Synthesis and antifungal activities of novel nicotinamide derivatives containing 1,3,4-oxadiazole

Background Pant diseases in agriculture are extremely difficult to control, which bring about for billions of dollars in economic losses worldwide each year; Nicotinamide derivatives has attracted more and more attention in the field of pesticide due to their broad bioactivities. In an effort to discover new molecules with good antifungal activity, a series of nicotinamide derivatives containing 1,3,4-oxadiazole were synthesized and bio-assayed. Results Bioassays demonstrated that some of the title compounds exhibited favorable antifungal activities against Gibberella zeae, Fusarium oxysporum, and Cytospora mandshurica. compounds 7b, 7c, 8, 9a, 9b, and 9c displayed 46.4%, 39.6%, 53.0%, 43.2%, 58.3%, 45.6% activities against G. Zeae, respectively; the activities of compounds 7a, 7b, 8, 9a, 9b, and 9c against F. oxysporum were 55.2%, 51.1%, 58.9%, 63.2%, 53.3%, and 47.6%, respectively; whereas inhibitory rates of compounds 7a, 7b, 7c, 8, 9a, 9b, and 9c on C. mandshurica were 53.1%, 49.9%, 44.9%, 52.8%, 59.8%, 54.5%, and 49.3%, respectively. Conclusion A series of the novel nicotinamide derivatives containing 1,3,4-oxadiazole were synthesized and bio-assayed. The results of antifungal tests revealed that the synthesized nicotinamide derivatives possessed weakly to moderately antifungal activities against G. zeae, F. oxysporum, and C. mandshurica. Most of the synthesized compounds exhibited similar activities as (or higher than) these of hymexozol on their corresponding fungus, and compounds 7a, 8, 9a, and 9b showed considerable prospects for further optimization. Primary structure-activity relationships revealed that the introduction of the groups of 4-chloro-6-methyl on benzene, the hydrazone group containing N,N-dimethylamino, and acetyl at -NH2 could enhance the antifungal activity.


Background
In recently years, plant diseases in agriculture are extremely difficult to control due to the continued moist weather during the crop growing season and the resistance of fungicides in the pathogen population, which bring about for billions of dollars in economic losses worldwide each year [1]. Take fusarium head blight (FHB) as example, it was caused by Gibberella zeae and was endemic in the wheat producing areas of many provinces of China, this kind of disease have caused an estimated 30 to 50% of reduction and even completely failure of harvests in many wheat-producing areas [2]. Therefore, the controlling of plant pathogens (such as G. zeae) has become more and more difficult, and the development of novel fungicidal molecules for these pathogens has been attracted more and more attention.
1,3,4-oxadiazole derivatives, another important heterocyclic derivative, has attracted more and more attention in development of fungicide, in the past few years, a lots of 1,3,4-oxadiazole derivatives with good antifungal activity have been developed by our group, some of the developed compounds processed good prospects for commercialization [19][20][21][22][23][24]. New fungicidal molecules are developed in the present work on the basis of the following: incorporation of the sub-structural unit of 1,3,4-oxadiazole into the backbone of boscalid and structural variation by the introduction of different kinds of moiety, resulting in nicotinamides containing a 1,3,4-oxadiazole substructure with broad-spectrum activity. Based on this hypothesis, a series of novel nicotinamide derivatives containing 1,3,4-oxadiazole substructure were designed and synthesized. Biological assays revealed that most of the synthesized compounds were provided with weak to moderate activities against G. Zeae, F. oxysporum, and C. mandshurica at 50 mg/L. Some of the synthesized compounds exhibited similar or higher activities as that of hymexozol on their corresponding fungus, compounds 9a, 7a, 8, and 9b showed considerable prospects.
The structures of the synthesized compounds (5-9) were established on the basis of their spectroscopic data. The IR spectra showed absorption bands around near 3230-3400 cm -1 , 2990-3150 cm -1 for the N-H, and Ar-H stretching vibrations, respectively. The absorption bands near 1670-1710 cm -1 were the presence C = O functional groups, the absorption bands of 1450-1620 cm -1 were the skeletal vibration of benzene ring. In the 1 H-NMR spectra of the title compounds, the -CONHAr proton appeared as a broad singlet at 11.88-10.27 ppm range; the -CH = N-proton for compounds 9a-9d mainly appeared as a singlet near the 8.50 ppm; the protons at 6 and 4 positions of pyridine ring were occurred as double-double peaks near 8.57-8.51 and 7.61-8.33, respectively; and the protons at 5 position of pyridine is
Primary structure activity relationships (SAR) revealed that antifungal activities on the three fungus can be improved to a certain extent by introduction of N,Ndimethylamino. For instance, compound 9a showed much higher activities against both G. Zeae and F.  oxysporum than that of 7a, and compounds 9b-9d also showed slightly better activities than these of 7b-7d. In addition, the introduction of acetyl can enhance the antifungal activity when chlorine was at 4-position of benzene, that why compound 8 showed higher activity than that of compound 7b. Moreover, the substituent on benzene play an important role when the group on 1,3,4-oxadiazole was "-NH 2 ", as we can see that compound 7a (R = 4-chloro-6-methyl) showed higher activity than that of 7b (R = 4-chloro), and compound 7c (R = 6-methyl) showed slightly lower activity, and the activity of 7d (R = H) was the lowest one. So the active order was 7a > 7b > 7c > 7d; a similar case can be found when -NH 2 group was changed to -N = CHN (CH 3 ) 2 (9a > 9b > 9c > 9d). Finally, the compound with thiol (5) displayed much lower activity than that of the compound with amino group, but the introduction of -CH 3 group (compound 6) could increase the activity against all the test fungus. Further structural modification is currently underway basing on the primary SAR to discover the potential nicotinamides containing 1,3,4-oxadiazole.

Chemistry
Unless otherwise stated, all the reagents and reactants were purchased from commercial suppliers; melting points were uncorrected and determined on a XT-4 binocular microscope (Beijing Tech Instrument Co., China). The 1 H-NMR and 13 C-NMR spectra were recorded on a JEOL ECX 500 NMR spectrometer (JEOL Ltd., Japan) at room temperature operating at 500 MHz for 1 H-NMR and 125 MHz for 13 C-NMR by using CDCl 3 or DMSO as solvents and TMS as an internal standard; infrared spectra were recorded in KBr on a IR Pristige-21 spectrometer (Shimadzu corporation, Japan); elemental analysis was performed on an Elemental Vario-III CHN analyzer (Elementar, German). The course of the reactions was monitored by TLC; analytical TLC was performed on silica gel GF 254. Intermediates 3 and 4 were prepared according to the reported methods [14] and used without further purifications, the process for preparing of them can be found in Additional file 1.

Antifungal biological assay
All the compounds (5, 6, 7a-7d, 8, 9a-9d) were evaluated for their antifungal activities against G. zeae, F. oxysporium, and C. mandshurica in vitro as described in literature [31,32]. The results of preliminary bioassays were compared with the experimental data of the commercial agricultural fungicide, hymexazol and carbendazim. All the compounds were dissolved in dimethyl sulfoxide (DMSO, 10 mL) before mixing with potato dextrose agar (PDA, 90 mL). The compounds were tested at a concentration of 50 mg/L. All fungal species were incubated in PDA at 27 ±1°C for 5 days to obtain new mycelium for antifungal assay. Mycelia dishes approximately 4 mm in diameter were cut from the culture medium. One of the specimens was picked up with a sterilized inoculation needle and then inoculated in the center of the PDA plate aseptically. The inoculated plates were incubated at 27 ±1°C for 5 days. DMSO in sterile distilled water served as the control, whereas hymexazol and carbendazim acted as the positive control. Three replicates were conducted for each treatment. The radial growth of the fungal colonies was measured on the sixth day and the data were statistically analyzed. The in vitro inhibiting effects of the test compounds on the fungi were calculated by the formula: I(%) = [(C-T)/(C-0.4)] × 100, where C represents the diameter of fungal growth on untreated PDA, T represents the diameter of fungi on treated PDA, and I is the inhibitory rate.

Conclusion
In conclusion, a series of the novel nicotinamide derivatives containing 1,3,4-oxadiazole were designed and synthesized. The synthesized compounds were characterized by spectral data ( 1 H NMR, 13 C NMR, IR) and elemental analysis. All of the compounds were subjected to fungicidal activities in vitro against G. zeae, F. oxysporum, and C. mandshurica. The results showed that the synthesized nicotinamide derivatives possessed weakly to moderately antifungal activities against the tested fungus. Most of the synthesized compounds exhibited similar activities as (or higher activities than) that of hymexozol on their corresponding fungus, and the compounds 9a, 7a, 8, and 9b showed considerable prospects for further optimization. Primary structure activity relationships revealed that the introduction of the groups of 4-chloro-6-methyl on benzene, the hydrazone group containing N,N-dimethylamino, and acetyl at -NH 2 could enhance the antifungal activities against G. zeae, F. oxysporum, and C. mandshurica. However, the structures of the synthesized compounds need to be optimized. Further structural modification and biological evaluation are currently underway to explore the full potential of this kind of nicotinamides containing 1,3,4-oxadiazole.

Additional file
Additional file 1: Experimental details and spectroscopic data of the intermediates 3-4 and the title compounds 5-9. Which includes the experimental procedure and the 1 H NMR of intermediates (3a-3d and 4a-4d), the experimental protocols and the spectroscopic data of title compounds 5, 6, 7a-7a, 8, and 9a-9d. All the copies of 1 H NMR and 13 C NMR for the title compounds were also presented in this additional file.

Competing interests
The authors declare that they have no competing interests.

Authors' contributions
The current study is an outcome of constructive discussion with BAS, SY and DYH; JW carry out their synthesis and characterization experiments was also involved in the drafting of the manuscript. JY performed the antifungal activities; JM and QC Shi carried out the 1 H NMR, 13 C NMR spectral analyses; SHK and LJL carried out the elemental analysis and infrared spectroscopy. JW and SY were involved in revising the manuscript. All authors read and approved the final manuscript.