Pyrimidines as Potent Antioxidant Agents

 

K. Ishwar Bhat¹, Abhishek Kumar^2*

¹Department of Pharmaceutical Chemistry, NGSM Institute of Pharmaceutical Sciences,

Nitte (Deemed to be University), Paneer, Deralakatte-575018, Mangalore, Karnataka.

²Assistant Professor, Department of Pharmaceutical Chemistry, NGSM Institute of Pharmaceutical Sciences Paneer, Deralakatte, Mangalore 575018, Karnataka, India.

 

ABSTRACT:

A series of novel pyrimidine-2-thiol derivatives (PM₁-PM₆) have been synthesized upon refluxing 1-(4-nitrophenyl)-3-phenyl substituted prop-2-en-1-one in presence of thiourea and glacial acetic acid. 1-(4-nitrophenyl)-3-phenyl substituted prop-2-en-1-one were prepared by condensing 4-nitroacetophenone with substituted benzaldehydes in presence of 20% NaOH as base. The structures of newly synthesized compounds were characterized by IR, ¹H NMR and mass spectroscopic studies. Newly synthesized compounds were screened for their antioxidant studies. Most of the compounds exhibited excellent antioxidant activity compared to the standard drug ascorbic acid. 

 

 

 

INTRODUCTION:

Pyrimidine are the most important six membered heterocyclic compounds having two nitrogen atoms. Pyrimidine moiety is present in living systems in the form of nucleic acids and vitamins; since it is the basic nucleus in DNA and RNA and possess diverse biological activities. They are known for the broad spectrum of biological activities such as antimicrobial¹, antitumor², anti-inflammatory³, antibacterial⁴, anti tubercular⁵ and cytotoxic⁶ activity. Pyrimidine hetero cycles possessing hydroxyl and thiol group has a unique place in medicinal chemistry and also plays a vital role in biological processes as well as synthetic drugs. Chalcones act as intermediates in the biosynthesis of various flavonoids and also have been used as intermediates in the synthesis of various pharmacologically significant heterocyclic molecules such as pyrimidines, pyrazolines, isoxazolines and benzodiazepines.

 

Prompted by these literatures, we planned to synthesize some novel pyrimidine derivatives derived from chalcones and to explore their biological and pharmacological activities. Herein, the synthesis of some derivatives of title structure compounds containing pyrimidine moiety is reported in an attempt to significantly improve the biological spectrum of pyrimidines. All the novel compounds were evaluated for their antioxidant activity studies.

 

MATERIALS AND METHODS:

All the chemicals were of analytical grade: 4-nitroacetophenone, substituted benzaldehyde, thiourea, glacial acetic acid, ethanol and sodium hydroxide.

 

Melting points were determined by open capillary method and are uncorrected. Purity of the intermediates and final compounds were monitored by thin layer chromatography (TLC) using silica gel G plates. The spots were visualized under UV light and by the exposure to iodine vapors. Chloroform: Ethylacetate (9:1) was used as solvent for running the TLC of these compounds. All IR spectra were recorded in Alpha Bruker using ATR method. ¹H NMR spectra were recorded on Bruker spectrophotometer (400 MHz) in DMSO solvent using tetra methyl silane (TMS) as an internal standard. Mass spectra was recorded by using a Jeol-D 300 mass spectrometer (70 eV), Shimadzu (Japan) by FAB.

 

General Procedure:

Synthesis of 1-(4-nitrophenyl)-3-phenyl substituted prop-2-en-1-one⁷ (CM₁-CM₆):

A mixture of 4-nitroacetophenone (0.01 mol) and substituted benzaldehyde (0.01 mol) in ethanol were stirred for 24 hrs in presence of 20% NaOH. The completion of the reaction was monitored by TLC. The mixture was poured into crushed ice and acidified with 5% HCl. The product was filtered and recrystallised from ethanol.

 

Synthesis of 4-(4-nitrophenyl)-6-phenyl substituted pyrimidine-2-thiol⁸ (PM₁-PM₆):

A mixture of substituted chalcones (0.01 mol) in ethanol/glacial acetic acid and thiourea (0.01 mol) in 20% NaOH was refluxed for 20 hrs. After completion of the reaction, reaction mixture was poured into ice cold water, filtered and recrystallised from ethanol. Purity of the compound was monitored by TLC using Chloroform: ethylacetate (1:9) as solvent.

 

 

 

Figure 1: Reaction scheme for Pyrimidine derivatives

 

 

 

 

 

Spectral data

4-(4-chlorophenyl)-6-(4-nitrophenyl)pyrimidine-2-thiol (PM₁)

 

IR KBr (cm^-1): 1510(C=C str), 830(Ar C-H bending), 3025(Ar C-H str), 1684(C=N str), 2056(S-H str), 1348(C-N str), 740(C-Cl str).

 

¹H NMR (400 MHz, DMSO): δ 7.2-7.6 (m, 10H, Ar-H), 9.3 (s, 1H, SH).

MS (M+1): m/z 346.

 

4-(4-fluorophenyl)-6-(4-nitrophenyl)pyrimidine-2-thiol (PM₂)

IR KBr (cm^-1): 1505(C=C str), 825(Ar C-H bending), 3022(Ar C-H str), 1678(C=N str), 2049(S-H str), 1351(C-N str), 1295(C-F str).

 

¹H NMR (400 MHz, DMSO): δ 7.3-7.6 (m, 10H, Ar-H), 9.2 (s, 1H, SH).

 

MS (M⁺): m/z 327.

 

4-(2-mercapto-6-(4-nitrophenyl)pyrimidin-4-yl)phenol (PM₃)

 

IR KBr (cm^-1): 1502(C=C str), 821(Ar C-H bend), 3018(Ar C-H str), 1659(C=N str), 2052(S-H str), 1348(C-N str), 1428 (Ar-NO₂ str), 3482 (O-H str).

¹H NMR (400 MHz, DMSO): δ 7.3-7.7 (m, 10H, Ar-H), 9.3 (s, 1H, SH).

MS (M+1): m/z 325.

 

Evaluation of Antioxidant Activity:

DPPH radical scavenging activity:

The stable radical DPPH was used as the reagent in this spectrophotometric assay⁹. 3 ml of 0.1 mM solution of DPPH and 1 ml of the compounds containing various concentrations (10-200 μg/ml) was mixed and preserved in dark for 30 min. Then it was incubated at 30 min at room temperature. Then the absorbance was measured against blank at 517 nm.

 

Nitric oxide free radical scavenging method:

By using this reagent sodium nitroprusside, nitric oxide radicals (NO) were generated. To this different concentrations (10-200 μg/ml) of the test compounds, 1 ml of sodium nitroprusside (10 mM) and 1.5 ml of phosphate buffer saline (0.2 M, pH 7.4) were added. The resulting mixtures were incubated for 150 min at 25⁰C and 1 ml of the reaction mixture was treated with 1 ml of Griess reagent. The absorbance was measured at 546 nm. The results obtained were expressed for both antioxidant methods as percentage of inhibition, which was calculated according to the following equation¹⁰.

% Inhibition= Control absorbance - Test absorbance

Control absorbance

RESULTS AND DISCUSSION:

Table 1: Physicochemical data of the synthesized Pyrimidine derivatives (PM₁-PM₆)

Comp. code	R	Mol. wt	M.P oC	Physical state	% Yield
PM1	4-Cl	345	232-234	White crystals	68
PM2	4-F	327	205-207	White crystals	71
PM3	3-OH	325	225-227	White crystals	62
PM4	4-CH3	329	258-260	Brown crystals	72
PM5	3-NO2	354	236-238	Yellow crystals	68
PM6	4-CN	334	245-247	White crystals	69

 

 

Table 2: Antioxidant activity of substituted Pyrimidines (PM₁-PM₆) by DPPH radical scavenging method.

Compound	R	% inhibition at different concentrations (μg/ml)
		10	50	100	200
PM1	4-Cl	40	51	64	68
PM2	4-F	55	61	77	89
PM3	3-OH	34	46	52	61
PM4	4-CH3	39	47	54	59
PM5	3-NO2	51	64	72	88
PM6	4-CN	53	65	74	89
Standard	Ascorbic acid	65	83	88	90

 

Table 3: Antioxidant activity of substituted pyrimidines (PM₁-PM₆) by Nitric oxide free radical scavenging method.

Compound	R	% inhibition at different concentrations (μg/ml)
		10	50	100	200
PM1	4-Cl	44	57	66	71
PM2	4-F	53	66	76	83
PM3	3-OH	40	51	62	68
PM4	4-CH3	39	47	56	63
PM5	3-NO2	54	67	76	89
PM6	4-CN	59	69	78	89
Standard	Ascorbic acid	65	83	88	90

 

Antioxidant studies:

The test compounds were subjected to in vitro antioxidant activity by using both DPPH radical scavenging and nitric oxide free radical scavenging method by using ascorbic acid as standard drug. Compounds PM₂, PM₅, PM₆ showed very good activity compared to standard drug ascorbic acid. Radical scavenging activity in DPPH and nitric oxide methods increases with concentration and % inhibition at highest concentration 200 μg/is shown in the Table 2 and Table3.

 

CONCLUSION:

This study reports the successful synthesis of various substituted pyrimidine derivatives with good yields and most of the compounds showed potent antioxidant activity.

ACKNOWLEDGEMENTS:

The authors are thankful to Nitte University for providing the necessary facilities to carry out this research. The authors are grateful to Head, SAIF, Panjab University, Chandigarh for the spectroscopic data.

 

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Accepted on 22.02.2018          © RJPT All right reserved

Research J. Pharm. and Tech 2018; 11(5):1927-1929.