including 2D-NMR (COSY, HMBC and NOESY) and HRMS. Compound 1 is a new chlorinated briarane while compound 3 contains a rare methyl ester at C-16. The anti-inflammatory activities tested on superoxide anion generation and elastase release by human neutrophils in response to FMLP/CB were evaluated.


Introduction
Gorgonian corals of the genus Junceella (Ellisellidae) are common in subtropical and tropical waters in a number of places around the world, such as the South China Sea and Indo-Pacific Ocean, and are well known as a source of highly oxidized diterpenes of the briarane class (3,8-cyclized cembranoids) [1]. Many in vitro and in vivo studies on diterpenes isolated from gorgonians showed a variety of biological activities including anti-tumor, anti-inflammatory, antiplasmodial, antibacterial, antiviral, antimalarial OPEN ACCESS and antioxidant, as well as ecologically relevant activities such as fish-feeding deterrence. Diterpenes isolated from gorgonian corals have a large structural diversity with 40 different diterpene classes being represented [2]. Recently, three new 8-hydroxybriarane diterpenoids (junceols A-C) and four new briarane diterpenoids (juncenolides H-K) were reported from a chemical investigation of Junceella juncea Pallas collected off the southern Taiwan coast, and some of these metabolites were found to exhibit inhibitory effects on superoxide anion generation and elastase release by human neutrophils [3,4]. Fourteen new briarane diterpenes, juncins O-ZII, were isolated from the EtOH/CH 2 Cl 2 extract of a South China Sea sample of J. juncea and some of them have been shown to exhibit potent antifouling and antifeedant activities [5][6][7]. A bioassay-guided fractionation of the acetone extract of a Taiwanese collection of J. juncea led to the identification of seven new diterpenoids, juncenolides A-G [8][9][10][11]. Moreover, a chemical investigation of the Indian Ocean gorgonian J. juncea resulted in the isolation of eight new briarane-type diterpenoids, juncins G-N [12][13][14]. A new briarane diterpenoid with antifungal activity was also isolated [15]. In continuation of our research programs oriented towards discovering new metabolites from the gorgonians collected off Taiwanese waters, we reinvestigated J. juncea. Examination of different chromatographic fractions of an AcOEt-soluble extract of the Taiwanese J. juncea Pallas resulted in the isolation of three new briaranes, designated juncenolides M-O (1-3) (Figure 1). Their structures were elucidated through detailed spectroscopic analyses, mainly 2D NMR experiments ( 1 H, 1 H COSY, HQMC, HMBC). The relative stereochemistry of the chiral centers and the geometry of the double bonds were deduced from NOESY spectra.
The isolated briaranes 1-3 were tested on inhibitory effects of superoxide anion generation and elastase release by human neutrophils in response to FMLP/CB at a concentration of 10 μg/mL. As illustrated in Table 3, compounds 2 and 3 showed moderate inhibitory activities against elastase release with 29.0 ± 5.6%, and 35.9 ± 7.4%, respectively. Furthermore, compound 3 also exhibited moderate inhibitory activity against superoxide anion with 27.6 ± 7.0%. Table 3. Effects of compounds 1-3 on superoxide anion generation and elastase release by human neutrophils in response to FMLP/CB a .

General
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Animal Material
The gorgonian Junceella juncea Pallas (Ellisellidae) was collected in Tai-Tong County, Taiwan, by scuba diving at a depth of 15 m, in November 2006. The fresh gorgonian was immediately frozen after collection and kept at −20 °C until processed. This species was identified by one of the authors (C.-C.L). A voucher specimen (WSG-5) was deposited in the School of Pharmacy, College of Medicine, National Taiwan University, Taiwan.

Anti-Inflammatory Assays
Neutrophils were obtained by means of dextran sedimentation and Ficoll centrifugation. Superoxide generation and elastase release were carried out according to a procedure described previously [20]. Superoxide anion production was assayed by monitoring the superoxide dismutase-inhibitable reduction of ferricytochrome c. Elastase release experiments were performed using MeO-Suc-Ala-Ala-Pro-Valp-nitroanilide as the elastase substrate. Genistein was used as a positive control.

Conclusions
Three new diterpenoids, named juncenolides M-O (1-3), were isolated from the Taiwanese gorgonian Junceella juncea Pallas. Compound 1 is a new chlorinated briarane, compound 2 is a new brierane with a free hydroxy at C-2, while compound 3 contains a rare methyl ester at C-5. The anti-inflammatory activities tested on superoxide anion generation and elastase release by human neutrophils in response to FMLP/CB were evaluated. As a result, compounds 2 and 3 showed moderate inhibitory activities against elastase release at 10 μg/mL.