Synthesis of N-{ [ 5-aryl / alkyl-1 , 3 , 4-oxadiazol-2-yl ] methyl } pyridin-2-amines as antimicrobial and anticancer agents

A new series of oxadiazole analogues was synthesized starting from 2aminopyridine. The compounds were characterized by infrared (IR), nuclear magnetic resonance (NMR), and mass spectral analyses followed by determination of their anticancer and antimicrobial activities. Three compounds were tested for in vitro anticancer activity against NCI-60 human cell lines of nine different panels including leukemia, non-small lung cancer, colon cancer, CNS cancer, melanoma, ovarian cancer, renal cancer, prostate cancer, and breast cancer according to the National Cancer Institute (NCI, USA) Protocol at 10 μM. The compounds N-{[5-(4chlorophenyl)-1,3,4-oxadiazol-2-yl]methyl}pyridin-2-amine (5c), N-{[5-(4-methoxyphenyl)-1,3,4-oxadiazol-2-yl]methyl}pyridin-2-amine (5f), and N-{[5-(3,4-dimethoxyphenyl)-1,3,4-oxadiazol-2-yl]methyl}pyridin-2-amine (5g) showed anticancer with higher selectivity towards HOP-92 (Non-Small Cell Lung Cancer). N-{[5-(4Fluorophenyl)-1,3,4-oxadiazol-2-yl]methyl}pyridin-2-amine (5b) showed maximum antibacterial activity with minimum inhibitory concentration (MIC) of 4-8 μg/mL, while N-{[5-(4-methoxyphenyl)-1,3,4-oxadiazol-2-yl]methyl}pyridin-2-amine (5f) showed maximum antifungal activity with MIC 4 μg/mL.

In the present investigation the scaffold was designed containing oxadiazole nucleus attached to hydrophobic aryl ring (Zibotentan contains both oxadiazole and pyridine rings) through a methylene group, which imparts flexibility to the molecule due to sp 3 hybridization, so that the compound may well accommodate in their target sites, with the hope of increased biological activity.
The NH attached to the oxadiazole ring through linker (-CH 2 -) is expected to play an important role in reducing toxicity (Fig. 1).Some of the antibacterial/ antimycobacterial drugs (sulfapyridine, sulfasalazine, isoniazid etc.) also contain pyridine.Similarly furamizole that contains oxadiazole ring exhibits a strong antibacterial activity.

Chemistry
The 2,5-disubstituted-1,3,4-oxadiazole analogues (5a-j) described in this study are shown in Table 1 and the reaction sequence for their synthesis is shown in Scheme 1.
The purity of the synthesized compounds was checked by elemental analysis.Both the analytical and spectral data of the compounds were in full agreement with the proposed structure.The IR spectra of final compounds showed oxadiazole stretching at 1152-1169 cm -1 , and NH band at 3191-3209 cm -1 , while the C=N stretching was observed at 1531-1547 cm -1 .The proton NMR spectra confirmed the structures on the basis of the chemical shift, multiplicity and coupling constants in DMSO-d 6 .The spectra showed a triplet at δ 1.24-1.29 ppm corresponding to CH 3 ; a multiplet at δ 2.54-2.56corresponding to CH 2 group; a singlet at δ 3.32-3.35ppm corresponding to CH 2 (methylene linker); a singlet at δ 3.80-3.81ppm corresponding to OCH 3 ; a singlet at δ 8.58-8.99ppm corresponding to the aromatic NH; a singlet at δ 10.52-11.11ppm corresponding to the OH phenolic group, while aromatic peaks were observed as singlet, doublet and multiplet in the aromatic region according to the nature of protons.The molecular mass (M + ) and (M+2) + were observed in the mass spectra.

Antibacterial activity
Some of the compounds showed significant antibacterial activity.Compound 5b showed maximum antibacterial activity with minimum inhibitory concentration (MIC) of 8 µg/mL against S. aureus and B. subtilis and 4 µg/mL against E. coli.The compounds 5c, 5d, 5e, 5f, and 5h showed moderate activity with MIC between 32 and 64 µg/mL while compounds 5a, 5i and 5j showed lower activity.The activity of compound 5b was found to be nearly equal to the standard drug ciprofloxacin.The antibacterial activity of the compounds is reported in Table 3. Electron withdrawing groups like 4-fluoro, 4-chloro increased the activity in a greater extent than electron releasing the hydroxy and methoxy groups.The furyl and ethyl substitutions at position 5 of oxadiazole ring were not favorable.

Antifungal activity
Some of the compounds (5a-j) showed significant antifungal activity.The compound 5f showed maximum antifungal activity with MIC of 4 µg/mL against A. niger and C. albicans followed by compound 5g with MIC of 8 µg/mL against A. niger and 16 µg/mL against C. albicans.The antifungal activity of compounds is reported in Table 3 with fluconazole as the standard drug.Compounds 5d, 5f and 5g showed significant antifungal activity while rest of the compounds showed moderate to low antifungal activity.The electron releasing methoxy and hydroxy groups confers better antifungal activity than electron withdrawing fluoro and chloro groups.The activity was found to be less important with furyl or ethyl substituents at position 5 of the oxadiazole ring.

Conclusions
All the synthesized compounds were obtained in satisfactory yields and evaluated for their anticancer and antimicrobial activities.The compound 5f, which expressed maximum anticancer activity on human cancer cell lines at 10 µM concentration could be considered as lead for further optimization and drug discovery.Similarly compounds 5b and 5f showed maximum antimicrobial activity.All these derivatives can be further modified to exhibit more potency.Synthesis of other series of oxadiazole analogues is in progress in our laboratory.The oxadiazole derivatives discovered in this study may provide valuable information in the field of drug design and discovery.
a big apprehension nowadays in both anticancer and antimicrobial therapy.The indiscriminate use of antibiotics causes attribution to the emergence of drug resistance to majority of antibacterial agents .On the other hand fungal infections like Candidiasis, Crytococcosis and Aspergillosis are more common in immuno-compromised patients.Owing to this increased microbial resistance, new classes of antimicrobial agents with novel mechanisms of action are today need to fight against the multidrugresistant infections.Cancer causes nearly 13 percent total deaths globally surpassing cardiovascular disease.In India the total number of cancer cases are likely to go up from 979,786 cases in the year 2010 to 1,148,757 cases in the year 2020.A total of 1,658,370 new cancer cases and 589,430 cancer deaths are projected to occur in the United States in 2015 and it is expected that the new cases of cancer will jump to 19.3 million worldwide by 2025.

Fig. 1 .
Design of newer oxadiazole scaffolds as biologically active agents

Table 2 .
In vitro anticancer activity of N-{

Table 3 .
Antimicrobial activity of N-{