New azaphilone, isocoumarin and α-pyrone derivatives from the marine-derived gut fungus Paraphaeosphaeria sp. XZD2-1
Graphical abstract
Six new compounds including two azaphilones, lunatoic acids D–E (1, 2), three isocoumarins, lunatinins B–D (3-5), and one α-pyrone derivative, lunatinin E (6), as well as four known ones, lunatoic acid A (7), lunatinin (8), penicipyran D (9) and chaetoquadrin F (10) were isolated from the rice medium of the gut fungus Paraphaeosphaeria sp. XZD2-1.
Introduction
In the research and development process of lead compounds for drugs, marine-derived fungi are an abundant source for their novel and bioactive secondary metabolites (Blunt et al., 2018). The genus Paraphaeosphaeria which has reported to yield many new metabolites were often found as plant endophytic fungi (de Amorim et al., 2019; Huang et al., 2017), insect gut fungi (Shao et al., 2016) or marine-derived fungi (Tsuda et al., 2003). In our study for new natural products from marine-derived fungi, one strain Paraphaeosphaeria sp. XZD2-1 was isolated from the gut of a sea shrimp. OSMAC strategy indicated the fungus mainly produced diketopiperazine alkaloids in T.G. + salt liquid medium (Figure S64), but showed rich secondary metabolites when cultured on solid rice medium. A chemical investigation of the EtOAc extract from the rice medium resulted in the isolation of six new compounds, lunatoic acids D–E (1, 2), lunatinins B–E (3-6). The remaining four known compounds were identified as lunatoic acid A (7) (Nukina and Marumo, 1977), (+)-lunatinin (8) (Cai et al., 2018; Yamaguchi et al., 2004), penicipyran D (9) (Ma et al., 2017) and chaetoquadrin F (10) (Fujimoto et al., 2003), as shown in Fig. 1, by comparison of their NMR, MS data and optical rotations (Table S3) with those reported.
Section snippets
Results and discussion
Lunatoic acid D (1) was obtained as a yellowish oil. HRESIMS analysis gave an ion peak at m/z 405.1551 [M−H]−, indicating a molecular formula of C21H26O8, corresponding to 9 degrees of unsaturation. The IR spectrum showed the presence of a hydroxyl (3242 cm-1) and a conjugated carbonyl (1648 cm-1). The 1H NMR spectrum (Table 1) showed four olefinic proton signals (δH 6.12, 6.31, 6.42, 7.11), and four methyl signals (δH 0.84, 0.85, 1.05, 1.70). The 13C NMR spectrum exhibited two ketone carbonyls
General experimental procedures
Optical rotations were measured on a Rudolph Research Analytical Autopol I automatic polarimeter. UV spectra were recorded on a Shimadzu UV-1800 spectrophotometer. IR spectra were taken on a Thermo Scientific Nicolet iS10 spectrometer using an attenuated total reflectance (ATR) method. CD spectra were measured on a JASCO J-1500 CD spectrometer. HRESIMS spectra were performed on an Agilent 6224 TOF LC-MS spectrometer. NMR spectra were recorded on a JEOL 600 MHz NMR spectrometer with TMS as an
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
Acknowledgment
This work was supported by the National Natural Science Foundation of China (NSFC No.41406141). We thank Jianyang Pan (Pharmaceutical Informatics Institute, Zhejiang University) and Ying Liu (Ocean College, Zhejiang University) for performing NMR spectrometry.
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