Design and synthesis of novel 3-(thiophen-2-yl)-1,5-dihydro-2H-pyrrol-2-one derivatives bearing a hydrazone moiety as potential fungicides

Background Tetramic acid, thiophene and hydrazone derivatives were found to exhibit favorable antifungal activity. Aiming to discover novel template molecules with potent antifungal activity, a series of novel 3-(thiophen-2-yl)-1,5-dihydro-2H-pyrrol-2-one derivatives containing a hydrazone group were designed, synthesized, and evaluated for their antifungal activity. Results The structures of 3-(thiophen-2-yl)-1,5-dihydro-2H-pyrrol-2-one derivatives bearing a hydrazone group were confirmed by FT-IR, 1H NMR, 13C NMR, 1H-1H NOESY, EI-MS and elemental analysis. Antifungal assays indicated that some title compounds exhibited antifungal activity against Fusarium graminearum (Fg), Rhizoctorzia solani (Rs), Botrytis cinerea (Bc) and Colletotrichum capsici (Cc) in vitro. Strikingly, the EC50 value of 5e against Rs was 1.26 µg/mL, which is better than that of drazoxolon (1.77 µg/mL). Meanwhile, title compounds 5b, 5d, 5e–5g, 5n–5q and 5t exhibited remarkable anti-Cc activity, with corresponding EC50 values of 7.65, 9.97, 6.04, 6.66, 7.84, 7.59, 9.47, 5.52, 6.41 and 7.53 µg/mL, respectively, which are better than that of drazoxolon (19.46 µg/mL). Conclusions A series of 3-(thiophen-2-yl)-1,5-dihydro-2H-pyrrol-2-one derivatives bearing a hydrazone group were designed, synthesized and evaluated for their antifungal activity against Fg, Rs, Bc and Cc. Bioassays indicated that some target compounds exhibited obvious antifungal activity against the above tested fungi. These results provide a significant basis for the further structural optimization of tetramic acid derivatives as potential fungicides. Electronic supplementary material The online version of this article (10.1186/s13065-018-0452-z) contains supplementary material, which is available to authorized users.


Background
An emergence of resistant fungi is a huge impetus to the development of agricultural fungicides with novel molecular structures and unique mechanisms [1]. In this regard, the structural optimization of natural heterocycles plays a important role in the searching for bioactive lead compounds [2,3]. As attractive nitrogenous heterocycles, tetramic acid derivatives are widely researched for some reasons. First, tetramic acid derivatives exist in secondary metabolites from various terrestrial and marine organisms and have favorable compatibility with the environment [4]. Second, tetramic acid derivatives contain a unique pyrroline-2-one or pyrrolidine-2,4-dione substructure that is easy to synthesize to some extent [5]. Third, tetramic acid derivatives are reported to exhibit various agricultural bioactivities including fungicidal [6], herbicidal [7], insecticidal [8], antibacterial and antiviral [9] properties. Encouraged by the above findings, series of tetramic acid derivatives bearing amino [10], strobilurin [6], phenylhydrazine [11], oxime ether [12] and pyrrole [13] groups were synthesized and reported for their antifungal activity against plant fungi in our previous work. However, the potential application of Open Access *Correspondence: ycl@njau.edu.cn 1 Jiangsu Key Laboratory of Pesticide Science, College of Sciences, Nanjing Agricultural University, Nanjing 210095, China Full list of author information is available at the end of the article tetramic acid derivatives as agricultural fungicides was greatly limited by their unsatisfactory curative rates [6,[10][11][12][13].

Configuration confirmation of title compounds
As shown in the 1 H NMR and 13 C NMR spectra of title compounds, these 3-(thiophen-2-yl)-1,5-dihydro-2H-pyrrol-2-one derivatives containing a hydrazone group does present itself via one single molecular structure. Aiming to further understand the structural characteristics of title compounds, the configuration of compound 5f was studied as an example by a 1 H-1 H NOESY analysis [30]. As shown in Fig. 2, the chemical shifts of H f , H j and H k protons were 5.39, 10.10 and 7.26 ppm in the NOESY spectrum of compound 5f (DMSO-d 6 ), respectively. The obvious NOE phenomena between H j and H f , and between H j and H k indicated that these protons close with each other, which typically revealed the double bond C=NNH of title compound 5f possesses the cis-configuration.
Encouraged by the above preliminary bioassays, the EC 50 values of some compounds that exhibited fine antifungal activity against Rs, Cc and Fg at 10 μg/ mL were determined and are summarized in Table 2. Table 2 shown that the EC 50 values of the selected compounds ranged from 1.26 to 9.89 µg/mL against Rs, from 5.52 to 9.97 µg/mL against Cc and from 6.02 to 8.85 µg/mL against Fg. Strikingly, the EC 50 value of the title compound 5e against Rs was 1.26 µg/mL, which is better than that of drazoxolon (1.77 µg/mL). Meanwhile, the title compounds 5b, 5d, 5e-5g, 5n-5q and 5t had remarkable EC 50 values of 7.65, 9.97, 6.04, 6.66, 7.84, 7.59, 9.47, 5.52, 6.41 and 7.53 µg/mL against Cc, respectively, which are better than that of drazoxolon (19.46 µg/mL). The above results also indicates that 3-(thiophen-2-yl)-1,5-dihydro-2H-pyrrol-2-one derivatives containing a hydrazone group can serve as potential structural templates in the search for novel and highly efficient fungicides.

Structure-activity relationships
As indicated in Tables 1 and 2, the antifungal effects of title compounds were greatly affected by structural variations. Some structure-activity relationships (SAR) analyses were discussed as below. First, Tables 1 and 2 show that most of title compounds exhibited better antifungal activity against Rs than that against Bc, Cc and Fg. For example, Table 1 presents that the anti-Rs effects of title compounds 5b, 5d, 5f, 5h, 5i, 5j, 5l, 5m, 5p, 5s, 5u, 5v and 5w are better than the corresponding effects against Bc, Cc and Fg at 10 μg/mL. Table 2 also exhibits that title compounds 5b, 5d, 5e, 5f, 5g, 5n, 5o, 5p and 5q have better EC 50 values against Rs than that against Cc and Fg. Second, introducing methyl into the R 1 position is disadvantageous for the antifungal activity of title compounds against the tested four fungi. For instance, Table 1 shows that the inhibition rates of compounds 5e, 5j and 5p (R 1 = H) are obviously better than that of compounds 5v, 5w and 5u (R 1 = Me) against the tested four fungi at 10 μg/mL. Third, when the R 2 was substituted by 4-Me, 4-F, 2-Br and 4-OMe groups, the corresponding title compounds 5e, 5n, 5p and 5t exhibited overall better antifungal activity than that of compounds 5l, 5m, 5o and 5q-5s against Rs, Bc and Fg at 10 μg/mL. Finally, a presence of 4-F, 4-Cl and 4-Br groups at the R 3 position can effectively enhance the antifungal activity of title compounds against Rs, Bc and Fg. For example, the inhibition effects of compounds 5e, 5f and 5g were overall better than that of compounds 5a-5d and 5h-5k against Rs, Bc and Fg at 10 μg/mL.

General
Reagents and solvents used without further purification are analytically or chemically pure. Melting points (m.p.) were determined on an uncorrected WRS-1B digital melting point apparatus (Shanghai Precision and Scientific Instrument Corporation, China). The FT-IR spectra were recorded on a Thermo Nicolet 380 FT-IR spectrometer (Thermo Nicolet Corporation, America). 1 H NMR, 13 C NMR, and 1 H-1 H NOESY spectra were collected on a Bruker AV 400 MHz spectrometer (Bruker Corporation, Germany) at room temperature with DMSO-d 6 as a solvent. Mass spectra were recorded on a TRACE 2000 spectrometer (Finnigan Corporation, America). Elemental analyses were determined on an Elementar Vario EL cube analyzer (Elementar Corporation, German). Reactions were monitored by thin layer chromatography (TLC) on silica gel GF 245 (400 mesh). The tested strains Fg, Rs, Bc and Cc were provided by the Laboratory of Plant Disease Control at Nanjing Agricultural University.

General procedures for intermediates 4
A mixture of a intermediate 2 (10 mmol), a intermediate 3 (11 mmol) and triethylamine (11 mmol) in acetone (50 mL) was stirred at room temperature for 4 h. After that, the white solid appeared in the reaction solution was filtered, washed with water and diethyl ether to obtain a intermediate 4.