Abstract
Six new 9-hydroxybriarane diterpenoids, briarenolides ZI–ZVI (1–6), were isolated from a gorgonian coral Briareum sp. The structures of briaranes 1–6 were elucidated by spectroscopic methods and by comparison of their spectroscopic data with those of related analogues. Briarenolides ZII (2) and ZVI (6) were found to significantly inhibit the expression of the pro-inflammatory inducible nitric oxide synthase (iNOS) protein of lipopolysaccharide (LPS)-stimulated RAW264.7 macrophage cells.
Highlights
The briarane-type diterpenoid (3,8-cyclized cembranoid), 2β-acetoxy-2-(debutyryloxy)-stecholide E, was first isolated from the gorgonian coral Briareum sp. in 1996 [1]
The methine unit at δC 56.2 was more shielded than expected for an oxygenated carbon and was correlated to the methine proton at δH 5.54 (H-6) in the heteronuclear multiple quantum coherence (HMQC) spectrum, and this proton signal was 3J-correlated with H-7 in the 1H–1H COSY spectrum, which proved that a chlorine atom was attached at C-6
At a concentration of 10 μM, briarenolides ZII (2) and ZVI (6) were found to significantly reduce the levels of inducible nitric oxide synthase (iNOS) to 47.2% and 55.7%, respectively, in comparison to the control cells stimulated with LPS only (Figure 4 and Table 7)
Summary
The briarane-type diterpenoid (3,8-cyclized cembranoid), 2β-acetoxy-2-(debutyryloxy)-stecholide E, was first isolated from the gorgonian coral Briareum sp. in 1996 [1]. The remaining hydroxy and acetoxy groups had to be positioned at C-11 and C-14, respectively, as indicated by analysis of 1H–1H COSY correlations and characteristic NMR signal analysis. The methine unit at δC 56.2 was more shielded than expected for an oxygenated carbon and was correlated to the methine proton at δH 5.54 (H-6) in the heteronuclear multiple quantum coherence (HMQC) spectrum, and this proton signal was 3J-correlated with H-7 (δH 4.73) in the 1H–1H COSY spectrum, which proved that a chlorine atom was attached at C-6 These data, together with the HMBC correlations between H-17/C-9, -18 and -19 and H3-18/C-8, -17 and -19, established the molecular framework of 1. It was found that the spectral data (IR, 1H and 13C NMR) of 5 (Table 5) were similar to those of a known briarane, excavatolide Z (9) [8] (Figure 1), except that 9 exhibited signals representing an n-butyrate substitution, which were replaced by a hydroxy group in 5. 8.7 ̆ 3.8 100 ̆ 0 91.4 ̆ 16.6 47.2 ̆ 7.2 63.7 ̆ 12.0 76.4 ̆ 13.0 74.0 ̆ 9.4 55.7 ̆ 6.1 14.2 ̆ 7.3 a Dexamethasone (DEX) was used as a positive control; COX-2: cyclooxygenase 2; iNOS: inducible nitric oxide synthase; LPS: liposaccharide
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