Microwave assisted efficient one-pot synthesis of 3,5,6-trisubstituted-1,2,4-triazines from fatty acid hydrazides under solvent-free conditions and their antimicrobial activity

Rapid and efficient solvent-free synthesis of 3,5,6-trisubstituted-1,2,4-triazines from fatty acid hydrazides under microwave irradiation is described. The one-pot synthesis on solid inorganic support provides the products in good yields. The newly synthesized compounds are screened for antimicrobial activity. The structural features of the synthesized 1,2,4-triazines were characterized by IR, 1 H NMR, 13 C NMR, mass and elemental analysis.


Introduction
Microwave assisted organic synthesis (MAOS) continues to affect synthetic chemistry significantly by enabling rapid, reproducible and scaleable chemistry development. 1The use of microwave irradiation is an established tool in organic synthesis for achieving better selectivity, rate enhancement and reduction of thermal degradation byproducts. 2,3However these procedures are practically limited as solvents in a microwave oven at elevated temperatures create high pressure, which may cause explosion.One of the advances to overcome this problem is inorganic 4,5 solid supported organic synthesis which attracts attention because of enhanced selectivity, milder reaction conditions and associated ease of manipulation.It also provides an opportunity to work with open vessels and enhanced possibility for scaling up reactions. 6,7umerous reactions including condensations, cycloadditions, heterocycle formation, and metal-catalyzed cross-coupling have been explored under microwave conditions, some of which have been applied to medicinal chemistry and total synthesis of natural products. 8MAOS can facilitate the discovery of new reactions and reduce cycle time in optimization of reactions.In addition it serves to expand chemical space in compound library synthesis.Moreover it is an acknowledged quick alternative and green technology in synthetic organic chemistry that also provides easier work-up procedures.
As can been seen from Tables 1 and 2, the scope of the reaction using saturated, olefinic (internal and terminal) and hydroxy fatty acid hydrazides was found to be good.Ph Ph The yield of 3,5,6-trisubstituted-1,2,4-triazines did not depend on the length of the fatty acid chain hydrazide Table 2.In order to determine the optimum conditions for the synthesis of triazines, variations in molar ratios of reagents and the irradiation time and power level of microwave set-up were investigated.After some experimentation, we found a set of conditions that generally provides products in good yield.The optimum conditions employed are that the molar ratio of fatty acid hydrazide and 1,2-diketone is 1:1 and irradiation time and power level of microwave set up are 7-12 min, 60 and 100% power.The synthesized compounds were identified on the basis of IR, 1 H NMR, 13 C NMR, mass spectra and elemental analysis.All reactions were carried out using fatty acid hydrazides (1 eq) with respect to 1,3-diketone under microwave irradiation.b Microwave equipment multimode was used.c Full power is 1.35 kHz.d Monitored by TLC. e All yields refer to isolated products and the products were characterized by IR, 1 H NMR, 13 C NMR, MS and elemental analysis.
purified by column chromatography.The esters of fatty acids were prepared by refluxing the fatty acid in methanol in the presence of catalytic amount of sulfuric acid.General (GR) grade of solvents were employed for the extraction purposes and when required solvents were dried and distilled before use.Homogeneity of the product was observed on TLC. 1 H NMR spectra were recorded in CDCl 3 on a Bruker DRX-400 instrument.The chemical shifts (δ) were measured relative to TMS as an internal standard.Coupling constants (J) are expressed in Hz.Mass spectra were obtained on a Jeol SX-102 (FAB) spectrometer.IR spectra were obtained on Shimadzu 8201 PC FT-IR using KBr pellet with absorption given in cm -1 .The microwave irradiations were carried out using an unmodified domestic oven (LG, Model MC-808WAR, 1.35 KW, 2450MHz).

General procedure for preparation of fatty acid hyrazides
The hydrazides of the fatty acids were prepared by refluxing the methyl esters (0.01 mmol) with hydrazine hydrate (0.02 mmol) in methanol at about 130 o C for 2 h. 42The mixture was then cooled to room temperature.The hydrazide solidified and was filtered, washed with water and recrystallized from ethanol.The fatty acid hydrazides were characterized by melting points and mixed melting points and spectral data.
General procedure for synthesis of 3,5,6-trisubstituted-1,2,4-triazines A mixture of fatty acid hydrazide (2 mmol), diketone (2 mmol) and silica gel was ground in a pestle, NH 4 OAc and Et 3 N were added in catalytic amounts and the prepared mixture in an open pyrex beaker was subjected to microwave irradiation for the appropriate time (Table 2).After complete conversion as indicated by TLC, the mixture was extracted with petroleum ether (3x50 ml) and washed with water (3x50 ml).Then the solvent was evaporated in vacuum and the product was purified by column chromatography.

Conclusions
The preparation of.3,5,6-trisubstituted-1,2,4-triazines derivatives of fatty acid hydrazides is a valuable addition to the synthesis and manipulation of fatty acid derivatives.This work could be extended to some naturally occurring unusual fatty acids.Biological evaluation of these derivatives may furnish some other important applications.

Table 2 .
Optimization for the reaction of fatty acid hydrazides with 1,2-diketones under microwave irradiation

Table 3 .
and SD plates for fungi were incubated for 5-6 days at 25 o C. The activity was determined by measuring the zone of growth inhibition of the test organism.Antibiotics, chloramphenicol and nystatin (Hi-Media Pvt Ltd, Mumbai, India) were used in the test system as positive controls.The compounds were dissolved in DMF.The results are shown in Table3.The compounds showed good activity against H. oryzae, S. aureus, E. coli and better results were obtained against C. albicans, Penicillium, A. niger.The moderate activity of compounds was observed against S. typhimurium and B.subtilis.Antimicrobial activity of 3,5,6-trisubstituted-1,2,4-triazines 5CFU/ml) of test organism was spread on nutrient agar (NA) and sabaroud dextrose (SD) agar plates.Sterile filter paper disc (8 mm) (Hi-Media Pvt Ltd, Mumbai, India) impregnated with 50 µg of the compound and a disc without compound was used as a negative control.The NA plates were incubated for 18 h at 37 o C for test bacteria, C.ARKAT USA, Inc. albicans