Microwave Assisted Synthesis and Antimicrobial Evaluation of Schiff Bases of Indole-3-aldehyde

In order to develop new antimicrobial agents, a series of Schiff bases of indole-3-aldehyde were synthesized by microwave assisted synthesis by taking DMF as solvent and evaluated for their antimicrobial activity. All the synthesized compounds were characterized by IR, H NMR and mass spectral analysis. All compounds were tested against five gram positive and five gram negative bacterial strains and one fungal strain. All compounds exhibited better activity against gram positive strains than against gram negative strains and the compounds were found more active against S.aureus and B.subtilis.


Introduction
A Schiff base (or azomethine) is a functional group that contains a carbon nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group but not hydrogen 1 .Schiff bases are usually synthesized from the condensation of primary amines and active carbonyl group 2 .Schiff bases have been reported to possess antimicrobial properties [3][4][5][6][7] .Schiff bases are characterized by the −N=CH− (imines) group which is important for elucidating the mechanism of transamination and racemisation reactions in biological systems and are also known to have biological activities such as antimicrobial 8 antifungal 9 , antitumor 10 and herbicidal 11 activity.Indole derivatives found to possess antibacterial 12 , anticonvulsant 13 and antihypertensive activity.These observations led to the conception that Schiff bases of indole-3-aldehyde would possess potential antimicrobial properties.Microwave assisted organic synthesis (MAOS) accelerate the course of many organic reactions, producing high yields and higher selectivity, lower quantities of side products and consequently, easier work-up and purification of the products 14 .In the present study a series of Schiff base of Indole-3-aldehyde were synthesized by use of conventional microwave and characterized by IR, 1 H NMR and mass spectroscopy.The compounds were screened for antibacterial and antifungal activities.The minimum inhibitory concentrations of the compounds were also determined by serial dilution method.
Indole-3-aldehyde on condensation with different aromatic or aliphatic amine in presence of DMF as solvent yields Schiff bases (Scheme 1).

Experimental
A mixture of 2 mmole of indole-3-aldehyde and 2 mmole of different aryl or alkyl amines was taken and triturated in a mortar pestle.Then above mixture was transferred to a vessel which was then kept in microwave for synthesis.4 to 5 mL of DMF was also added to mixture before putting it in microwave.

Physicochemical and spectroscopic determination of synthesized compounds
The melting points were taken in open capillary tube and were uncorrected.Reactions were monitored by thin layer chromatography using silica gel-GF 254 as adsorbent on glass plate.The spots were applied on silica gel plate and the plate was run in (ethyl acetate: hexane (3:7)) in a closed chamber.The spots on the plate were detected in UV cabinet.The differences in R f value between starting compound and product were indicative of the conversion of starting compound into the product.
IR spectra were recorded on FTIR-8400F model in KBr.NMR spectra were recorded on Broker A VANCE DPX 300 instrument using TMS as internal reference and chemical shift value are expressed in delta units.The mass spectra (ESI) were recorded on Waters @Micromass Q-Tof Micro.It showed fragmentation pattern as m/z values.All physical and spectral data are given in Table 2.

Results and Discussion
We have synthesized a series of Schiff bases of indole-3-aldehyde by microwave assisted synthesis.This protocol presented many advantages, such as good to excellent yields, much shorter reaction time

Conclusion
The structures of synthesized compounds were confirmed by IR and NMR spectroscopy.All compounds exhibited significant antibacterial activity but they showed moderate antifungal activity.Further bioassay, optimization and structure-activity relationship of the title compounds are underway.

Figure 1 .
Figure 1.Scheme for the synthesis of Schiff bases
Microwave was run at 400-480 W for different time for different reaction mixtures.Reaction completion was monitored continuously after each run by TLC.Then product was washed with ethanol , solvent was evaporated, dried and recrystalized with ethanol.Structures, time in microwave assisted synthesis and % yield are given in Table1Table1.Structures, time in microwave assisted synthesis and % yield of synthesized compounds.

Table 2 .
Physical and spectroscopic data of synthesized compoundsContd…inhibition & minimum inhibitory concentrations of the synthesized compounds were determined by serial dilution method.Ciprofloxacin, ampicilin and fluconazole were used as reference standards for antibacterial and antifungal activity respectively.The lowest concentration of the test compounds showing no visible microbial growth were considered as minimum inhibitory concentration.The observed zone of inhibition and MIC values for bacterial and fungal strains are given in Table3 and 4respectively.

Table 3 .
Antibacterial of the synthesized compoundsIn vitro activity-zone of inhibition in mm, (MIC in µg/mL) for gram positive strains

Table 4 .
Antifungal activity of the synthesized compounds