Design, synthesis and biological potentials of novel tetrahydroimidazo[1,2-a]pyrimidine derivatives

Background A novel series of 5-(substituted aldehyde)-7-methyl-3-oxo-N-phenyl-2-((3,4,5,6-tetrahydroxytetrahydro-2H-pyran-2-yl)methylene)-1,2,3,5-tetrahydroimidazo[1,2-a]pyrimidine-6-carboxamide analogues (1–24) was synthesized using the Biginelli condensation. Results and discussion The synthesized compounds were screened for their in vitro antimicrobial potential against Gram (positive and negative) bacterial and fungal strains by tube dilution technique. In the series, compound 15 exhibited significant antimicrobial activity against Candida albicans and Aspergillus niger with MIC value = 1.04 × 10−2 µM/ml and compound 2 was found to be most active antioxidant agent with IC50 value = 46.31 using DPPH assay. Anticancer activity results indicated that compound 23 displayed better anticancer activity against human breast cancer cell line (MCF-7) with GI50 value = 34.78 using SRB assay. Conclusions All synthesized derivatives exhibited good antimicrobial, antioxidant and anticancer activity using specific method and compared with standard drugs, especially compounds 2, 15 and 23 displayed more activity than reference drugs. Structure activity relationship demonstrated that presence of electron releasing groups of the synthesized compounds enhanced the antibacterial activity against Escherichia coli as well as antioxidant activity and electron withdrawing groups improved the antimicrobial as well as anticancer activity against human breast (MCF-7) cancer cell line.


Background
Pyrimidines are obtained from the various natural resources and synthethic reaction in medicinal chemistry [1]. They are also known as m-diazine or 1,3-diazone can be considered as cyclic amine. Heterocyclic compounds are used in agricultural and medicinal reasons using biological and chemical studies. Pyrimidine derivatives play a vital role in several biological activities i.e. antihypertensive, anticancer, antimicrobial, anti-inflammatory, antifungal, analgesic, antioxidant, anticonvulsant and antiviral [2]. Antimicrobials agents are one of the most important weapons in the resistance of infection caused by bacterial strains [3]. In the past few years, increase the resistance of microorganisms toward antimicrobial agents become a serious health problem so there is a need of safe, potent and novel antimicrobial agents [4]. Pyrimidine derivatives showed most antimicrobial activity against Gram +ve and Gram -ve microorganism [5]. At that time, many antimicrobial drugs are present in the market but due to the indiscriminate use of antimicrobial agents often followed the development of resistant strains of microorganism so there is a need for the development of new class of active antimicrobial drugs with lesser or no side effects [6]. Pyrimidine agents recently attracted medicinal chemist in exploring their potential as antioxidant agents. Oxidative stress appears to play an important role in many human diseases, including cancers. The use of antioxidants in pharmacology is intensively studied, particularly for stroke and neurodegenerative diseases [7]. Antioxidants are the agents that neutralize free radicals, which scavenge reactive oxygen species may be high potent value in preventing the onset and propagation of oxidative diseases like neurovascular, autoimmune and cardiovascular diseases [8].
Cancer is one of the most serious medical problem and second leading cause of death in the world, characterized by a deregulation of the cell cycle which mainly results in a progressive loss of cellular differentiation and uncontrolled cellular growth. The current situation highlights the need for discovery and development of small molecule anticancer drugs with improved tumor selectivity, efficacy and safety remains desirable [9]. Many pyrimidine derivatives were reported to be active against various forms of cancer. Due to less effective, more side effect and lack of a broad range of anticancer agents there is a need of anticancer agents have motivated the idea of researchers toward the discovery of novel anticancer agents [10]. Owing to the pharmacological significance of pyrimidine derivatives so, we have planned to synthesize some new pyrimidine derivatives and evaluate for their antimicrobial, antioxidant and anticancer activities.

Antimicrobial activity
The in vitro antimicrobial activity of synthesized compounds against Gram-positive bacteria: Staphylococcus aureus (MTCC 3160), Bacillus subtilis (MTCC 441), Gram-negative bacterium: Escherichia coli (MTCC 443) and fungal: Candida albicans (MTCC 227) and Aspergillus niger (MTCC 281) strains was examined by tube dilution method [11]. Norfloxacin and fluconazole used as standard for antibacterial and antifungal activities respectively. Dilutions of test and standard compounds were prepared in double strength nutrient broth for bacterial strains and sabouraud dextrose broth for fungal strains [12]. The samples were incubated at 37 ± 1 °C for 24 h (for bacterial species), at 25 ± 1 °C for 7 days (A. niger) and at 37 ± 1 °C for 48 h (C. albicans) respectively and the results were recorded in terms of MIC (the lowest concentration of test substance which inhibited the growth of microorganisms). In case of Gram positive bacteria, compounds 12 and 14 (MIC sa = 2.14 × 10 −2 µM/ ml) having significant activity against S. aureus and compound 18 (MIC bs = 0.58 × 10 −2 µM/ml) exhibited most potent against B. subtilis. In case of Gram negative bacterium, compound 21 (MIC ec = 1.10 × 10 −2 µM/ml) displayed more potent activity against E. coli. Compound 15 (MIC ca & an = 1.04 × 10 −2 µM/ml) was found to be most potent against C. albicans and A. niger. These compounds may be taken as lead to discovery novel antimicrobial agents. The presented results are showing in Table 2.

Antioxidant activity
The antioxidant activity of the synthesized compounds was evaluated with spectrophotometrically using free radical scavenging DPPH assay. The DPPH is a stable free radical with maximal absorption at 517 nm and is reduced to a corresponding hydrazine when it reacts with hydrogen donors. When DPPH reacts with an antioxidant agent, it can donate hydrogen get reduced and deep violet colour of DPPH change to yellow, showing a considerable decreased in absorption at 517 nm. DPPH solution (3 μg/ml) was prepared in methanol (methanol: DPPH in 1:1) for blank reference. Four types of dilutions were prepared in the methanol of the synthesized derivatives and standard (ascorbic acid) in the concentration of 25, 50, 75 and 100 μg/ml and then 1 ml of each concentration was added to 1 ml of DPPH solution. The solution mixture was shaken vigorously and kept in dark place for 30 min at room temperature and absorbance was measured by UV at 517 nm [13]. Free radical DPPH inhibition in percentage (%) was calculated as follows: where, A Blank = absorbance of the blank reaction, A Sample = absorbance of the test compound IC 50 value was calculated from the graph plotted between % inhibition and synthesized compound (Figs. 1,2,3). Antioxidant activity demonstrated, compounds 2 and 16 exhibited excellent activity at absorbance 517 nm with IC 50 values = 46.31 and 48.81 respectively and compared with ascorbic acid as standard drug. These compounds may be used as a lead for development of new antioxidant agents. The presented results are showing in Table 3. 17. -- 24.

Experimental section
Synthesized pyrimidine derivatives followed the general procedure discussed in synthetic (Scheme 1). All reagents and solvents used in study were of both laboratory and analytical grade and procured from commercial market.

General procedure for synthesized pyrimidine analogues
Step 1: intermediate-I A mixture of 3-oxo-N-phenylbutanamide (0.02 mol), guanidine nitrate (0.030 mol) and corresponding aldehyde (0.02 mol) in the round bottom flask with 100 ml methanol and then added aluminum chloride (0.006 mol) with 4-5 drops of concentrated hydrochloric acid after that the reaction mixture was refluxed for 10-11 h. before completion of the reaction we had been checked the reaction with every 30 min by TLC plats with suitable solvent system (benzene). After completion of the reaction the reaction mixture was cooled at room temperature and poured into ice cold water with vigorous stirring, filtered and recrystallized with methanol [11].

Table 4 Percentage (%) control growth against human breast cancer cell line MCF-7
The significance of italic values was found to be most active against human breast (  (0.030 mol) were taken in round bottom flask and refluxed with for 6-7 h (controlled temperature at 140-142 °C) before completion of the reaction, we had been checked the reaction with every 30 min by TLC plats with suitable solvent system (benzene). After completion of the reaction the reaction mixture was cooled at room temperature and poured into ice cold water to yielded solid precipitate, filtered and recrystallized with methanol.