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  • The molecular formula of was determined

    2021-10-19

    The molecular formula of 2 was determined to be C20H26O5 on the basis of HRESI (−) MS data at m/z 345.1705 [M–H]− (calcd 345.1702 for C20H25O5). Detailed analysis of the 1H and 13C NMR data of 2 revealed similar fragments compared with desoxyarticulin (Faini et al., 1987) except for an additional hydroxyl group present in 2. The chemical shift value of the 16-methylene signal at δH 4.74 (doublet) in desoxyarticulin was down-fielded to δH 5.87 (singlet) in 2 implying that the additional moiety was anchored at C-16. This suggestion was further confirmed by a 2D NMR experiment. The relative configuration of 2 was determined to be the same as that of compound 1 by the NOESY correlations. Compound 2 also showed negative Cotton effect at 244nm in the CD spectrum as 1, suggesting the absolute configuration of C-5 as 5S. Thus, the absolute configuration of compound 2 was determined as 5S, 8R, 9R, 10R. The molecular formula of compound 6 was established to be C18H28O4 from the [M–H]− peak at m/z 307.1908 (calcd 307.1909 for C18H27O4) in the HRESIMS. Analysis of the 1H, 13C NMR spectral data of 6 revealed similar characteristics with those of norhardwikiic auz methyl ester (Avila et al., 1992) except for a missing methoxyl group in 6. The HMBC correlations from the methoxyl protons at δH 3.67 to C-13 confirmed that C-13 was connected to a methoxyl group, and that the C-18 was caryboxyl group. The relative configuration of 6 was determined by a NOESY experiment in a similar fashion as that for the norhardwikiic acid methyl ester. The molecular formula of 7 was assigned as C20H26O6 by HRESIMS ([M–H]−; m/z 361.1657; calcd 361.1651). The NMR spectroscopic spectra suggested the presence of same structural features as those of 16(R and S)-3,13-Z-kolavadien-16,15-olide-2-one (Kijjoa, et al.,1990) except that the 18-methyl was oxygenated to the corresponding carboxyl group, and this hypothesis was confirmed by 2D NMR data. The relative configuration of 7 was determined by the NOESY experiment. The HRESIMS (m/z 345.1704, [M–H]−) of dichrocephnoid E (8) gave a molecular formula of C20H26O5. The 1H and 13C NMR data of 8 revealed the presence of the same hexahydronaphthalen-2(1H)-one as that of 7, where the acetal proton H-16 in compound 7 was found to be an oxymethyene unit in compound 8. In the 1H–1H COSY NMR the additional oxymethene unit displayed a correlation with the olefinic proton H-4, which suggested that the carboxyl group in 7 was reduced to an oxymethene, and the acetal carbon (C-16) was oxygenated to the corresponding carboxyl moiety. This result was confirmed by HMBC correlations. The relative configuration of 8 was determined by a NOESY experiment. Considering the nearly similar NMR data and same biosynthesis (Zdero et al., 1991), the absolute configurations of 6–8 were postulated to be same as their analogues. The three known compounds, mkapwanin (3) (Omosa et al., 2010), marrubiastrol (4) (Rudolf and Bernhard, 1978), and desoxyarticulin (5) (Faini et al., 1987) were identified by comparing their 1H and 13C NMR data with the reported literature. The inhibitory HIV-1 integrase activity of all the isolated compounds was evaluated. The results displayed that compounds 1–6 showed potential inhibitory activity against HIV-1 integrase with IC50 values of 36.82±2.43μM, 34.66±1.69μM, 39.70±2.23μM, 33.36±2.16μM, 35.72±2.63μM and 12.35±1.27μM, respectively (Table 3). In summary, five new clerodane diterpenoid dichrocephnoids A–E (1,2, 6–8) together with three known analogues were isolated from the whole herb of D. benthamii C.B. Clarke. All of the isolated compounds exhibited inhibition activity against HIV-1 integrase, and dichrocephnoid C (6) showed the most potent inhibition with IC50 value of 12.35±1.27μM. To the best of our knowledge, it is the first time to report this class of diterpenoids with inhibition activity against HIV-1 integrase.