A number of secondary metabolites with potential pharmacological activities such as cytotoxic, antiviral, anti-inflammatory, and insecticidal effects were discovered from marine organisms [1]. Marine diterpenoids of the class briarane have been investigated with great interest owing to their novel structures and interesting bioactivities [2–5]. The gorgonian of the genus Junceella grown in the tropical and subtropical waters of Indo-West Pacific regions are well known as a source of highly oxidized briarane-type diterpenoids with a γ-lactone moiety [6–9]. In continuation of our study on the chemistry and biological activities of briarane diterpenoids [10–16], we investigated the Taiwanese gorgonian J. fragilis. A chemical investigation of the acetone extract has yielded four new 8-hydroxybriarane diterpenoids, designated as frajunolides L–O (1–4). In this paper, we report the isolation, structural elucidation, and anti-inflammatory activity as tested by superoxide anion generation and elastase release by human neutrophil in response to fMLP/CB, of these compounds.

Compound 1 was deduced to have the molecular formula C₂₈H₃₈O₁₁ with ten degrees of unsaturation from high-resolution ESI mass spectrometry. The IR absorptions were observed at 3439, 1768 and 1735 cm⁻¹ suggesting the presence of hydroxyl, γ-lactone and ester groups, respectively. The ¹H-, ¹³C-NMR and DEPT spectroscopic data (Table 1) revealed that compound 1 possessed four acetyl groups (δ_H 1.94, 1.98, 2.13, and 2.21), two tertiary methyl protons (δ_H 1.15, Me-15; δ_H 2.03, Me-16), a doublet methyl (δ_H 1.15, d, J = 6.9 Hz, Me-19), five oxygenated methine protons (δ_H 4.94, t, J = 3.3 Hz, H-2; δ_H 5.28, d, J = 9.6 Hz, H-7; δ_H 5.62, d, J = 5.1 Hz, H-9; δ_H 5.32, m, H-12; 4.77, br s, H-14), a trisubstituted olefinic group (δ_H 5.58, d, J = 9.6 Hz, H-6; δ_C 120.0, C-6; δ_C 145.2, C-5), an oxygenated quaternary carbon (δ_C 82.6, C-8), an exocyclic double bond (δ_H 5.34, 5.30, H₂-20; δ_C 118.3, C-20; 146.3, C-11), two methine carbons (δ_C 40.6, C-10; δ_C 43.2, C-17), three methylene carbons (δ_C 30.8, C-3; δ_C 29.0, C-4; δ_C 33.5, C-13), along with a γ-lactone carbonyl carbon (δ_C 175.9, C-18). The proton and carbon assignments of 1 were completely established by using 1D- and 2D NMR experiments, including ¹H-¹H COSY, HSQC, and HMBC (Figure 1). The ¹H-¹H COSY spectrum exhibited four sets of correlations (H-2/H-3/H-4, H-6/H-7, H-9/H-10, and H-12/H-13/H-14). The HMBC correlations of Me-15 (δ_H 1.15, s)/C-1 (δ_C 47.0), C-2 (δ_C 74.2), C-10 (δ_C 40.6), C-14 (δ_C 73.6); Me-16 (δ_H 2.03, s)/C-4 (δ_C 29.0), C-5 (δ_C 145.2), C-6 (δ_C 120.0); Me-19 (δ_H 1.15, d, J = 6.9 Hz)/C-8 (δ_C 82.6), C-18 (δ_C 175.9); H-9 (δ_H 5.62, d, J = 5.1 Hz)/C-8, C-7 (δ_C 78.0); H-10 (δ_H 3.57, d, J = 5.1 Hz)/C-1, C-2, C-11 (δ_C 146.3), C-12 (δ_C 71.5); H₂-20 (δ_H 5.34, s; 5.30, s)/C-11, C-12; H-13 (δ_H 1.95, m; 2.20, m)/C-14, C-1 established the connectivities from C-1 to C-20 unambiguously, and revealed that compound 1 belongs to 8-hydroxybriarane diterpenoids with a γ-lactone ring [11]. The four acetate groups of 1 were assigned at C-2, C-9, C-12, and C-14 positions by the HMBC correlations between the acetate carbonyl carbons (δ_C 170.4 × 2, 170.3, and 168.9) and four oxygenated methine protons (δ_H 4.94, H-2; δ_H 5.62, H-9; δ_H 5.32, H-12; 4.77, H-14). Thus the planar structure of 1 was completely established.

Our results showed that the planar structure of compound 1 is the same as frajunolide A, but differing in the ¹H- and ¹³C NMR data of the methylenecyclohexane ring, especially at C-12 and C-20 positions [10]. The ¹³C NMR chemical shift of C-12 (δ_C 71.5) was shifted downfield in comparison with frajunolide A (δ_C 67.3), suggesting that the relative stereochemistry of H-12 was α-orientation [11]. The relative configuration of 1 was determined by NOESY correlations (Figure 1) and MM2 minimized energy calculated molecular modeling, and comparison with other naturally occurring briarane diterpenoids [2–5]. Briarane-type diterpenoids were reported to have the Me-15 in the β-orientation and H-10 in the α-orientation. In the NOESY of 1, H-10 showed correlations with H-2, H-9, H-12, suggesting that these protons are located on the α-face. In addition, the correlation between H-9 and Me-19 indicated that Me-19 is α-oriented too. However, correlation of H-17/H-7 suggested that H-7 and H-17 are on the β-face. Moreover, NOESY correlation of H-14/Me-15 suggested that 14-acetoxy group is located on the α-face. The Z-configuration at C-5 was elucidated by the observation of NOESY correlation between H-6 and Me-16. On the basis of the above interpretation, the structure of compound 1 was elucidated. The name frajunolide L was given.

Compound 2 had the molecular formula C₂₈H₃₈O₁₁, the same as that of 1, as determined by HRESIMS, suggesting that the structure of 2 was similar to 1. The IR spectrum of 2 also displayed strong absorptions at 3429, 1776 and 1735 cm⁻¹ indicating that compound 2 contained hydroxyl and carbonyl groups of five-membered γ-lactone ring and ester groups. Both ¹H- and ¹³C NMR spectroscopic data (Table 1) of 2 were found to be similar to those of 1. These signals include four acetyl group (δ_H 1.92, 2.01, 2.07, and 2.16), two tertiary methyl protons (δ_H 0.99, Me-15; δ_H 1.99, Me-16), a methyl doublet (δ_H 1.17, d, J = 6.8 Hz, Me-19), and a methine quartet (δ_H 1.15, q, J = 7.2 Hz). However, 1D- and 2D-spectroscopic data of 2 revealed that the exocyclic double bond (C-11/C-20) in 1 shifted to C-12 (δ_C 127.6)/C-11 (δ_C 134.2) and an acetate group was found to locate at C-20 (δ_C 68.7). The gross structure of 2 was further deduced from the ¹H-¹H COSY, HMQC, HMBC correlations (Figure 2). The relative configuration of 2 was determined by NOESY correlations (Figure 2) and application of MM2 molecular modeling together with comparing the NMR spectra of 2 with those of 1. The NOESY correlations of H-10/H-2, H-9, and H-9/Me-19 suggested that the configurations of H-2, H-9, H-10, and Me-19 were in α-orientation while correlations of H-7/H-6, H-17, and H-14/Me-15 agreed with β-disposition of H-7, H-14, Me-15 and H-17.

The HRESI mass spectrum of 3 gave a quasi-molecular ion peak at m/z 589.2266 [M + Na]⁺, indicative of a molecular formula C₂₈H₃₈ClO₁₂ (calc. for m/z 589.2261), consistent with 10 degrees of unsaturation. The presence of a chloride was evident from the fragment [M + Na]⁺ at m/z 589 and the isotope fragment [M + Na + 2]⁺ at m/z 591 in ESIMS, with the typical ratio of relative intensity (3:1) in the mass spectrum. In the infrared spectrum, strong absorption bands were found at 3436, 1735 and 1780 cm⁻¹ characteristic for hydroxyl, ester carbonyl (acetyl) and five-membered γ-lactone ring, suggesting a briarane-type diterpenoid similar to compounds 1 and 2. It was found that the ¹H- and ¹³C NMR spectra of 3 in CDCl₃ showed mostly broad peaks and in some cases, certain peaks were not observed. In order to mark more optimum signals of the NMR spectra, compound 3 was dissolved in pyridine-d₅. The ¹H- and ¹³C NMR data (Table 1) of 3 revealed the presence of four acetate groups (δ_H 1.99, 2.11, and 2.30 × 2; δ_C 172.5, 171.1 × 2, 170.3, 22.2, 21.9, 21.8, and 21.5), an exocyclic double bond (δ_H 5.83, 5.42, H₂-16; δ_C 144.0, C-5; 125.5, C-16) and a γ-lactone carbonyl carbon (δ_C 176.4, C-19). Judging from the molecular formula and NMR data of 3, six degrees of unsaturation were counted for, indicating that compound 3 contained a tetracyclic system including an exocyclic epoxide (δ_H 2.84, d, J = 4.0 Hz; 2.59, br s, H₂-20; δ_C 58.2, C-11; 52.6, C-20). The HMBC correlations (Figure 3) between H-2 (δ_H 6.68, d, J = 8.5 Hz), H-4 (δ_H 5.90, d, J = 10.5 Hz), H-9 (δ_H 6.30, s), and H-14 (δ_H 5.25, s) with one of ester carbonyl carbons, respectively, revealed that four acetyl groups were connected to the C-2, C-4, C-9, and C-14 positions. By interpretation of the NMR spectroscopic data, the planar structure of compound 3 was elucidated. The relative configuration of 3 was determined by NOESY (Figure 3) and detailed comparison with known compounds [10]. The chemical shift of C-11 (δ_C 58.2) and C-20 (δ_C 52.6), and the NOESY correlations between H₂-20 and Me-15 agreed with β-face of H₂-20, 11R-configuration regarding the exocyclic epoxide, and chair conformation of the cyclohexane ring. Furthermore, NOESY correlations of H-10/H-2, H-4/H-2 and H-10/H-9 suggested that H-2, H-4 and H-9 were located on the same face and could be assigned as α.

Compound 4 showed a pair of quasi-molecular ion peaks at m/z 607 and 609 [M + H]⁺ with a ratio of 3:1 in the ESIMS, indicating the presence of a chlorine atom. Moreover, a molecular formula C₂₈H₃₇ClO₁₁ was established by HRESIMS and confirmed by ¹H- and ¹³C NMR spectroscopic analysis (Table 1). The IR absorption bands at 3467, 1780 and 1739 cm⁻¹ indicated that 4 contained hydroxyl, γ-lactone, and ester carbonyl functionalities similar to 1–3. Detailed inspection of ¹H- and ¹³C NMR spectroscopic data revealed the presence of the key structural feature of a 8-hydroxybriarane diterpenoid with two exocyclic double bonds. The locations of the two exocyclic double bonds were confirmed by the HMBC experiment (Figure 4), which showed correlations of H₂-16 (δ_H 4.92, s; 5.49, s)/C-4 (δ_C 33.4), C-5 (δ_C 144.9), and C-6 (δ_C 56.2), and H₂-20 (δ_H 4.92, s; 5.19, s)/C-10 (δ_C 44.6), C-11 (δ_C 147.0), and C-12 (δ_C 38.6), respectively. In addition, the oxygenated methine proton H-2 (δ_H 6.62, d, J = 8.0 Hz), H-9 (δ_H 6.28,), H-13 (δ_H 5.72, ddd, J = 12.0, 5.2, 3.2 Hz), and H-14 (δ_H 5.66, s) showed HMBC correlations with the acetate carbonyl carbons (δ_C 171.8, 170.9, 170.8, 170.3). Furthermore, detailed analysis of 2D NMR spectroscopic data (¹H-¹H COSY and HMBC) established the planar structure of 4. The configuration of compound 4 was determined on the basis of NOESY correlations (Figure 4). The NOESY correlations of Me-15 (δ_H 1.27, s)/H-14 and H-13/H-14 implied that H-13 and H-14 are on the β-face while correlations of H-2/H-10, H-10/H-9, H-9/Me-19, H-17/H-7 and H-6/H-7 confirmed that the configuration of these protons are identical to those of compound 3.

General pharmacological study of the anti-inflammatory activities of compounds 1–4 were evaluated by measuring superoxide anion generation and elastase release by human neutrophils in response to fMet-Leu-Phe (fMLP)/Cytochalasin B (CB) [17]. The results are illustrated in Table 2. Compounds 1 and 4 showed mild inhibitory effects on both superoxide anion generation and elastase release at 10 μg/mL. It is notable that compound 3 exhibited selective but modest inhibition of elastase release in vitro.

Chemical investigation of the Taiwanese gorgonian Junceella fragilis has resulted in the isolation of four new briarane diterpenoids, frajunolides L–O (1–4). Among them, compounds 1, 3 and 4 exhibited mild or selective anti-inflammatory activity.

Supplementary data associated with this article can be found in the online version.