Synthesis and antimicrobial activity of new Schiff bases containing coumarin moiety and their spectral characterization

New Schiff base 2-[(4-Methyl-2-oxo-2H-chromen-7-yl)oxy]-N'-(substitutedmethylene)aceto hydrazides (4a-p) were synthesized by the condensation of aryl/hetero aromatic aldehydes with 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy]acetohydrazide under conventional and microwave conditions and characterized through IR, H NMR and Mass spectral data. The synthesized compounds have been screened for antimicrobial activity.


Introduction
Azomethine group (-C=N-) containing compounds typically known as Schiff bases have been synthesized by the condensation of primary amines with active carbonyls.8] They have been studied extensively as a class of ligands [9][10][11] and are known to coordinate with metal ions through the azomethine nitrogen atom.
Schiff base complexes play a vital role in designing metal complexes related to synthetic and natural oxygen carriers. 12Metal complexes make these compounds effective as stereospecific catalysts towards oxidation, reduction, hydrolysis, biological activity and other transformations of organic and inorganic chemistry. 13In organic compounds the presence of -C=N-along with other functional groups form more stable complexes compared to compounds with only -C=Ncoordinating moiety.
Similarly coumarin derivatives have been of great interest because of their role in natural and synthetic organic chemistry.Many products which contain a coumarin subunit exhibit biological activity such as molluscicides, 14 anthelmintic, hypnotic, insecticidal 15 activity and some are serving as anticoagulant agents 16

Scheme 2
The synthesis of 4a was optimized by varying the solvents, reaction time, temperature and the results are given in Table 1.It was found that the yield was up to 83.4 % when the reaction mixture was refluxed for 30 min. in a chloroform/methanol mixture.In microwave assisted synthesis, the mixture of 3 with various aryl/hetero aromatic aldehydes is irradiated in an unmodified domestic microwave oven (800W) at 10 % intensity for 2-3 min.to yield the range of desired products 4a-p depicted in Table 2. Optimal reaction time for the synthesis was 2-3 min.at 80W of microwave irradiation power.Similarly the synthesis of compounds 2 and 3 was also attempted under the microwave conditions and detailed procedures are included in the experimental part.The study clearly showed that all the steps involved in this synthesis resulted in higher yields under micro wave conditions than the conventional method.

Antimicrobial activity
Compounds 4c-l were screened for their antibacterial activity against gram-positive Staphylococcus aureus, Bacillus subtilis and gram-negative Escherichia coli, Pseudomonas aeruginase, Klebsiella pseumoniae by the disc diffusion method in Mueller-Hinton agar medium at three concentrations (150, 200, 1000 ppm) in DMSO.These solutions were added to each filter disc, and the plates were incubated at 37 °C and examined for zone of inhibition around each disc after 48 hours.Results were compared with the activity of the standard antibiotic Ciproflaxacin.The results are summarized in Table 3.
According to preliminary antibacterial screening by the paper disc method, compounds 4c, 4e, 4g, 4h, 4j and 4l were found to be active against all the bacterial strains.The antibacterial activity of the synthesized compounds against standard and pathological strains was found to be similar (statistically insignificant difference Compounds 4k, 4l and 4e were found to be the most active compounds against the screened gram positive and gram negative standard and pathological bacterial strains.Based on the above results, we can conclude that compounds 4g, 4l and 4k are most effective as it inhibits 18mm, 15mm and 16mm diameter growth of Staphylococcus aureus, Pseudomonas auregenosa.

Conclusions
To conclude, the modified procedure for the conversion of 1 into 2 with dry DMF at 80 o C instead of dry acetone resulted with improved yield of 82 %.The same conversion also effected by sodium bismuthate as a catalyst instead of K

Experimental Section
General Procedures.Infrared (IR) spectra were recorded at room temperature from 4000 cm -1 to 400 cm -1 with KBr pellets at a resolution of 4cm -1 , using Avatar 330 equipped with DTGS detector.Most of the obtained vibrational bands of the IR spectrum were identified and compared with those available in literature.The 1 H NMR was measured on a Bruker AMX-400 instrument at room temperature using the X-WIN NMR version X-WIN NMR 1.3 cn drx software.The 1 H NMR was measured for ~ 0.03 M solutions in DMSO-d 6 using TMS as internal reference.The accuracy of the 1 H shifts is considered to be 0.02 ppm.The coupling constants J are in Hz.Mass spectra were obtained using LC-MS-MS (3200 Q-trap).Melting points were determined in open capillaries and are uncorrected.All reagents were purchased from Aldrich and Qualigens and used without further purification.

Procedure for the preparation of Ethyl 2-[(4-Methyl-2-oxo-2H-chromen-7-yl)oxy]acetate
(2) (a) Conventional method using K 2 CO 3 .Method A. To a solution of 7-Hydroxy-4-methyl-2Hchromen-2-one 1 in dry DMF, anhydrous potassium carbonate (1.0 molar equiv) and ethyl chloro acetate (1.0 molar equiv) were added.The resultant mixture was stirred at 80 o C for 10h, cooled and then the reaction mixture was added to a large amount of water.The solid separated was filtered, washed with excess of water.The crude product was purified by crystallization from ethanol.The yield of product was 81-82 % [lit.yield 40 %] 24 .(b) Conventional method using NaBiO 3 .Method B. To a solution of 7-Hydroxy-4-methyl-2Hchromen-2-one 1 in dry DMF, anhydrous sodium bismuthate (1.0 molar equiv) and ethyl chloro acetate (1.0 molar equiv) were added.The resultant mixture was stirred at 80 o C for 14h, cooled, filtered the reaction mixture and then the filtrate was added to a large amount of water.The solid separated was filtered and then the filtrate was extracted with ethyl acetate.The organic layer was separated and evaporated the solvent.The crude product was purified by crystallization from ethanol.The yield of product was 41-42.5 % [lit.yield 40 %] 24 .(c) Solvent free microwave irradiation method.Method C.This involves the irradiation of a mixture of 7-Hydroxy-4-methyl-2H-chromen-2-one 1 (0.01 mole), chloroethyl acetate (0.01 mole) and potassium carbonate (0.01 mole) in a microwave oven (MW domestic type oven 800W SANYO) at 10 % intensity for 12 min.(six pulses each of 2 min.).After completion of reaction (by TLC), the mixture was poured into ice-cold water.The separated solid was filtered, washed with excess of cold water and dried at room temperature.Microwave.This involves the irradiation of a mixture of compound 3 (0.01 mole) and aryl / heterocyclic aldehyde (0.01 mole) in a microwave oven (MW domestic type oven 800W SANYO) at 10 % intensity for 2-3 min.(four to six pulses each of 30 sec.) and the product was set aside to cool.The separated solid was filtered, washed with excess of methanol and dried.
When solid aldehyde was used, the mixture of compound 3 (0.01 mole), aryl / hetero aromatic aldehyde (0.01 mole) and alumina (1.0g, mesh: 70-290 ASTM, P H = 4.5) were finely ground in mortar and pestle then, the mixture was irradiated in microwave oven (MW domestic type oven 800W, SANYO) at 10 % intensity for 2-3 min.(four to six pulses each of 30 sec.) and set aside to cool.The resultant solid mixture was poured into 30 mL of methanol, and then solid separated was filtered, washed with methanol.

Table 1 .
Yield of 4a at different reaction conditions

Table 2 .
Comparison between microwave-assisted and conventional method of synthesis of 4a-p in terms of yield and time

Table 3 .
Antibacterial activity of compounds 4c-l

Table 3 .
Continued * Zone of inhibiton in mm. a In DMSO, Concentration in ppm.b Standard reference.
The resultant mixture was stirred at room temperature for 15 min, and the solid filtered with a sintered glass funnel.The residue was dried and then desiccated to afford a crystalline powder.The powder was recrystallised from chloroform/methanol and gave colorless needles.