Indian Journal of Chemistry Vol. 54B, October 2015, pp 1260-1274 Synthesis of Schiff bases of 4-amino-3-mercapto-5-pyridin-4yl-4H-1,2,4-triazole and their evaluation as SAR inducers Sujan Majumder a , Bishnu Maya Bashyal b & R L Gupta* †a a Division of Agricultural Chemicals and b Division of Plant Pathology, Indian Agricultural Research Institute, New Delhi 110 012, India E-mail: drgupta.rl@gmail.com Received 28 May 2015; accepted (revised) 7 September 2015 A series of twenty five Schiff bases 6a-y of 4-amino-3-mercapto-5-pyridin-4yl-4H-1,2,4-triazole having different substitution in the aryl ring attached to imine group designed incorporating the isonicotinic acid moiety present in INA, a known SAR inducer have been synthesized and characterized using 1 H and 13 C NMR, FT–IR spectroscopy and elemental analysis. All twenty five Schiff bases, 4-arylideneamino-3-mercapto-5-pyridin-4-yl-4H-1,2,4-triazoles have been screened for systemic acquired resistance (SAR) inducing activity against sheath blight of rice and five potential compounds viz. 6f, 6g, 6r, 6t and 6u analogues have been further evaluated. All the five compounds have considerably decreased the relative lesion height (RLH) as compared to control with maximum reduction in RLH shown by compound 6u (47.15%). These five potential compounds have been further studied for their effect on phenol content, PAL and peroxidase activity. The compound 6u has been identified as the most potent SAR inducer both based on phenotypic and biochemical study and also does not show direct fungicidal activity against R. solani. Its RI activity is found better than 2,6-dichloroisonicotinic acid (INA), a resistance inducing chemical used as standard. Keywords: Schiff bases, triazoles, INA, RI activity, SAR inducers, sheath blight of rice, R. solani 1,2,4-Triazoles and their derivatives show wide spectrum of bioactivities. Besides various pharmacological activities, these compounds have shown activity as plant protection agents i. e. fungicides 1-3 , herbicides 3-5 , insec- ticides 6 , etc. Some of these compounds have been commercialized as excellent fungicides 7 . The increasing importance of 1,2,4-triazoles as agrochemicals has led to some recent reports of synthesis and pesticide activity of some 4-amino-5-substituted aryl-3-mercapto-1,2,4- tria-zoles 8-11 . Further, Schiff bases obtained by reaction of aldehydes with such 4-amino-1,2,4-triazoles have been reported to find application as fungicides 12 , nematicides 13 , plant-growth regulators 14 and for various pharmacological activities 15-17 . Plant diseases caused by fungi, bacteria and viruses cause dramatic losses in agriculture and horticulture all over the world. Traditional fungicides play a crucial role in the integrated disease management by killing or controlling target fungi directly, sometimes causing adverse effects on the environment, and often leading to fungicide resistance. It is always of interest to seek out new environment-benign methods for plant protection. In this context, the prospect of broad-spectrum disease control using the plant’s own resistance mechanisms has led to increasing interest in the development of agents which can mimic natural inducers of resistance 18 . The use of chemicals to activate SAR (systemic acquired resistance) thus provides a novel alternative for plant disease control 19 . The first and best studied synthetic chemical developed as a SAR activator is acibezolar- S-methyl 20 , marketed in Europe as BION and as ACTIGARD in the United States. At low rates, it activates resistance in many crops against a broad spectrum of diseases, including fungi, bacteria and viruses. 2,6-Dichloroisonicotinic acid (INA) 21 is another well known SAR activator analogous in action to salicylic acid 22 . 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 thus designed and synthesized. The micro- wave assisted synthesis and SAR inducing activity of these compounds against sheath blight of rice have been reported in this paper. ______________ †Address correspondence: Dr. R. L. Gupta, B-410 Mahalaxmi Apts, Plot No. 4, Sector 2, Dwarka, New Delhi 110 075.