Antibacterial Screening of some Heterocyclic Compounds Containing Quinoline Nucleus

. In this work, some novel derivatives of Cyanopyridines and Isoxazoles were synthesized using Vilsmeier-Haack reagent and their structures were confirmed by FTIR, 1 H NMR and mass spectroscopic methods. The antibacterial activities of these synthesized compounds were studied in DMSO and DMF by agar well diffusion method against some Gram positive and Gram negative bacteria. It is observed that activity depends upon three S: solvent, strain and structure.

Recently, more and more new simple and elegant syntheses of substituted quinolines have been described [19][20][21][22][23][24][25]. The synthesis of quinoline derivatives thus continues to be an attractive area of research and the synthesis of various substituted quinolines have been largely described in the literature through different strategies [26][27][28]. Some of these substituted quinolines are important constituents of several pharmacologically active compounds. A number of biological activities [29][30][31][32][33][34] have also been associated with quinoline containing compounds such as antiinflammatory, antiallergic, antimalarial, antibacterial, antiproliferative, anticancer and antiparasitic activities.
Thus, aim of the present work is to synthesize some new nitrogen containing heterocyclic compounds and to study their antibacterial activity. Some novel derivatives of cyanopyridine and isoxazole bearing quinoline nucleus are synthesized and their structures were elucidated on the basis of IR, MASS and 1H NMR spectral data. All the compounds have been screened for their invitro biological activity by agar well diffusion method.

Spectral analysis:
The structure of all the synthesized compounds was confirmed by spectroscopic techniques.
The IR spectra were recorded on Shimadzu FT-IR-8400 instrument using KBr pellet method. The Mass spectra were recorded on Shimadzu GC-MS-QP-2010 model using direct inlet probe technique. 1 H NMR was determined in DMSO solution on a Bruker Ac 400 MHz spectrometer.
Antibacterial studies: The antibacterial activities of all synthesized compounds were studied in DMF and DMSO. Both the solvents were fractionally distilled by standard method [35].
In the present work, the antibacterial activities of the synthesized compounds have been screened against some Gram positive and Gram negative bacterial stains using agar well diffusion method.

Preparation of test compounds:
The solutions were prepared at a concentration of 1 mg/μl for all the compounds.
Preparation of the plates and microbiological assay: The antibacterial evaluation was done by agar well diffusion method [36,37] using Mueller Hinton Agar No.2 as the nutrient medium. The agar well diffusion method was preferred to be used in this study because it was found to be better than the disc diffusion method as suggested by Parekh et al. [37]. The bacterial strains were activated by inoculating a loop full of test strain in 25 ml of N-broth and the same was incubated for 24 h in an incubator at 37 o C. 0.2 ml of the activated strain was inoculated in Mueller Hinton Agar. Mueller Hinton Agar kept at 45 o C was then poured in the Petri dishes and allowed to solidify. After solidification of the media, 0.85 cm ditch was made in the plates using a sterile cork borer and these were completely filled with the test solution. The plates were incubated for 24 h at 37 o C. The mean value obtained for the three wells was used to calculate the zone of growth inhibition of each sample. The controls were maintained for each bacterial strain and each solvent. The inhibition zone formed by these compounds against the particular test bacterial strain determined the antibacterial activities of these synthesized compounds.

Results
In both Cyanopyridine and Isoxazole series, ten compounds are synthesized. The synthesized compounds were purified through column using chloroform: methanol (80:20 v/v). The purity of compounds was checked by TLC using appropriate solvent systems.
The physical parameters of synthesized cyanopyridine and isoxazole compounds are listed in Tables 1 and 2.    However, in DMF, all the compounds exhibited inhibition against B. cereus and S. aureus as shown in Fig. 4. Further, maximum activity is for NCP-8. Against M. flavus, only NCP-9 and NCP-10 exhibited inhibition and inhibition is same for both the compounds.    In DMSO, for B. cereus, only NISO-2, NISO-4, NISO-5 and NISO-6 showed inhibition. Other compounds had no effect at all. In DMF, against B. cereus, all compounds exhibited inhibition and maximum inhibition is for NISO-6.  In DMSO, against P. mirabilis only NISO-1, NISO-2 and NISO-7 showed inhibition which is of same magnitude. Others compounds showed no inhibition at all. For S. typhimurium NISO-1, NISO-5, NISO-6 and NISO-8 showed inhibition. Other compounds had no effect on this bacterium. Maximum inhibition is for NISO-1 and minimum for NISO-8. No inhibition is observed against C. freundii by any compounds. In DMF, all compounds showed almost similar inhibition against P. mirabilis. Against S. typhimurium, NISO-8 show maximum activity followed by NISO-2 whereas NISO-3, NISO-4 and NISO-9 show similar minimum activity while NISO-1, NISO-5, NISO-6, NISO-7 and NISO-10 shows no activity. Against C. freundii, all the compounds show almost similar average activity except NISO-4. Thus, all substitutions are effective for C. freundii except NISO-4.

Cyanopyridines:
The results indicate that inhibition depends on three S: solvent, strain and structure. All the compounds have the same central moiety but different substitutions as given in Table 1. Comparison of inhibition against Gram positive bacteria of studied compounds in DMF and DMSO solvents shows that DMF is good solvent.
In DMSO, against B. cereus except NCP-4, other compounds exhibited inhibition. Table 1 shows that NCP-4 contains nitro group at para position. Thus, presence of nitro group could not affect this bacterial strain. The compound NCP-8 containing chloro group at para position is most effective. Against M. flavus and S. aureus, not a single compound is effective.
However, in DMF, again NCP-8 containing chloro group at para position exhibited maximum inhibition against B. cereus and S. aureus. Against M. flavus, hydroxyl group at ortho position (as in NCP-9) and only hydrogen (as in NCP-10) exhibited inhibition to the same compound. Other substitutions had no effect at all.
Thus, overall p-chloro substitution (as in NCP-8) is most effective against studied Gram positive bacteria.
In case of Gram negative bacteria, in DMSO, against S. typhimurium and C. freundii, studied compounds had no effect. However, for P. mirabilis only NCP-3, NCP-4 and NCP-8 containing bromo, nitro and chloro substitutions at para position respectively showed inhibition to the same extent.
In DMF, all the substitutions are found to be effective against all the three gram negative bacteria. The compound having no substitution i.e., NCP-10 is most effective against S. typhimurium whereas nitro group at meta position (as in NCP-5), hydroxyl group at para position (as in NCP6) and methyl group at para position ( as in NCP-7) are least effective. For P. mirabilis, all substitutions are almost equally effective. However, when there is no substitution group present