Synthesis , Characterization , and Antimicrobial Activity of Methyl-2-aminopyridine-4-carboxylate Derivatives

e compounds containing an azomethine group (–CH=N–) are important in elucidating the mechanism of transamination and racemisation reactions in biological systems [1, 2]. Due to the great exibility and diverse structural aspects, a wide range of Schiff bases have been synthesized and their complexation behaviors have been studied [3]. ey have been synthesized from a variety of compounds, such as amino thiazoles, 2-hydroxy-1-napthalaniline, amino sugars, aromatic aldehydes, ketones, isatin, triazole ring, thiosemicarbazides, amino acids, and pyrazolone [4, 5]. Antibacterial, antifungal, antitumor, and anticancer activities of Schiff bases have been reported [6, 7] and they are active against a wide range of organisms. Antibacterial activity has been studied more than antifungal activity, because bacteria can achieve resistance to antibiotics through biochemical and morphological modi cations [8, 9]. Some Schiff bases bearing aryl groups or heterocyclic residues possess excellent biological activities have attracted the attention of many researchers in recent years [10–12]. e Schiff bases formed from aromatic aldehydes or aromatic ketones and their derivatives are quite stable. Due to the great exibility and diverse structural aspects of Schiff bases, a wide range of these compounds have been synthesized and their activities have been studied [13, 14]. Antimicrobial and antifungal activities of various Schiff bases have also been reported [15]. Many Schiff bases are known to be medicinally important and are used to design medicinal compounds [16]. In connection with such studies, the present paper reporting for the rst time on the synthesis of methyl2-aminopyridine-4-carboxylate derivatives, 3a–f, which are formed during the reaction of methyl-2-aminopyridine-4carboxylate (1) with different aldehydes (2a–f).ese synthesized compounds were characterized by elemental analyses, FT-IR, and H NMR studies. Antibacterial and antifungal activities were also reported in this paper.


Introduction
e compounds containing an azomethine group (-CH=N-) are important in elucidating the mechanism of transamination and racemisation reactions in biological systems [1,2].Due to the great exibility and diverse structural aspects, a wide range of Schiff bases have been synthesized and their complexation behaviors have been studied [3].ey have been synthesized from a variety of compounds, such as amino thiazoles, 2-hydroxy-1-napthalaniline, amino sugars, aromatic aldehydes, ketones, isatin, triazole ring, thiosemicarbazides, amino acids, and pyrazolone [4,5].Antibacterial, antifungal, antitumor, and anticancer activities of Schiff bases have been reported [6,7] and they are active against a wide range of organisms.Antibacterial activity has been studied more than antifungal activity, because bacteria can achieve resistance to antibiotics through biochemical and morphological modi cations [8,9].Some Schiff bases bearing aryl groups or heterocyclic residues possess excellent biological activities have attracted the attention of many researchers in recent years [10][11][12].e Schiff bases formed from aromatic aldehydes or aromatic ketones and their derivatives are quite stable.Due to the great exibility and diverse structural aspects of Schiff bases, a wide range of these compounds have been synthesized and their activities have been studied [13,14].Antimicrobial and antifungal activities of various Schiff bases have also been reported [15].Many Schiff bases are known to be medicinally important and are used to design medicinal compounds [16].
In connection with such studies, the present paper reporting for the rst time on the synthesis of methyl-2-aminopyridine-4-carboxylate derivatives, 3a-f, which are formed during the reaction of methyl-2-aminopyridine-4carboxylate (1) with different aldehydes (2a-f).ese synthesized compounds were characterized by elemental analyses, FT-IR, and 1 H NMR studies.Antibacterial and antifungal activities were also reported in this paper.

Experimental
All chemicals used were of the analytical reagent grade (AR).All solvents and reagents were purchased from Sigma Aldrich Chemicals Pvt Ltd.Melting range was determined by Veego Melting Point VMP III apparatus.Elemental analyses were recorded on Perkin Elmer 2400 Elemental Analyser.e FT-IR spectra were recorded using KBr discs on FT-IR Jasco 4100 infrared spectrophotometer. 1 H NMR spectra were recorded on Bruker DRX-400 MHz spectrometer and TMS as an internal standard.in minimum volume of ethanol, and then 2-3 drops of concentrated sulfuric acid are added to the reaction mixture.e solution was stirred for 4-5 hr at room temperature and the progress of the reaction was followed by TLC until the reaction was complete.It was cooled to 0 ∘ C, the precipitate was ltered, washed with ethanol, and recrystallized from methanol.Methyl-2-aminopyridine-4-carboxylate derivatives (3a-f) were synthesized by the method summarized in Scheme 1. e chemical structures and physical data of all the synthesized compounds are tabulated in Table 1.
2.9.In Vitro Antifungal Activity.e synthesized compounds were screened for their antifungal activity against Candida albicans in DMF by poisoned food technique [18].Potato Dextrose Agar (PDA) media was prepared and about 15 ml of PDA was poured into each petri plate and allowed to solidify.5 mm disc of seven days old culture of the test fungi was placed at the center of the petriplates and incubated at 26 ∘ C for 7 days.Aer incubation the percentage inhibition was measured and three replicates were maintained for each treatment.Activity of compounds was compared with standard drug nystatin.All the synthesized compounds were tested (at the dosage of 500 L of the novel compounds/petriplate, where concentration was 0.1 mg/mL) by poisoned food technique.

Results and Discussion
3.1.Schiff Base Characterization.Schiff bases of methyl-2-aminopyridine-4-carboxylate derivatives, 3a-f, were synthesized from the reaction of methyl-2-aminopyridine-4carboxylate with different aldehydes.e elemental analyses data showed good agreement between the experimentally determined values and the theoretically calculated values within the limits of permissible error.e absorptions around 3000 cm −1 in synthesized compounds con rm the aromatic stretching vibrations, and the appearance of a medium to strong absorption bands above 1600 cm −1 due to a stretching vibration of the azomethine (C=N) bond formation in synthesized compounds via condensation.e proton spectral data agree with respect to the number of protons and their chemical shis with the proposed structures.e proton spectral data of methyl-2-aminopyridine-4-carboxylate (1) shows resonance at  5.50 ppm (s, 2H, -NH 2 ).In compounds 3a-f, the above resonance disappeared and additional resonances assigned to the -CH=N ( 7.10-8.18ppm) were observed, which con rmed the condensation between the amino group and carbonyl group.e 1 H NMR spectra of 3c and 3d are shown in Figures 1 and 2, respectively.antibacterial activity against four pathogenic bacterial strains.Compounds 3a and 3b showed weak antibacterial activity against S. aureus and X. campestris.

Antifungal Activity.
All the synthesized compounds 3a-f were also tested against C. albicans.e compounds 3c and 3d showed good antifungal activity than other compounds in the series against tested strain.Compound 3c showed inhibition in the range of 16 mm when compared with other compounds in the series against C. albicans.From the results, it is evident that compounds 3a and 3b are moderately active.e antimicrobial activity results of synthesized compounds were compared with standard drugs as depicted in Table 2. Compounds 3a-f showed antimicrobial activity in the order of 3c > 3d > 3f > 3e > 3b> 3a against tested bacterial and fungal strains.Initial structure activity relationship can be drawn for the compounds 3a-f.In the present study, different electron withdrawing and electron donating groups attached to aromatic ring as substituent were linkage to azomethine group.e close survey of antimicrobial efficacy indicated that the inhibition values of all the compounds exhibited a varied rang (11-26 mm and 10-16 mm) of antibacterial and antifungal activities against all the tested microbial strains.e electron donating hydroxyl group and methoxy group of phenyl ring in 3c produces enhanced activity.e electron withdrawing pyridine ring in 3f and electron donating ethoxy group in 3e showed moderate activity, whereas the indole in 3d produces good antimicrobial activity.e compound 3a is weakly active and this may be due to the absence of phenyl ring.However, 3b showed better activity compared to 3a probably due to the presence of benzene in 3b.e above SAR correlation studies reveal that the nature of the linkage (substituent on aromatic ring) in uences the antimicrobial activity.

Conclusion
In conclusion, in a series of methyl-2-aminopyridine-4carboxylated derivatives, 3a-f were synthesized in good yield, characterized by different spectral studies, and their antimicrobial activities were determined against clinically important pathogens.Compounds 3c and 3d demonstrated good inhibition against bacterial and fungal strains tested.e SAR studies revealed that the substituents on aromatic ring are responsible for the antimicrobial activity of these classes of agents.On the basis of their activity, these derivatives were identi ed as viable leads for further studies.
of Gram positive and Gram negative bacteria.Sterile discs of 10 mm diameter (Hi-Media) were made in each of the petriplates, to which 50 L (1 mg/mL, i.e., 50 g/disc) of the different synthesized compounds were added.e treatments also included 50 L of DMF and streptomycin as negative and positive control for comparison.