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EHOOPrAHH^ECKAa XHMH3, 2014, moM 40, № 2, c. 226-233
SYNTHESIS AND EVALUATION OF IN VITRO ANTIOXIDANT PROPERTIES OF NOVEL 2,5-DISUBSTITUTED 1,3,4-OXADIAZOLES
© 2014 Lakshmi Ranganatha V, Shaukath Ara Khanum#
Department of Chemistry, Yuvaraja's College, University of Mysore, Mysore, Karnataka, 570005 India Received July 26, 2013; in final form, October 28, 2013
2,5-Disubstituted 1,3,4-oxadiazole compounds are one of the most attractive heterocyclic compounds for researchers due to their biological activities. In the undertaken research, a number of potential 2,5-disubstitut-ed 1,3,4-oxadiazole analogues were synthesized through multi step reaction and characterized by FT-IR, 1H NMR, mass spectra, and also by elemental analysis. Further benzophenone tagged indole acetohy-drazides and 2,5-disubstituted 1,3,4-oxadiazoles were evaluated for antioxidant potential, through different in vitro models such as DPPH, nitric oxide and hydrogen peroxide methods. In the series of compounds some of them had shown good to moderate in vitro antioxidant potential compare to the standard drug ascorbic acid in all the above three methods.
Keywords: antioxidant assay, benzophenone, N-methyl indole, 1,3,4-oxadiazole.
DOI: 10.7868/S0132342314020080
INTRODUCTION
Man has always been fascinated with things that would help him remain youthful and healthy. This desire of mankind is answered through antioxidants. The key role played by antioxidants is in achieving optimum health and longevity. It is well known that free radicals play an important role in the inflammatory process. Superoxide anion radicals, hydrogen peroxide and hydroxyl radicals, produced by activation of phagocytes, are considered to be involved in inflammation and tissue destruction. Free radicals are also involved in the biosynthesis of prostaglandins, important mediators of inflammation. Compounds with an-tioxidant properties are generally expected to protect against inflammation. Several substituted oxadiazole derivatives are reported to possess anti-inflammatory and/or antioxidant activities [1].
The much hype about antioxidants among people is attributed to their ability to conflict deadly diseases effectively. They help to prevent cancer, heart disease, allergies, inflammation and respiratory diseases such as bronchitis and asthma [2]. Hence efforts are going on continuously to discover more efficient antioxidant drugs in order to accomplish our health goals [3, 4]. Synthetic organic chemistry has always played a vital role in drug discovery process. Many of the drugs in use in the last fifty years or more have been of synthetic or semi-synthetic origin. Among the several synthesized novel compounds, oxadiazoles draw the atten-
# Corresponding author (phone: 0091-99018 88755; fax: 08212419239; e-mail: shaukathara@yahoo.co.in).
tion for their wide applications. The capacity of1,3,4-oxadiazole nucleus to undergo a variety of chemical reactions have made it medicinal backbones on which number of potential molecules can be constructed. In fact there are a number of drugs available in the market like raltegravir, an antiretroviral drug, nesapidil, a class IV antiarrhythemic drug, furamizole, a nitro furan derivative possessing a strong antibacterial activity, tioda-zosin, an antihypertensive drug, fenadiazole, a hypnotic drug, BB-83698 an antibacterial agent, all of which incorporate the oxadiazole ring [5]. Several substituted 1,3,4-oxadiazole exhibit antimicrobial [6, 7] pesti-cidal [8], antimycobacterial [9], and antifungal [10] activities.
By observing the potentiality of the indole derivatives [11—13] and benzophenone analogues [14—17] we have planned and synthesized a series of 1,3,4-ox-adiazoles containing 2-N-methyl indole-5-substitut-ed benzophenone moiety (9a—f) in a single framework. Besides to evaluate the antioxidant potentiality of all the analogues (8a—f) and (9a—f) were subjected for antioxidant potential through different in vitro models such as DPPH, nitric oxide and hydrogen peroxide free radical-scavenging activity.
RESULTS AND DISCUSSION
A series of 2,5-disubstituted 1,3,4-oxadiazole derivatives containing N-methyl indole and benzophe-none moiety were efficiently synthesized in moderate to good yields, and were screened for antioxidant activity. The series of compounds were prepared in a
,R
R2
K2CO3 ClCH2COOC2H5_ Dry Acetone Refflux 1 8-10 h
r
R
(3a—f)
H
H3C (9a-f)
Scheme. Synthesis of 2,5-disubstituted 1,3,4-oxadiazole analogues.
multi-step process. The reaction sequence for different title compounds (9a—f) was outlined in Scheme. The starting materials substituted phenyl benzoates (3a—f) were synthesized according to a reported procedure through the reaction of substituted phenol (1) with substituted acid chlorides (2a—f) in the presence of10% sodium hydroxide. The phenyl benzoates (3a— f) on subjecting to Fries rearrangement afforded 4-hy-
droxy benzophenones (4a—f) [14]. Ethyl (2-aroyl-4-methylphenoxy) acetates (5a—f) were achieved in excellent yield by reacting compounds (4a—f) with ethyl chloroacetate in the presence of anhydrous potassium carbonate and dry acetone. Compounds (5a—f) on treated with 99% hydrazine hydrate gave different hy-drazides (6a—f). Finally, compounds (6a—f) on couple with indole 3-carboxylic acid (7) in the presence of
Table 1. The in vitro antioxidant activity of compounds (8a— f) and (9a—f) in DPPH method
Compounds Concentration (^g/mL)
25 50 75 100 IC50
(8a) 62.56 ± 0.69 66.74 ± 1.30 70.69 ± 1.27 75.77 ± 1.32 18.78 ± 1.21
(8b) 51.87 ± 1.70 57.95 ± 1.00 60.68 ± 0.90 66.99 ± 1.58 22.83 ± 1.45
(8c) 78.85 ± 0.27 79.85 ± 0.48 82. 92 ± 0.59 84.89 ± 0.76 15.78 ± 0.26
(8d) 72.94 ± 0.17 76.78 ± 0.43 78.95 ± 0.44 82.79 ± 0.68 15.67 ± 0.58
(8e) 46.36 ± 0.57 51.21 ± 1.20 54.98 ± 0.53 60.31 ± 0.79 23.78 ± 1.01
(8f) 60.41 ± 1.18 65.76 ± 1.25 69.52 ± 0.89 73.64 ± 0.96 21.36 ± 0.77
(9a) 64.33 ± 0.31 66.74 ± 0.47 69.51 ± 0.66 73.49 ± 0.79 19.98 ± 1.27
(9b) 54.26 ± 0.90 57.13 ± 1.40 60.25 ± 0.79 65.64 ± 1.06 24.79 ± 0.43
(9c) 64.19 ± 1.08 67.83 ± 1.27 69.75 ± 0.58 72.36 ± 0.69 16.86 ± 0.53
(9d) 69.84 ± 0.24 73.85 ± 0.41 76.69 ± 0.63 80.56 ± 0.68 16.85 ± 0.59
(9e) 48.62 ± 0.60 52.85 ± 1.24 56.61 ± 0.55 61.92 ± 0.82 25.67 ± 1.10
(9f) 59.74 ± 1.17 64.47 ± 1.24 68.91 ± 0.88 72.17 ± 0.95 20.94 ± 0.78
Ascorbic acid 81.97 ± 0.11 82.89 ± 0.36 84.84 ± 0.42 86.78 ± 0.51 14.97 ± 0.42
Blank - - - - -
"-" Showed no scavenging activity. Values were the means of three replicates ± SD.
N,N,N ',N '-tetramethyl-o-(benzotriazol-1-yl)uroni-um tetrafluoroborate (TBTU) and 2,6-dimethyl pyridine (Lutidine) as a coupling agent furnished (8a—f) in excellent yield. Further final compounds were archived by intramolecular cyclization of (8a—f) in the presence of triflic anhydride, pyridine and DCM obtained (9a—f) in good yield. The structures of the newly synthesized compounds have been established on the basis of 1H NMR, IR, mass spectral and elemental analysis. The appearance of peaks at 1620 cm-1 (C=N) and 1155 cm-1 (cyclic C-O-C) confirmed the cyclization of compounds (8a—f) into 1,3,4-oxadi-azoles (9a—f). Also disappearance of a broad singlet at 10.1-10.5 ppm for NHNH protons in compounds (9a—f) further supported the formation of 1,3,4-oxa-diazoles. The disappearance of amide C=O and NHNH stretching bands of compounds (8a—f) and detection of strong C=N stretching band at about 1632 cm-1, are strong evidences for ring closure of 1,3,4-oxadiazoles (9a—f).
All the synthesized compounds (8a—f) and (9a—f) were screened for their possible in vitro antioxidant activity through different in vitro models such as di(phe-nyl) - (2,4,6-trinitrophenyl)iminoazanium (DPPH), nitric oxide and hydrogen peroxide free radical-scavenging activity. Observing the results indicated that, few of the tested compounds are significant in their antioxidant properties. Particularly compounds (8c), (8d), (9c) and (9d) were most efficient among the series with its IC50 value almost comparable with the standard drug ascorbic acid in all the above three methods. In DPPH method compounds please, go on (8c), (8d), (9c) and (9d) having IC50 values of 15.78,
15.67, 16.86 and 16.85 ^g/mL respectively, compared to the standard ascorbic acid 14.97 ^g/mL The same compounds (8c), (8d), (9c) and (9d) in nitric oxide scavenging assay shows IC50 values of 14.62, 15.98, 14.84 and 16.11 ^g/mL respectively, compared to the standard ascorbic acid 13.97 ^g/mL. Further, in hydrogen peroxide radical scavenging assay compounds (8c), (8d), (9c) and (9d) having IC50 values of 16.35, 17.54, 17.64 and 17.70 ^g/mL respectively, compared to the standard ascorbic acid 15.83 ^g/mL.
This property of the compounds (8c) and (8d) could be attributed to the methyl and methoxy group respectively, attached to the para position of the benzoyl ring in the benzophenone molecule. In compounds (9c) and (9d) the presence of one additional oxadiazole ring in between benzophenone and N-me-thyl indole is the most important reason for exhibiting antioxidant activity near to the standard drug.
EXPERIMENTAL
All solvents and reagents were purchased from Sigma Aldrich Chemicals Pvt Ltd. Melting points were determined on an electrically heated VMP-III melting point apparatus. The elemental analysis of the compounds was performed on a Perkin Elmer 2400 Elemental Analyser. The FT-IR spectra were recorded using KBr discs and Nujol on FT-IR Jasco 4100 infrared spectrophotometer. The 1H NMR spectra were recorded using Bruker DRX 400 spectrometer at 400 MHz with TMS as the internal standard. Mass spectra were recorded on LC-MS/MS (API-4000) mass spectrometer.
Table 2. The in vitro antioxidant activity of compounds (8a— f) and (9a—f) in nitric oxide (NO) method
Compounds Concentration (^g/mL)
25 50 75 100 IC50
(8a) 63.85 ± 1.16 68.95 ± 1.55 73.82 ± 1.39 78.92 ± 0.69 18.77 ± 1.21
(8b) 65.67 ± 1.65 68.78 ± 1.30 72.83 ± 1.40 77.56 ± 0.74 25.97 ± 1.19
(8c) 77.58 ±
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