Microwave-Assisted Synthesis of Some Quinoxaline-Incorporated Schiff Bases and Their Biological Evaluation

Quinoxaline-incorporated Schiff bases (4a–j) were synthesized by the condensation of 2-[(3-methylquinoxalin-2yl)oxy]acetohydrazide (3) with indole-3-carbaldehyde, furfuraldehyde, 5-(4-nitrophenyl)-2-furfuraldehyde, and substituted benzaldehydes under conventional and microwave irradiation methods. e microwave method was found to be remarkably successful with higher yields, less reaction time, and environmentally friendly compared to conventional heating method. e chemical structures of the synthesized compounds have been con�rmed by analytical and spectral data. All the compounds have been evaluated for antitubercular and anti-in�ammatory activities.


Introduction
Quinoxaline derivatives have been reported to possess a wide range of biological properties such as being anticancer [1,2], antihistaminic [3], antiviral [4], antimicrobial [5,6], antifungal [7], antitubercular [8,9], and anti-in�ammatory [10].Schiff bases possess antimicrobial [11], antitubercular [12], anti-in�ammatory [13], and analgesic [14] properties.In view of these facts, we herein report the facile synthesis of a new series of quinoxaline-incorporated schiff bases and screen for antitubercular and anti-in�ammatory activities.In recent times, microwave-assisted organic reactions have become very popular and gain special attention due to their ecofriendly nature, safety, less reaction time, and higher yields.In continuation of our research on quinoxaline derivatives, in the present study, we made an attempt to synthesize the title compounds using conventional and microwave irradiation methods and compared the reaction time and percentage yields.

Experimental
Melting points were determined using open capillary tubes on ANALAB melting point apparatus and are uncorrected.IR spectra were recorded on FTIR spectrophotometer (SHI-MADZU 8400 series) using KBr pellets.

Results and Discussion
ion peak at m/z 233 (100%) corresponding to its molecular weight in mass spectrum con�rmed the structure of the compound (3).From compound (3), ten different Schiff bases have been synthesized (Scheme 1) by condensing with indole-3-carbaldehyde, furfuraldehyde, 5-(4-nitrophenyl)-2furfuraldehyde, and substituted benzaldehydes using conventional and microwave irradiation methods.e physical data of Schiff bases, time and yields in both the methods, were compared and mentioned in the Table 1.

Antitubercular Activity.
All the newly synthesized compounds were evaluated for their possible in vitro antitubercular activity at a concentration of 6.25 g/mL against Mycobacterium tuberculosis H 37 RV (ATCC 27294) in BACTEC 12B medium using both micro dilution assay and the microplate alamar blue assay (MABA).Compounds exhibiting �uorescence were tested in the BACTEC 460 radiometric system.e antitubercular activity data were compared with standard drug, Rifampin at a concentration of 0.25 g/mL.e compounds 4c, 4d, 4i, and 4j exhibited substantial antitubercular activity, particularly 4h showed 75% inhibition (MIC > 6.25 g/mL) and emerged as the most active compound in the present series.e MIC and percentage inhibition data of the compounds that have shown activity against Mycobacterium tuberculosis are presented in Table 2.

3.�. Anti-�n�ammatory Activity.
Male Wistar rats (140-200 g) of both sexes were used for the studies.e rats were obtained from Mahaveer enterprises, Hyderabad.e animals were divided into groups of six each and fasted for 12 hr before the experiment.e ethical guidelines prescribed for the investigation of animals used in experiments were followed in all tests.Carrageenan (0.1 mL, 1%) was administered into the plantar surface of the right hind paw of each rat, 1 hr aer the administration of the test compounds and standard drug ibuprofen (100 mg/kg).One group was kept as control, received only 0.5% CMC solution.Before injection of carrageenan, the average volume (  ) of the right hind paw of each rat was calculated.Aer injection, the paw volume (  ) was measured aer 3rd hr with the aid of a plethysmograph.e edema was expressed as an increase in the volume of paw, T 1: Characterization data of compounds 4(a-j).and percentage inhibition of acute edema was obtained as follows:

Comd
where Among all the compounds, compounds 4a, 4b, 4d, 4g, 4h and 4i exhibited signi�cant activity at 3rd hr (Table 3).Results are presented as Mean ± SEM (standard error of mean) of six rats.Statistical analysis was performed using one-way analysis of variance (ANOVA) followed by Dunnett's test for multiple comparisons, using Graph-pad Soware.

Toxicity Study.
Acute toxicity of schiff bases was determined in albino mice with the staircase method.Each group of 5 animals was fasted for 24 hr prior to the administration of the test compounds.e test compounds, 4a, 4b, 4d, 4g, 4h

Conclusion
A novel series of schiff bases were synthesized by using conventional and microwave irradiation methods and all the compounds were characterized by physical and spectral data.Compounds 4c, 4d, 4h, 4i, and 4j exhibited potent antitubercular activity.Compounds 4a, 4b, 4d, 4g, 4h, and 4i have shown signi�cant protection against the edema formation.�ith further molecular modi�cation and manipulation of these compounds, several other promising bioactive molecules can be developed in future.

3 -
methylquinoxalin-2-ol (1) was synthesized by condensing o-phenylenediamine with ethyl pyruvate.Compound (1) was converted into ethyl [(3-methylquinoxalin-2-yl)oxy]acetate (2) by heating under re�ux with ethyl chloroacetate in dry acetone and anhydrous potassium carbonate.e structure of compound (2) was con�rmed by the carbonyl peak of ester at 1740 cm −1 in IR, and a mass ion peak at m/z 247 (100%) in mass spectroscopy.e reaction of the compound (2) with hydrazine hydrate (99%) in ethanol under re�ux for 5-6 hr gave 2-[(3-methylquinoxaline-2-yl)oxy]acetohydrazide (3).e carbonyl peak of amide at 1680 cm −1 and NH, NH 2 stretching at 3330 cm −1 and 3226 cm −1 in IR and a mass 1H NMR spectra were recorded in CDCl 3 /DMSO-d 6 solvents unless otherwise mentioned.Chemical shis are reported on AVANCE 300 MHz and INNOVA 400 MHz relative to TMS internal standard on the -scale.Mass spectra of the compounds were recorded on Mass spectrometer (Agilent 1100 series; EI/ES-MS) at Indian Institute of Chemical Technology, Hyderabad.e syntheses were carried out in scienti�c microwave oven supplied by Catalyst, India.All the reactions were monitored by thin layer chromatography on 60 F 254 0.25 mm precoated aluminum plates (Merck) and visualized with UV light.