Synthesis and antioxidant activity of some new substituted pyrazolo [4, 5-e]-4H-pyrimido[2,3-b] benzimidazoles

 

Prashant N. Ubale, Sambhaji P. Vartale*, Sandip G. Sontakke

PG Research Centre, Department of Chemistry, Yeshwant Mahavidyalaya Nanded-431602 (MS), INDIA

*Corresponding Author E-mail: spvartale@gmail.com

2-Amino benzimidazole condensed with ethyl-2-cyano-3,3-bis(methylthio)acrylate using catalytic amount of potassium carbonate and DMF as solvent to form 3-cyano-2-metylthio-4-oxo-4H-pyrimido [1, 2-a] benzimidazole (3). The formed compound (3) reacted with differently substituted aryl / heteryl hydrazino compounds then it gives series of substituted pyrazolo [4, 5-e]-4H-pyrimido [2, 3-b] [1, 3] benzimidazoles (5a-h). Structure of compounds were confirmed by spectroscopic techniques IR, mass, ¹H &¹³C NMR and analytical properties. all synthesized compounds were screened for their antioxidant activity.

 

 

 

INTRODUCTION:

Pyrimidine fused with benzimidazole to form pyrimido benzimidazole. Pyrimido benzimidazole have wide range of application in medicinal chemistry such as antimicrobial¹, antimalerial², T-cell activation³, P-receptor binding agent⁴, Anti-inflammatory, antiamoebic and analgesic activity⁵etc. Because of these distant importances against life threatening diseases pyrimido benzimidazole have very good medicinal value.

 

Pyrazole is five membered heterocycles in which two nitrogen are present on adjacent position. Pyrazole ring is present in different functional drugs such as Phenyl butazole, forbisen, oxyphenbutazole, sulphinpyrazone, feprazone, phenyl butazone, analgin etc. Despite of these, pyrazoles and its derivatives also used to cure anti-bacterial⁶, anti-fungal⁷, anti-leukemic agent⁸, antianalgesic⁹, anticancer¹⁰, anti-viral agent¹¹.

 

When pyrimido benzimidazole fused with pyrazole to form pyrazolo pyrimido benzimidazole. This moity contain three basic heterocyclic ingredients as pyrazole, pyrimidine and benzimidazole which contain five nitrogens in the fused ring system. Very few references are available on synthesis of pyrazolo pyrimido benzimidazole. Okamoto et.al¹² synthesized pyrozolo[3’,4’:4,5] pyrimido[1,6-a]benzimidazole by refluxing 4-(2-benzimidazolyl)-1-methyl-5-aminopyrazole with trialkylorthoformates. I.B.Dzvinchuk et.al¹³ reported the synthesis of Pyrazolo[5′,1′:2,3]pyrimido[1,6-a]benzimidazole by cyclization of cyanoethylhydrazones of 2-arylmethyl-1H-benzimidazoles with trifluroacetyl anhydride. Abdou O. Abdelhamid et al¹⁴ prepared the substituted pyrazolo [3',4':4,5] pyrimido[1,6-a] benzoimidazolo-5-one.In the present work, we synthesized the substituted pyrazolo pyrimido benzimidazole by condensing pyrimido[1,2-a] benzimidazole with substituted aryl / heteryl hydrazine compounds. And the study of antioxidant activity of these prepared compounds.

 

MATERIAL AND METHODS:

Material:

All the chemicals used in present works are from Sigma Aldrich, SD-fine and Spectrochem and used without any further purification. Melting points of the products are determined in open capillary tubes on an electrothermal melting point apparatus and were uncorrected. All the reactions were regulated by TLC. IR, ¹H-NMR and Mass spectra were recorded on Shimadzu FT-IR spectrophotometer, Bruker advance spectrophotometer 500 MHz in DMSO-d₆ using tetramethylsilane as an internal standard and GC-MS spectrometer using the ESI technique respectively.

 

General Procedure:

Synthesis of 3-Cyano-2-metylthio-4-oxo-4H-pyrimido [1, 2-a] benzimidazole (3)

The parent compound, 3-cyano-2-metylthio-4-oxo-4H-pyrimido [1, 2-a] benzimidazole(3)prepared by treating mixture of 2-amino benzimidazole (0.01mole) and ethyl-2-cyano-3,3-bis(methylthio)acrylate (0.01mole) in 20 ml of N, N-dimethyl formamide (DMF) and anhydrous potassium carbonate (10mg) was refluxed for 5 to 6 hours. Then reaction mixture poured into ice cold water. The solid obtained recrystallized from ethanol-DMF mixture (7:3) (Scheme I).

 

Synthesis of 3-amino-4-oxo-2-N-(substituted) pyrazolo [4, 5-e] 4H-pyrimido [2, 3-b] [1, 3] benzimidazole (5a-h):

An equimolar mixture of 3-cyano -2-metylthio-4-oxo-4H-pyrimido [1, 2-a] benzimidazole (0.01mole) is condensed with different substituted aryl / heteryl hydrazino compounds (0.01mole) in presence of catalytic amount of anhydrous potassium carbonate and N, N'- dimethyl formamide as reaction solvent (25 ml). The reaction mixture was cooled to room temperature and poured in to ice cold water. The separated solid product was filtered and then washed with water and recrystalized from ethanol and DMF mixture (7:3) to get corresponding substituted pyrazolo [4, 5-e] 4H-pyrimido [2, 3-b] [1, 3] benzimidazole (5a-g) (Scheme-II).

 

RESULT AND DISCUSSION:

a] Chemistry:

In present work, we have reported one pot synthesis of 3-amino-4-oxo-2-N-(substituted) pyrazolo [4, 5-e] 4H-pyrimido [2, 3-b] [1, 3] benzimidazole (5a-g). Reaction started with 2-amino benzimidazole (1) and ethyl 2-cyano-3, 3-bis (methylthio) acrylate (2) were refluxed in N, N’-dimethyl formamide and catalytic amount of anhydrous potassium carbonate to afford compound (3) shown in scheme-I.

 

 

Compound (3) contain bis electrophilic centres which will give nucleophilic substitution with -SCH₃ group followed by nucleophilic attack of –NH- of hydrazine on nucleophilic centre of –CN to form new five membered cyclic fused pyrazole heterocycles (scheme-II).

 

 

b] Spectral and physical data:

The structures of newly synthesized compounds were proved on the basis of spectral analysis such as IR, ¹H NMR, ¹³C NMR & mass spectral data. Compounds 5a-g shows absence of IR absorption band in the range of 2200-2270 cm^-1 of –CN stretching. Carbonyl strech occurs between 1600 cm^-1 to 1670 cm^-1. ¹H NMR is one of the very important tools for verification of organic molecules in that we observed the characteristic peaks from δ 6.50 to 8.70 ppm due to aromatic =C-H proton in all the structures. Similarly, -NH and -NH₂ proton shows singlet between δ 3.8 to 4.5 ppm. Molecular ion peaks of mass spectra are also in good agreement with the molecular weight of structures.

 

3-Cyano-2-metylthio-4-oxo-4H-pyrimido[1,2-a] benzimidazole (3)

IR (KBr/cm-1): 2214 cm^-1 (CN), 1666 cm^-1 (CO), ¹H NMR (400 MHz, DMSO-d6) δ 3.8 (s, 1H, -NH), 2.3 (s, 3H, SCH₃), 7.2-8.4 (m, 4H,Ar-H) ppm; EI-MS (m/z:RA%): 258 (M+1).

 

3-amino-4-oxo-2-N-(4^’-methylphenyl)-pyrazolo[4,5-e]-4H-pyrimido[2,3-b] [1,3] benzimidazole (5b):

IR: 3305, cm^-1 (-C-NH₂), 1458-1647, (aromatic C=C); ¹H NMR: (500 MHz, DMSO) δ: 3.85 (s, 1H, aromatic C-NH), 4.1(s, 2H, -NH₂) ppm; Mass: m/z = 231 (M+1).

 

3-amino-4-oxo-2-N-(2’-benzothiazolyl)-pyrazolo[4,5-e]4H-pyrimido[2,3-b] [1,3] benzimidazole (5e):

IR:  3315, cm^-1 (-C-NH₂), 1640 cm^-1 (-CO), 1458-1650, (aromatic C=C strech); ¹H NMR: (500 MHz, DMSO) δ : 3.81 (s, 1H, aromatic C-NH), 4.5 (s, 2H, C-NH₂), 7.2-8.4(m, 8H, Ar-H) ppm; Mass: m/z = 374 (M+1).

 

Table: 1 Physico-chemical data of newly synthesized compounds.

 

 

c] Anti-oxidant activity:

1] DPPH (1, 1-diphenyl-2-picrylhydrazyl) radical scavenging assay:

DPPH (1, 1-diphenyl-2-picrylhydrazyl) radical scavenging assay was carried out as per reported method. In brief, 1 ml (1 m Mol) of the test compound is added to equal quantity of 0.1 m Mol solution of DPPH in ethanol. After 20 min of incubation at room temperature, the DPPH reduction was measured by reading the absorbance at 517 nm. Ascorbic acid (1 m Mol) was used as the reference compound. The overall DPPH radical scavenging activity of tested substituted pyrazolo [4, 5-e] pyrimido [2, 3-b] [1, 3] benzimidazole (5a-g) were in the range of 21.1 + 0.231 to 30.2 + 0.422 as compared to the standard ascorbic acid 78.48 + 0.13.

 

2] OH Radical scavenging activity.

The OH radical in which oxygen species are highly reactive, proton radical scavenging action is known as an important mechanism of antioxidants, study of table clearly indicates comparatively moderate OH radical scavenging activity of newly synthesized imino pyrimido[1,2-a] benzimidazole derivatives in a range of 15.9+ 0.241 to 35.1 + 0.537 as compared with standard ascorbic acid (02.67 + 0.24%).

 

Table: 2 Antioxidant potentials of newly synthesized compounds.

 

CONCLUSION:

In the present work, we synthesized different substituted pyrazolo [4, 5-e] pyrimido [2, 3-b] [1, 3] benzimidazole (5a-g) in good yields. Selected newly synthesized derivatives were evaluated in vitro to determine their free radical scavenging activities using DPPH free radical model. It is important to note that the series novel pyrazolo [4,5-e] pyrimido [2,3-b] [1,3] benzimidazole derivatives were comparatively moderate in stabilizing the DPPH and hydroxyl free radical as compared with the standard ascorbic acid. It can be positively concluded that from first to last modification of novel series to design more effective antioxidants.

 

ACKNOWLEDGEMENTS:

The authors are grateful to Principal, Yeshwant Mahavidyalaya, Nanded for providing laboratory facilities. We also thankful to Director CIF, Savitribai Phule Pune University, Pune and Vishnu Chemicals Ltd. for providing spectral data and Department of Botany, Indira Gandhi Mahavidyalaya, CIDCO, Nanded for evaluation of antioxidant activity.

 

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Received on 13.06.2017         Modified on 10.07.2017

Accepted on 20.08.2017         © AJRC All right reserved