Synthesis of Schiff Bases of 4-Amino-3-Mercapto-5-Pyridin- 4yl-4H-1,2,4-Triazole and Their Evaluation as Resistance Inducers and Antifungal Agents

Schiff bases are the products of aldehyde and primary amine and characterized by N=CH– group. Schiff bases comprising of various heterocyclic moiety have found use as drugs and also been reported as pesticides. Schiff bases of several series of 4-Amino-1,2,4-triazoles have already been synthesized and their antifungal, antiviral and PGR activity have been reported. Some 5–Aryl-4-amino 3-mercapto triazoles and their schiff bases have been reported as fungicides and nematicides. Schiff bases of 4–amino-3-mercapto-5-pyridin-2yl-4H-1,2,4-tiazole have recently been reported as nematicides. The use of chemicals to activate SAR provides a novel alternative to plant disease control. 2,6-Dichloro isonicotinic acid (INA), is a well known systemic acquired resistance (SAR) activator analogous in action to salicyclic acid. A series of twenty five Schiff bases of 4-Amino-3-mercapto-5-pyridin-4yl-4H-1,2,4-triazole, incorporating the isonicotinic acid moiety and having different substitution in the aryl ring attached to imine group were designed and synthesized following the synthesis scheme which involved five steps starting with isonicotinic acid which was first converted to→ ethyl ester→ hydrazide →dithiocarbazinic salt→ triazole and then finally to Schiff bases. The products were characterized by elemental analysis, FT–IR, 1 HNMR and 13 CNMR spectroscopy. Three of the five steps of the synthesis i.e ethyl ester to hydrazide; dithiocarbazinic salt to triazole and triazole to Schiff bases were accomplished using microwave. The microwave synthesis have been found must faster(1-1.5 min) and efficient than the conventional method (2-4 hr). The Schiff bases were obtained in good yields 44.4 to 86.9% (conventional) and 69.7 to 94.9% (microwave). All twenty five Schiff bases, 4-Arylidenamino-3-mercapto-5-pyridin-4-yl-4H-1,2,4triazoles were screened for systemic acquired resistance (SAR) inducing activity against sheath blight of rice and five potential compounds viz 3-NO2 (GS6), 4-NO2 (GS7), 3-Br (GS18), 2-F (GS20) and 4-F (GS21) analogs were further evaluated. All the five compounds considerably decreased the relative lesion height (RLH) as compared to control with maximum reduction in RLH shown by compound No. GS21 (47.15%). These five potential compounds were further studied for their effect on phenol content, PAL and peroxides activity. The compound GS21, 4-(4-Flurobenzylidenamino)-3-mercapto-5-pyridin-4-yl-4H-1,2,4-triazole was identified as the most potent SAR inducer both based on phenotypic and biochemical study. Its RI activity was found better than 2,6-dichloroisonicotinic acid (INA), a resistance inducing chemical used as standard. The twenty five Schiff bases, 4-Arylidenamino-3-mercapto-5-pyridin-4-yl-4H-1,2,4triazoles were also evaluated in vitro for antifungal activity against R. solani and F. moniliforme. The compounds in general showed moderate fungicidal activity against both the fungi. The highest antifungal activity against both the fungi was exhibited by the compound GS19, 4-(4Bromobenzylidenamino)-3-mercapto-5-pyridin-4-yl-4H-1,2,4-triazole (ED50 (μg/ml)= 358, R. solani and 460, F. moniliforme). The compound GS21, 4-(4-Flurobenzylidenamino)-3mercapto-5-pyridin-4-yl-4H-1,2,4-triazole, however, exhibited least activity (ED50 994 μg/ml) against R. solani. The Schiff base GS21, 4-(4-Flurobenzylidenamino)-3-mercapto-5-pyridin-4yl-4H-1,2,4-triazole which exhibited the least fungicidal activity against R. solani was found to be most potent and promising SAR inducer against sheath blight of rice. 4&vehuks&3&ejdSIVks&5&fijhMhu&4&by&4&,p&1]2]4&Vakb,tksy fÓQ cslksa dk la'ys"k.k ,oa mudk izfrjks/k mRizs jd o dodjks/kh ds :Ik esa ijh{k.k