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Published October 31, 2014 | Version v1
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Thiophene acetylenes and furanosesquiterpenes from Xanthopappus subacaulis and their antibacterial activities

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Zhang, Li, Chen, Chao-Jun, Chen, Jia, Zhao, Qian-Qian, Li, Ya, Gao, Kun (2014): Thiophene acetylenes and furanosesquiterpenes from Xanthopappus subacaulis and their antibacterial activities. Phytochemistry 106: 134-140, DOI: 10.1016/j.phytochem.2014.07.014, URL: http://dx.doi.org/10.1016/j.phytochem.2014.07.014

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urn:lsid:plazi.org:pub:7E3BFFB0FF80A560FFEA2F62FFA3D136

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Figure: 10.5281/zenodo.10489482 (DOI)
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References

  • 10,11-Cis -xanthopappin B (3) had the same molecular formula of C13H11OSCl as known compound 4, deduced by the analysis of their HRESIMS, 13C NMR, and DEPT spectroscopic data (Table 1 and Table 2). The chlorine atom in the molecule of 3 was clearly confirmed by the ion peak intensity ratio (3:1) between [M+] m/z 250 (95) and [M+2]+ 252 (32) in the EIMS. The detailed structural elucidation of known compound 4 was described previously (Tian et al., 2006). Analysis of the 1H NMR and 13C NMR spectra of 10,11-cis -xanthopappin B (3) established that 3 and 4 shared similar structure motifs, and the only significant difference in 3 was that the di-substituted olefinic proton signals of H-10 and H-11 exhibited a coupling constant of 10.8 Hz, which consequently suggested a cis double bond (Fusetani et al., 1989) C-10/11 for 3. Therefore, compound 3 was determined to be 5-(2-chloro-1- hydroxyethyl)-2-(Z)-hept-5-ene-1,3-diynylthiophene.
  • Compound 5 has been isolated from Leuzea carthamoides as an isomeric mixture of 5 and 6 (Laszlo et al., 1984). Compound 5 also possessed the same molecular formula, C13H12O2S, as determined by HRESIMS, as that of known compound 6 (Tian et al., 2006). The 1H NMR and 13C NMR spectra of 5 were very similar to those of 6, except for the coupling constant (J = 10.8 Hz) of H-10 and H-11, which supported a cis double bond C-10/11 for 5. Thus, 5 was determined to be 5-(1, 2-dihydroxyethyl)-2-(Z)-hept-5-ene- 1,3-diynylthiophene.
  • Compounds 7 and 8 were shown to have the same molecular formula of C13H11OSCl by analysis of their HRESIMS, 13C NMR and DEPT spectra (Table 1). The presence of a chlorine atom in the molecule of 7 was clearly confirmed by the ion peak intensity ratio (3:1) between [M+H]+ m/z 251.3 (95) and [M+H+2]+ 253.3 (32) in the ESIMS (Tian et al., 2006). The 1H NMR spectrum exhibited signals at δ H 5.68 (1H, dq, J = 10.8, 1.6 Hz), 6.10 (1H, dq, J = 10.8, 6.8 Hz) and 1.94 (3H, dd, J = 6.8, 1.6 Hz) for a cis -propenyl group. The presence of a pair of doublets at δ H 7.09 (1H, J = 3.6 Hz) and 7.02 (1H, J = 3.6 Hz) in the 1H NMR spectrum, two quaternary carbon resonances at δ C 126.1 and δ C 122.6, and two methine carbon signals at δ C 131.3 and δ C 133.1 in the 13C NMR spectrum of 7 indicated that there was a thiophene ring structural fragment and that the thiophene ring is di-substituted (Baruah et al., 1982) by electron-withdrawing groups. Moreover, the proton resonances at δ H 4.83 (1H, dd, J = 4 Hz, 6.8 Hz), 3.79 (1H, dd, J = 10.4, 4 Hz) and 3.74 (1H, dd, J = 10.4 Hz, 6.8 Hz), and the ESIMS fragment ions at m/z 201 [M-CH2Cl]+(100) and 171 [M-CH2ClCH(OH)]+(20) indicated the presence of a 2-chloro-1-hydroxyethyl group in the molecule (Bohlmann et al., 1981; Balza and Towers, 1990). This was further supported by the carbon signals at δ C 63.3 and δ C 48.9 (Table 1 and Table 2). The DEPT and 13C NMR spectra also showed four downfield-shifted alkynyl quaternary carbon resonances (δ C 79.6, 90.7, 86.4 and 91.6). The HMBC correlations H-3/C-6, H-4/ C-11, H-13/C-11, H-14/C-12, CH3-10/C-8, H-8/C-6 and H-9/C-7 indicated the attachment of the 2-chloro-1-hydroxyethyl group to C-5 through one alkyne chain and of the cis -propenyl group to C-2 through the other alkyne chain (Fig. 2). Therefore, compound 7 was determined to be 5-(but-4-chloro-3-hydroxy-1-ynyl)-2- (Z)-pent-3-ene-1-ynylthiophene. The 1H NMR and 13C NMR spectra of 8 were very similar to those of 7, with the only difference that the coupling constants (16 Hz) of the two olefinic protons H-8 and H-9 in 8 were larger than those in the cis isomer 7. Thus the structure of 8 was deduced as 5-(but-4-chloro-3-hydroxy-1- ynyl)-2-(E)-pent-3-ene-1-ynylthiophene.
  • Compound 10 was previously reported (Bohlmann and Zdero, 1967) but without any UV, IR, or [2]D values or detailed NMR spectroscopic data. The NMR spectra of 10 were nearly identical to those of compound 9 except for the resonances at δ H 5.69 (1H, dq, J = 15.6, 1.6 Hz), 6.26 (1H, dq, J = 15.6, 6.8 Hz) and 1.85 (3H, dd, J = 6.8, 1.6 Hz) for a trans -propenyl group. Thus, compound 10 was determined to be 5-(but-3,4-dihydroxy-1-ynyl)-2-(E)-pent-3- ene-1-ynylthiophene.
  • Compounds 3-10 showed positive or negative optical rotations ([2]D 25 = +10, +6, +30, -20, -10, +10, +40, +40 (c 0.1, acetone)), though the related compounds 4 and 6 were reported to be racemates (Tian et al., 2006). The absolute configurations of 3-10 remain undefined due to the limited amounts of these compounds obtained from the plant.
  • On the basis of the above work, a plausible biosynthetic pathway for thiophene acetylenes 1-10 is proposed (Scheme 1). Oleic acid is transformed into the key precursor A via repeated steps of desaturation and chain-shortening (Jacobs et al., 1995). Precursor A is then converted into the thiophene acetylenes (I a and I b) that accumulate in X. subacaulis; the key step in this conversion is the addition of H2S or its biochemical equivalent to conjugated triple bonds, which is most likely a two-step reaction (Bohlmann et al., 1973). It is reasonable to assume that I a and I b are converted into 7-10 and 3-6, respectively. The loss of one side chain from compounds 3-6 could plausibly lead to the formation of III, which would be further oxidised at the trans/cis double bond, producing a pair of erythro/threo vicinal diols (1 and 2).