Synthesis of Novel Thiazolyl Hydrazine Derivatives and Their Antifungal Activity

A series of novel thiazolyl hydrazine derivatives 3a–3o were synthesized and evaluated for their in vitro antifungal activity against six phytopathogenic strains, namely, Botryosphaeria dothidea ( B. d. ), Gibberella sanbinetti ( G. s. ), Fusarium oxysporum ( F. o. ), Thanatephorus cucumeris ( T. c. ), Sclerotinia sclerotiorum ( S. s. ), and Verticillium dahliae ( V. d. ), by the classical mycelial growth rate method. Biological assessment results showed that most of these target compounds showed good antifungal activity toward tested strains. Especially, compound 3l showed excellent antifungal activities against B. d. and G. s. with relatively lower EC 50 values of 0.59 and 0.69 µ g/mL, respectively, which were extremely superior to those of commercial fungicides ﬂuopyram, boscalid, and hymexazol and were comparable to those of carbendazim. Given the excellent bioactivity of designed compounds, this kind of thiazolyl hydrazine framework can provide a suitable point for exploring highly eﬃcient antifungal agents.


Introduction
Fungal infection is a serious disease that affects the appearance, yield, and quality of various plants and fruits including wheat, potatoes, rape, maize, soybeans, apple, peppers, and cucumbers, thereby attracting considerable attention in recent years [1][2][3][4][5][6][7]. Pesticides that are capable of attacking and annihilating intractable phytopathogenic fungi play an important role in safeguarding our crops [8]. However, due to the frequent and excessive usage of traditional fungicides, drug-resistant pathogens gradually increase year after year [9]. Additionally, plants do not have the same immune system as animals; therefore, it is urgent to develop new eco-friendly pesticides with high bioactivity and selectivity to ensure the quality and yield of agricultural products.
erefore, thiazole and hydrazide scaffolds were considered as important skeletons in the fabrication of active molecules. In our previous works, we found that 1,3,4oxadiazole hydrazide/sulfone/thioether derivatives showed good biological activity against plant microbial diseases [28][29][30]. In particular, 1,3,4-oxadiazole hydrazide derivatives performed a relatively high bioactivity against phytopathogenic oomycetes and fungi [30]. Encouraged by those studies, herein, a series of novel thiazole hydrazide derivatives were synthesized to continue the exploration of superior bioactive substrates. All the target compounds were screened for the in vitro activities against phytopathogenic fungi, including

Instrument and Chemicals.
e NMR spectra of the synthesized compounds were measured by Bruker Biospin-AG-400 apparatus (Bruker Optics, Switzerland). DMSO and TMS were used as the solvent and internal standard, respectively. All chemicals were purchased from Energy Chemical and used without further purification. All solvents meet the standard of analytical purity. e reaction process was detected by TLC.

General Procedures for Preparing Intermediate 1.
Different substituted thiobenzamide (0.01 mol), ethyl 2chloro-3-oxobutanoate (0.012 mol), and ethanol (50 mL) were added. After reacting for 3-4 h at 100°C, the organic layer was dried, and then 100 mL ethyl acetate was added. e organic layer was washed by water, brine, dried with sodium sulfate, and filtered, followed by the removal of the solvent under vacuum. Intermediate 1 was obtained by using column chromatography with the elution solvent of petroleum ether/ethyl acetate � 30/1.

Antifungal Activities of Target Compounds 3a-3o and Structure-Activity Relationship (SAR).
eir antifungal activity against aforementioned six kinds of plant pathogens was evaluated by using the mycelium growth rate approach, while the most applied antimicrobial drugs, namely, fluopyram (FP), boscalid (BS), hymexazol (HM), prochloraz (PC), and carbendazim (CB) were co-assayed for comparison. Table 1 reveals that some of the target compounds were identified with excellent antimicrobial activities at 25.0 µg/mL. For the biological effect against B. d. strain, compounds 3a, 3b, 3d, and 3g-3l gave the inhibition rate within 70.0-88.5%, which were better than positive drugs FP (35.3%), BS (36.5%), and HM (15.4%), but lower compared to CB (100%) and PC (100%). For the anti-G. s. activity, compounds 3a, 3b, 3d, 3k, and 3l afforded the inhibitory effect of 78.7%, 71.8%, 86.1%, 71.3%, and 80.5%, respectively. For the anti-V. d. activity, compounds 3b, 3d, and 3l gave the appreciable bioactivity with ratios of 79.7%, 81.1%, and 80.8%, respectively. By contrast, compound 3k was extremely bioactive against T. c. strain with the inhibition ratio of 98.7%. Simultaneously, this compound (3k) also displayed the best anti-S. s. activity with inhibitory rate of 85.5%. For the anti-F. o. activity, compound 3b yielded the best bioactivity with the rate of 76.1%. Among all the target molecules, compounds 3b, 3j, and 3k displayed the comprehensive bioactivity against all the tested fungal strains (B. d., G. s., V. d., T. c., S. s., and F   final bioactivity of target compounds could be significantly affected by a number of factors, such as the kind and electronic property of substitutional group, the type of halogen, and the location of halogen on the benzene ring. e detailed SAR is summarized as follows. (1) As R 1 � 4-Cl, introducing 2-F (3b) and 4-F (3d) on the benzene ring could improve the bioactivity against B. d., G. s., V. d., T. c., and S. s. strains compared to 3a (R 2 � H), while R 2 � 3-F and 4-Br normally gave a reduced tendency on the bioactivity.