A New Phenolic Acid Derivative from an Insect-Derived Fungus Nigrospora sphaerica (Strain No. ZMT05)
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A NEW PHENOLIC ACID DERIVATIVE FROM AN INSECT-DERIVED FUNGUS Nigrospora sphaerica (STRAIN No. ZMT05)
Weijia Ding, Zhihui Wu, Chan Qin, Jiayan Xie, Min Chen, and Chunyuan Li*
A new phenolic derivative, ethyl 4-ethoxy-2-hydroxy-3,6-dimethylbenzoate (1), was obtained from the fungus Nigrospora sphaerica ZMT05 isolated from Oxya chinensis Thunberg. Its structure was elucidated by comprehensive spectroscopic analyses. This compound showed high antifungal activity against Penicillium italicum and moderate activity against Fusarium oxysporum, Colletotrichum musae, and Fusarium graminearum. Keywords: phenolic acid, antifungal activity, Oxya chinensis Thunberg, Nigrospora sphaerica. Oxya chinensis Thunberg (Orthoptera: Acrididae) is a main insect pest in Oryza sativa L., occurring throughout rice-growing areas of China [1]. It feeds on different plants such as Oryza sativa L., Saccharum officinarum L., Zea mays L., Sorghum vulgare Pers., Glycine max (L.) Merr., Triticum aestivum L., Gossypium hirsutum L., and Phragmites communis Trin. [2]. In our ongoing project to search for leads of new agricultural fungicides from microorganisms [3–7], a fungus Nigrospora sphaerica (strain No. ZMT05) isolated from Oxya chinensis Thunberg caught our attention due to its antifungal activities against several plant pathogens in vitro [8, 9]. In this study, a new phenolic derivative, ethyl 4-ethoxy-2-hydroxy3,6-dimethylbenzoate (1), was purified from this strain. The isolation, structure elucidation, and antifungal activity bioassay of this compound are reported herein. Compound 1 was obtained as colorless needles. Its molecular formula was determined as C13H18O4 on the basis of HR-ESI-MS ([M + H]+ at m/z 239.1287, calcd 239.1283). The 13C NMR and HSQC spectra of 1 (Table 1) exhibited an ester carbonyl (δ 172.1, C-7), four methyls (δ 7.8, CH3-3; 14.3, C-9; 14.8, C-11; 24.7, CH3-6), two oxygenated methylenes (δ 61.1, C-8; 63.7, C-10), and a pentasubstituted benzene ring consisting of five quaternary (δ 105.4, C-1; 111.1, C-3; 140.0, C-6; 160.8, C-4; 162.2, C-2) and one methine (δ 106.8) olefinic carbons. HMBC correlations from H-9 (δ 1.41, t, J = 7.2 Hz) to C-8 and from H-11 (δ 1.42, t, J = 7.2 Hz) to C-10, together with the coupling relations between H-8 (δ 4.39, q, J = 7.2 Hz) and H-9 and between H-10 (δ 4.08, q, J = 7.2 Hz) and H-11, indicated the presences of two ethoxyl groups formed by the connectivity of C-8 with C-9 and of C-10 with C-11. Moreover, HMBC correlations from H-8 to C-7 suggested that C-8 and C-7 were combined through an oxygen bridge into a –COOCH2CH3 group. The 1H NMR spectrum revealed the presence of a hydrogenbonded hydroxyl group (δ 11.90, 2-OH) associated with the carbonylic oxygen, suggesting that the hydroxyl (2-OH) was adjacent to the –COOCH2CH3 (C-7, C-8, C-9). HMBC correlations from the proton of CH3-3 to C-2, C-3, and C-4 and from H-10 to C-4 suggest that CH3-3 was adjacent to both 2-OH and 4-ethoxyl (C-10, C-11). Furthermore, HMBC correlations from H-5 to C-1 and C-4 and from the proton of CH3-6 to C-1 and C
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