Synthesis and Antibacterial Studies of Schiff bases derived

Hanmanthu Guguloth¹, Srinivas Nerella², Shravankumar Kankala¹*, Ravinder Vadde¹*

¹Department of Chemistry, Kakatiya University, Warangal, India -506 009

²Department of Chemistry, Govt. Degree College, Jammikunta, Karimnagar, India

*Corresponding Author E-mail: shravankankala@yahoo.com

ABSTRACT:

A series of twelve new azomethine derivatives were synthesized by the condensation of o-phthalaldehyde with primary amine derivatives. The chemical structures of these compounds were confirmed by IR, ¹H, ¹³C NMR, Mass and elemental analysis. Antibacterial activities of these compounds have been tested against four strains of bacteria (Bacillus subtillus, Staphylococcus aureus, Escherichia coli and Klebsiella pneumonia) and found to possess potential antibacterial activity.

KEYWORDS: o-phthalaldehyde, primary amine, schiff base, antibacterial activity

INTRODUCTION:

Nucleophilic addition-elimination of primary amine for the construction of CH=N compounds such as imines, hydrazones, oxime-ethers are well established. Of particular interest would be the addition of carbonyls and aromatic amine compounds to give synthetically and pharmacologically valuable Schiff bases^{1, 2}. Schiff bases have several applications such as bactericidal^3-5, antipyretic⁶, anticancer⁷, antitumour⁸, antitubercular⁹ and sterease inhibitory¹⁰ activities. Recently Schiff base compounds have attracted more interest owing to their role in the biological activities. Some of these Schiff bases are reported to be used as chelating agent¹¹, and analytical reagents¹². The condensation of carbonyl compounds with primary amines was discovered in 1864s by Hugo Schiff¹³ and stability of aromatic Schiff bases is associated with conjugation. Schiff bases have CH=N and nitrogen as a donor atom, which can act as potential ligands and form complexes with various metals and show antibacterial activity.

Reactions of ortho-phthalaldehyde with nucleophiles were extensively reviewed by zuman¹⁴. Most of the substituted benzaldehydes showed a limited reactivity with weak nucleophiles like water¹⁵. However the strong reactivity towards nucleophiles was resulted when the second formyl group was introduced. As part of our interest it continued ¹⁶ in exploring to synthesize highly antibacterial compounds, we herein present the eco-friendly synthetic procedure of Schiff bases (scheme 1) in aqueous media with good yield (60-85%) and the reaction time as well as yield was depicted in Table 1.

EXPERIMENTAL:

General:

The solvents and organic reagents used were of AR grade. The solvents were dried and deoxygenated by refluxing and storing them over sodium. All the synthetic reactions were carried out in a nitrogen atmosphere using the schlenk technique. Elemental analyses were performed on a Perkin Elmer 2400 CHN analyzer. IR spectra were recorded on a Nicolet-740 spectrophotometer and Perkin-Elmer 580B spectrophotometer. Brucker WH 270 NMR and Brucker SXP-100 instruments were used to record ¹H, ¹³C and 31P-NMR spectra. Mass spectra were recorded on a VG-Micromass 7070H spectrometer operating at 70 eV. Hot air oven (Instrument and equipment Pvt. Ltd., Mumbai), incubator (Instrument and equipment Pvt. Ltd., Mumbai), laminar airflow unit (Clas Laminar Technologies Pvt. Ltd. Secunderabad), autoclave (Medica Instrument Mfg. Co., Mumbai) were used in the present investigations. Bacillus subtillus, Staphylococcus aureus, Escherichia coli and Klebsiella pneumonia cultures were obtained from (IMTECH, Chandigarh).

General Procedure for the Synthesis of Schiff bases (2a-l):

To a suspension of ortho-phthalaldehyde (OPA) (0.134 g, 0.01 mmol) was added to respective primary amine (0.02 mmol) in MeOH (absolute, 10 ml) in addition to molecular sieves and refluxed (oil bath at 40-70 °C) for 1h under N₂ (g). The reaction mass was allowed to attain room temperature and stirring was continued for 50 minutes. After conversion was complete, the mixture was evaporated under reduced pressure to afford a crude product which was subjected to recrystallization from MeOH to afford pure products (2a-l).

Compound (2a): Yield 72%, ¹H NMR (200 MHz, CDCl₃): δ 8.46 (2H, s, CH=N), 6.84-7.48 (14H, m, ArH) ppm. IR (ν _max, KBr, cm^-1): 1638, 1527, 1459, 1064; MS (EI, 70 eV): m/z (%) = 285 [M+H]⁺. EA calcd (%) for C₂₀H₁₆N₂: (284.13): C 84.48, H 5.67, N 9.85; found C 84.45, H 5.66, N 9.82.

RESULTS AND DISCUSSIONS:

The Schiff bases 2a-l were synthesized by the condensation of ortho-phthalaldehyde with various primary aromatic amines by reaction in hot methanol using molecular sieves as dehydrating agent. It is known that condensation of amines with aldehydes is favored by a polar medium.^{17, 18} (Scheme 1).

Scheme 1. Reaction of Ortho-phthalaldehyde with Aromatic amines

The structures of the title compounds (2a-l) were determined by IR, ¹H-NMR, and mass spectroscopy were in accord with the proposed structures. Compounds 2a-l illustrated in the IR spectra, an absorption band at 1615-1636 cm^-1, typical representation of the stretching vibrations of double CH=N bond. The absence of absorption in the regions of 1685 and 3420 cm^-1 indicates complete transformation of the C=O and NH₂ groups respectively.^{19, 20} However, compounds 2e-h, showed absorption bands at 3440, 2560, 1720 and 1724 cm^-1 indicating OH group, SH group o-COOH, and p-COOH groups respectively. The ¹H-NMR spectra of 2a-l hold multiplet signals due to aromatic protons in the region 6.55-8.20 ppm, and singlets at 8.25-8.68 ppm from the C-H protons of the CH=N groups. The ¹³C-NMR spectra of 2a-l Schiff base a downfield shift of CH=N group was observed in range 165.20-172.45 ppm suggest that complete condensation of orthophthalaldehyde with primary amines²¹. The formation of Schiff bases as shown in Table-1, the order of the yields j > f > e = I > l > c > k > a > h > b = g > d.

However, according to the literature reports, the formation of Schiff base with less sterically hindered and more electron donating substituents would improve the yields. If bulky and electron with drawing substituents gives less yields therefore the above order is in harmony with the literature. These synthesized variants of Schiff bases are screened for the antibacterial activities and resulted highly potent against both Gram positive and Gram negative bacteria.

Table 1. Synthesized Schiff bases of ortho-phthalaldehyde with Aromatic amines

^aAll products were characterized by IR, ¹H NMR and mass spectral analysis.

^bIsolated yields after recrystalization.

^cThe corresponding Schiff bases were isolated in 60-85% yields.

Antibacterial activity:

The in vitro antibacterial activity of the Schiff bases were tested against the gram positive and gram negative bacteria by Agar well diffusion method^{22, 23} as described by Desta (2005). Standard inoculums (1-2 x 107 cfu/ml 0.5 MC Farland strands) was introduced onto the surface of sterile nutrient agar plate, and evenly distributed by using a sterile glass spreader. Simultaneously, 8mm wells were bored on the surface of the solidified agar plate using a sterile cork borer. 70µl of Schiff bases dissolved in 10% DMSO from different concentration of 1, 2.5 and 5 mg/ml were introduced into each well and performed in triplicate. The Agar plates were incubated aerobically at 37 ^ºC. After 24h, the inhibition zones were measured and depicted in (Table 2) which is compared with the reference compound Chloramphenicol.

It can be seen that the antibacterial activity of Schiff bases c, e & f are more or less similar to that of the standards. Compounds d, g & h are moderately active and the other Schiff bases are less active. So, It is clear from the results the Schiff bases with pyridyl, hydroxyl, thiol and carboxylic acid substituents were found to possess antibacterial activity.

Table 2 Antibacterial activity of Schiff bases.

Sample	Concentration (mg/ml)	Test organisms
		Bacillus subtillus	Staphylococcus aureus	E. coli	K. pneumoniae
a	1 2.5 5	+ + ++	+ + +	+ + ++	0 0 +
b	1 2.5 5	+ + ++	+ + +	+ ++ ++	0 + +
c	1 2.5 5	++ +++ +++	+ ++ +++	++ +++ ++++	+ ++ +++
d	1 2.5 5	+ + ++	+ + +	+ ++ ++	0 + +
e	1 2.5 5	++ +++ +++	++ ++ +++	++ +++ +++	+ ++ ++
f	1 2.5 5	++ +++ +++	++ ++ +++	++ +++ +++	+ ++ ++
g	1 2.5 5	+ ++ ++	+ + +	+ ++ +++	0 + +
h	1 2.5 5	+ ++ ++	0 + +	+ ++ ++	+ + +
i	1 2.5 5	+ + +	+ + +	+ + ++	0 + +
j	1 2.5 5	+ + ++	+ + ++	+ ++ ++	0 + +
k	1 2.5 5	0 + ++	+ + +	+ + ++	0 + +
l m	1 2.5 5 1 2.5 5	0 + + + + ++	+ + + + + ++	+ + ++ + ++ ++	0 + + + + ++
St	1 2.5 5	++ +++ +++	++ ++ +++	++ +++ +++	++ +++ +++

m= Ortho-phthaladehyde, St. ═ reference standard; Chloramphenicol. The test was done using the diffusion agar technique. Well diameter 1 cm (100µl of each concentration was tested) 1 cm ═ 100. Inhibition values ═ 0.1 - 0.5 cm beyond control ═ +. Inhibition values ═ 0.6 – 1.0 cm beyond control ═ ++. Inhibition values ═ 1.1- 1.5 cm beyond control ═ +++. Inhibition values > 1.5 cm beyond control ═ ++++. 0 ═ not detected.

CONCLUSION:

In summary, a series of twelve new azomethine derivatives were synthesized by the condensation of o-phthalaldehyde with primary amine derivatives. These azomethine derivatives were screened for their antibacterial activity against four strains of bacteria (Bacillus subtillus, Staphylococcus aureus, Escherichia coli and Klebsiella pneumonia) and chemical structures of these compounds were confirmed by IR, ¹H NMR, and Mass. Antibacterial activities of these compounds have been tested and found to possess potential antibacterial activity.

ACKNOWLEDGEMENTS:

One of the authors S. Nerella thanks UGC, New Delhi for the UGC MRP 620.

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Received on 02.08.2015 Modified on 22.08.2015

Accepted on 25.08.2015 © AJRC All right reserved

Asian J. Research Chem. 8(8): August 2015; Page 530-534

DOI: 10.5958/0974-4150.2015.00084.X