Synthesis and biological activity of myricetin derivatives containing 1,3,4-thiadiazole scaffold

Background Myricetin and 1,3,4-thiadiazole derivatives were reported to exhibit favorable antiviral and antibacterial activities. Aiming to discover novel myricetin analogues with potent activities, a series of novel myricetin derivatives containing 1,3,4-thiadiazole moiety were synthesized, and their antibacterial and antiviral activities were evaluated. Result Bioassay results indicated that some target compounds exhibited potential antibacterial and antiviral activities. Among them, compounds 2, 3a, 3b, 3d, 3f, 3i, 3m and 3p exhibited excellent antibacterial activities against Xanthomonas oryzae pv. Oryzae (Xoo), with EC50 values of 42.7, 38.6, 20.8, 12.9, 22.7, 27.3, 18.3 and 29.4 μg/mL, respectively, which were better than that of thiadiazole-copper (94.9 μg/mL). Compounds 3b, 3d, 3e, 3f, 3i and 3o showed good antibacterial activities against Ralstonia solanacearum (Rs), with EC50 values of 37.9, 72.6, 43.6, 59.6, 60.6 and 39.6 μg/mL, respectively, which were superior to that of thiadiazole-copper (131.7 μg/mL). In addition, compounds 3d, 3f, 3i and 3m showed better curative activities against tobacco mosaic virus (TMV), with EC50 values of 152.8, 99.7, 127.1, and 167.3 μg/mL, respectively, which were better than that of ningnanmycin (211.1 μg/mL). Conclusions A series of myricetin derivatives containing 1,3,4-thiadiazole scaffold were synthesized, and their antibacterial activities against Xoo and Rs and their antiviral activity against TMV were evaluated. Bioassays indicated that some target compounds exhibited potential antibacterial and antiviral activities. These results indicated this kind of myricetin analogues could be further studied as potential alternative templates in the search for novel antibacterial and antiviral agents. Electronic supplementary material The online version of this article (doi:10.1186/s13065-017-0336-7) contains supplementary material, which is available to authorized users.


Background
The rational use of agrochemicals plays a pivotal role in agricultural production by effectively controlling plant diseases [1,2]. Unfortunately, the application of traditional pesticides is greatly limited due to their negative impacts on the environment and the rapid emergence of resistance [2,3]. Therefore, searching for high-efficiency and environmentally friendly agrochemicals remains an arduous challenge in pesticide chemistry [1,4]. In this process, natural products and their derivatives with new modes of action have been developed as pesticides that are safe to the environment [5,6].
As one of important natural products in medicinal chemistry, myricetin was reported to exhibit extensive bioactivities including antibacterial [7], antiviral [8], anticancer [9], anti-inflammatory [10], antioxidant [11], and hypoglycemic activities [12]. Our previous study extracted a mixture containing myricetin from the bark of Toona sinensis and found it to exhibit moderate antiviral activity against tobacco mosaic virus (TMV) [13]. Using natural myricetin as the lead molecule, some myricetin derivatives bearing Schiff-base moiety, which displayed good inhibitory activity against telomerase and excellent anticancer activity against human breast cancer cells MDA-MB-231, were synthesized by Xue et al. [14]. Furthermore, the acceptable antibacterial activities against Xanthomonas oryzae pv. oryzae (Xoo) and Ralstonia solanacearum (Rs) of myricetin derivatives containing acidamide moiety were also recently reported by us [15]. Obviously, myricetin derivatives as possible active ingredients play a key role in the searching for novel agrochemicals and pharmaceuticals (Fig. 1).
Considering these above results, we speculated that introducing 1,3,4-thiadiazole fragment into myricetin might generate novel lead compounds with greater biological activities. Thus, a series of myricetin derivatives containing 1,3,4-thiadiazole scaffold were synthesized (Scheme 1), and their antibacterial activities against Xoo and Rs and their antiviral activity against TMV were evaluated.

Results and discussion
Chemistry A series of myricetin derivatives containing thiadiazole moiety were successfully prepared in two steps in our current work. All of the target compounds 2, 3a-3q were characterized by infrared spectrum (IR), nuclear magnetic resonance (NMR) spectroscopy, and high resolution mass spectrum (HRMS) analysis. The IR spectral data of compounds 2, 3a-3q showed characteristic frequencies at 1723-1709 cm −1 and 1640-1621 cm −1 , which are assigned to the characteristic vibrations of C=O and C=N-, respectively. In the 1 H NMR spectra, the characteristic −CH 2 -groups between myricetin scaffold and 1,3,4-thiadiazole heterocycle was observed  13 C NMR spectra confirmed the existence of C=O and C=N-groups, respectively.

Antibacterial activity screening of the title compounds against Xac and Rs in vitro
Using Ralstonia solanacearum (strain MR111, Guizhou University, China) and Xanthomonas oryzae pv. oryzae (strain PXO99A, Nanjing Agricultural University, China) as the tested bacterial strains, the antibacterial activities of title compounds have been evaluated by the turbidimeter test [1,3,4,6], and the commercial agent thiadiazole-copper was tested as the control. Some compounds with good antibacterial activity against Xoo and Rs were tested at five double-declining concentrations (100, 50, 25, 12.5 and 6.25 μg/mL) to obtain the corresponding EC 50 values.
To further understand antibacterial activity of synthesized compounds, the EC 50 values of some target compounds, which exhibited better antibacterial activities against Xoo and Rs than thiadiazole-copper, were calculated and summarized in Table 2 39.6 μg/mL, respectively, which were superior to that of thiadiazole-copper (131.7 μg/mL). The inhibitory rates in Tables 1 and 2 indicated that most synthesized compounds bearing the same substituted fragment were found to exhibit better antibacterial activity against Xoo than Rs. For example, the EC 50 values of title compounds 3b, 3d, 3f and 3i against Xoo were respectively 20.8, 12.9, 22.7 and 27.3 μg/mL, which were better than that against Rs (37.9, 72.6, 59.6 and 60.6 μg/mL, respectively). The antibacterial results in Tables 1 and 2 also indicated that the different groups on R had significant effects on the antibacterial activity of the target compounds. Obviously, the presence of heterocycles can effectively enhance the antibacterial activity against Xoo. As examples of this phenomenon, the compounds 3m and 3p, which contain respectively 2-Cl-thiazol-5-yl and pyridin-3-yl groups, exhibited fine antibacterial activities against Xoo at 50 μg/mL, with the inhibition rates of 80.8 and 71.2%, respectively, which were superior to that of thiadiazole-copper (28.7%). Meanwhile, when R was substituted with 4-NO 2 Ph, 4-ClPh, 2-ClPh and 2,4-di-ClPh groups, the corresponding compounds 3b, 3d, 3f and 3i exhibit remarkable antibacterial activities against Xoo, with the EC 50 values of 20.8, 12.9, 22.7 and 27.3 μg/mL, respectively, which were better than that of thiadiazole-copper (94.9 μg/mL).

Antiviral activity screening of the title compounds against TMV in vivo
Using growing N. tobacum L. leaves at the same age as the test subjects, the curative and protective activities against TMV were evaluated based on the half-leaf blight spot method [25][26][27], and the commercial agent ningnanmycin was tested as the control under the same conditions. The antiviral activity against TMV in vivo at 500 μg/mL was listed in Tables 3 and 4. The preliminary bioassays results indicated that the inhibitory rates of title compounds against TMV at 500 μg/mL ranged from 18.2 to 68.4% in terms of their curative activity, and ranged from 21.5 to 60.8% in terms of their protective activity. Among them, the inhibitory rates of compounds 3d, 3f, 3i and 3m in curative activity were 59.8, 68.4, 66.8 and 57.1%, respectively, which were better than that of ningnanmycin (51.8%). Moreover, compounds 3c, 3i and 3m were found to exhibit significant protective activities (58.4, 60.8 and 56.7%, respectively), which were similar to ningnanmycin (58.3%).
The antiviral results in Tables 3 and 4 indicated that most of synthesized compounds bearing the same substituted fragment exhibited better protective activity than curative activity against TMV. Meanwhile, Results in Tables 3 and 4 also indicated that the different groups on R had significant effects on the anti-TMV activity of the target compounds. Obviously, the presence of benzyl chloride groups can effectively enhance the curative activity of title compounds against TMV. For example, compounds 3d, 3f, 3i and 3m, which contain respectively 2-ClPh, 4-ClPh, 2,4-di-ClPh and 2-Cl-thiazol-5-yl groups, exhibited excellent curative activities against TMV, with the EC 50 values of 152.8, 99.7, 127.1 and 167.3 μg/mL, respectively, which were better than that of ningnanmycin (211.1 μg/mL). Furthermore, when the R was 2-MePh, 2,4-di-ClPh and 2-Cl-thiazol-5-yl groups, the protective activities of corresponding compounds 3c, 3i and 3m at 500 μg/mL were 58.4, 60.8 and 56.7%, respectively, which were similar to that of ningnanmycin (58.3%).

General synthesis procedures for title compounds 3a-3p
To a solution of 2 (1.16 mmol) in acetonitrile (30 mL), sodium carbonate (1.74 mmol) and CH 3 I (1.74 mmol) were added, and the reaction mixture was stirred at 40 °C for 5 h. After the reaction was completed and cooled to room temperature, a solid precipitated was filtered and recrystallized with methanol to obtain the title compound 3a. Based on the similar method, the title compounds 3b-3p were prepared.