Synthesis, Spectral and Microbial Studies of Some Novel Schiff Base Derivatives of 4-Methylpyridin-2-amine

Schiff base derivatives of N-{(1E)-[3-(mono or di-substituted aryl)-1phenyl-1H-pyrazol-4-yl]methylene}-4-methylpyridin-2-amine were synthesized by the acid catalyzed condensation of 3-(monoor disubstituted aryl)-1-phenyl1H-pyrazole-4-carbaldehyde derivatives with 4-methylpyridin-2-amine. Schiff base derivatives were characterized by FT-IR, H-NMR, Mass spectral analysis and elemental analysis. All the synthesized compounds have been screened for their antimicrobial activities by using broth dilution method.


Introduction
Azomethines are generally known as Schiff bases to honour Hugo Schiff, who synthesized such compounds.These are the compounds containing characteristic -C=N-group.Several methods have been reported for the preparation of azomethines.Selvam et.al 1 have prepared sulfonamide and its derivatives as anti-HIV agents.More et.al 2 have marked the biological activity of Schiff bases synthesized from aminothiazoles.Ernst Bayer 3 has reported some metallocomplex Schiff bases derived from o-amino phenol.Schiff bases can be synthesized from an aromatic amine and a carbonyl compound by nucleophilic addition forming a hemiaminal, followed by a dehydration to generate an imine 4 .They are well known intermediates for the preparation of azetidinones, thiazolidinones, oxadiazolines and many other derivatives.Azomethines exhibit a wide range of pharmacological activities like antimicrobial 5 , antiparasitic 6 , antiinflammatory 7 , anticancer 8 etc.A large number of substituted pyrazole derivatives are prepared and tested for variety of biological activities like anti HIV 9 , antiinflammatory 10 , antimicrobial 11 , fungicidal 12 etc.Pyridine derivatives also possess wide therapeutic activities such as antiviral 13 , anti HIV 14 , anticancer 15 , antitumor 16 , antimicrobial 17 etc.

Experimental
The reagent grade chemicals were obtained from commercial sources and purified by either distillation or recrystallization before use.The purity of synthesized compounds was checked by thin layer chromatography (TLC) on silica gel plate using benzene: ethyl acetate (8:10).Melting points were determined by open capillary method and are uncorrected.IR spectra are recorded on FT-IR Perkin-Elmer spectrophotometer RX1 using KBr disc. 1 H-NMR spectra are recorded in CDCl 3 on a Bruker DRX-400 MHz using TMS as internal standard.The chemical shifts are reported as parts per million (ppm) and ESI MS were determined on Discovery Make Thermo Spectrometer.
As per the even electron rule the m/z 373 quasi ion cleaves to fragment with even mass at fragment peaks at m/z 359 formed by the loss of methyl (-CH 3 ) from the base peak.It indicated the presence of methyl group and m/z 111 due to the removal of chloro benzene.The mass spectrum of comp.(V 2 ) also shows the presence of 1-Cl atom in a molecule by the ratio of 373 and 375 is 9:3 as per the natural abundance.Intensity of 374 is 25.27% 373 peak from that there are 22 carbons present in the molecule.

Conclusion
The characterization of Schiff base was carried out for all ten Schiff base derivatives through elemental analysis, FT-IR, 1 H-NMR and Mass spectral analysis.Antibacterial and antifungal studies of these compounds indicated that some compounds show potential antimicrobial activity but over all the synthesized compounds were less active than the standard drugs.

Antimicrobial activity
Minimal bactericidal concentration (MBC) values of the synthesized compounds were determined by using broth dilution method.The synthesized compounds and references were dissolved in dimethylsulfoxide (DMSO), sterilized by filtration using sintered glass filter and store at 4 0 C.All the synthesized compounds were screened for their antibacterial and antifungal activities against the E. coil, P. auregenosa, S. aures, S. pyogenus and the fungi C. albicans, A. niger, and A. clavatus.The compounds were tested at 500, 250, 100 and 50 µg/mL concentration using nutrient agar tubes.The highest dilution showing at least 99% inhibition was taken as MBC (minimal bacterial concentration).Standard drugs were used as reference.
As shown in Table 2, some of the target compounds had good activity against gramnegative and gram-positive bacteria.The target compounds V 3 , V 4 were more active against S. Aureus than the standard drug ampicillin and the compounds V 5 (against E. coli), V 7 (against S. pyogenus ) were equal active as the standard drug ampicillin.But these compounds were less active against P. Aeruginosa a gram-negative bacteria.When the chemical structures of the active compounds were taken into consideration, the di-substituted o,p di-chloro derivative (V 3 ) was more active than the mono substituted p-chloro derivative (V 2 ) and the m-hydroxy derivative (V 7 ) was more active than the o-hydroxy and p-hydroxy derivatives (V 6 , V 8 ).
As shown in Table 3, the anti fungal activity data clearly show that the compounds V 7 having a hydroxy substituent in m-position of the phenyl ring exhibited good activity against C. albicans, A. niger, and A. clavatus.Compounds V 7 showed more antifungal activity as greseofulvin (against C. albicans) and less activity against A. niger and A. clavatus.Compounds V 4 and V 6 showed equal antifungal activity as greseofulvin (against C. albicans) but very low activity against A. niger, and A. clavatus.The result suggests that a hydroxy (-OH) group in m-position of the phenyl ring is suitable for antifungal activity.

Table 1 .
Physical and analytical data of title compound and its other derivatives (