Synthesis and antibacterial activity of novel 5,6,7,8-tetrahydroimidazo[1,2-a]pyrimidine-2-carbohydrazide derivatives

Background The intensely increasing multi-drug resistant microbial infections have encouraged the search for new antimicrobial agents. Hydrazone derivatives are known to exhibit a wide variety of biological activities including anti-microbial. In heterocyclic moiety, imidazo[1,2-a]pyrimidines are the subject of immense interest for their antimicrobial activity and also for their analgesic, antipyretic and anti-inflammatory properties. Results Condensation of 5,6,7,8-tetrahydroimidazo[1,2-a]pyrimidine-2-carbohydrazide 7 with aromatic aldehydes a–k in ethanol at reflux led to the generation of hydrazone derivatives 8a–k in 80–92% yield. The synthesis of carbohydrazide 7 was accomplished in six steps from commercially available 2-amino pyrimidine. The structures of the synthesized compounds were confirmed by 1H, 13C NMR, Mass and IR spectral data. All the synthesized hydrazone derivatives 8a–k were tested in vitro for their antibacterial activity. Compounds 8d, 8e and 8f exhibited excellent antibacterial activity with zone of inhibition 30–33 mm against E. coli (Gram negative bacteria) and S. aureus (Gram positive bacteria). These compounds also exhibited excellent antibacterial activity with zone of inhibition 22–25 mm against P. aeruginosa (Gram negative bacteria) and S. pyogenes (Gram positive bacteria). Conclusion Synthesized and recorded antibacterial activity of some new 5,6,7,8-tetrahydro-imidazo[1,2-a]pyrimidine-hydrazone derivatives.Graphical abstract: Synthesis of 5,6,7,8-tetrahydroimidazo[1,2-a]pyrimidine-2-carbohydrazide derivatives


Background
The fast resistance of bacteria against antibiotics has become a prevailing medical problem. Treatment options for these infections are often inadequate especially in immune compromised patients. The intensely increasing multi-drug resistant microbial infections in the past few decades have become a serious health issue. The exploration of new antimicrobial agents will always remain as a major challenging task.
Hydrazones constitute a versatile compound of organic class having the basic structure R 1 R 2 C=NNH 2 . Hydrazones are formed by the replacement of the oxygen of carbonyl compounds with the -NNH 2 functional group.
The α-hydrogen atom of hydrazones is more acidic as compared to ketones [1,2], because of its nucleophilic nature. Hydrazones are mainly synthesized by refluxing the appropriate quantity of substituted hydrazines/hydrazides with ketones and aldehydes in suitable solvents like ethanol, ethanol-glacial acetic acid, tetrahydrofuran, butanol, methanol, glacial acetic acid, etc. Hydrazone is a moiety that exhibits a wide variety of biological activities like anticonvulsant,antidepressant,analgesic,antimicrobial,antitumor,vasodilator,antiviral,anticancer,.
Imidazo [1,2-a]pyrimidines are evolving as potentially interesting drugs particularly with regard to their antimicrobial, analgesic, antipyretic, anti-inflammatory properties and also with regard to their activity against ulcers [13]. They are also important as benzodiazepine receptor Open Access *Correspondence: mtcharya@yahoo.com agonists, antiviral agents and calcium channel blockers [14].
Keeping in view of the biological importance of hydrazones and Imidazo [1,2-a]pyrimidines, in continuation to our research program about a potent antimicrobial agent, we report herein the synthesis, characterization and antibacterial activity of some new 5,6,7,8-tetrahydro-imidazo[1,2-a]pyrimidine-hydrazone derivatives. It is interesting to note that, a class of 5,6,7,8-tetrahydroimidazo[1,2-a]pyrimidine compounds is prescribed for treatment to reduce neurotoxic injury associated with anoxia or ischemia which typically follows stroke, cardiac arrest, hypoglycemia or perinatal asphyxia [11].

Antibacterial activity
The screening results of antibacterial activity of hydrazone derivatives 8a-k are summarized in Table 1. It is observed that compounds 8d, 8e and 8f revealed excellent antibacterial activity with zone of inhibition 30-33 mm against E. coli (Gram negative bacteria) and S. aureus (Gram positive bacteria) even in the case of P. aeruginosa (Gram negative bacteria) and S. pyogenes (Gram positive bacteria), compounds 8d, 8e and 8f displayed excellent anti-bacterial activity with zone of inhibition 22-25 mm. Compounds 8a, 8b and 8c showed appreciable activity while the compounds 8g and 8h showed moderate activity against all the tested bacterial strains. From the structural point of view of the scaffold, it may be generalized that scaffold with R = 4-CF 3 , 4-OCF 3 and 4-OCHF 2 showed excellent antibacterial activity, while R = 4-F, 2-CF 3 and 3-CF 3 showed good antibacterial activity and R = 2,4-difluoro and 3,4-difluoro showed moderate antibacterial activity. Furthermore, it is observed that within the series of the hydrazone derivatives 8a-k, compounds 8i (R = 2-Me, 3-F), 8j (R = 2-Cl, 3-F) and 8k (R = benzo[b]furan) showed no activity.

Experimental section
All commercial chemicals were taken as they are. The solvents underwent purification process as per standard procedures. For thin-layer chromatography (TLC) analysis, Merck pre-coated Plates (silica gel 60 F254) were used and spots were visualized with UV light. Merck silica gel 60 (230-400 mesh) was used for flash column chromatography and the eluting solvents were indicated in the procedures. Melting point (mp) determinations were performed by using Mel-temp apparatus and are uncorrected. 1 H NMR spectra were recorded in Varian MR-400 MHz instrument. Chemical shifts were reported in δ parts per million (ppm) downfield from tetramethylsilane (TMS) with reference to internal standard and the signals were reported as s (singlet), d (doublet), dd (doublet of doublet), t (triplet), q (quartet), m (multiplet) and coupling constants in Hz. The mass spectra were recorded on Agilent ion trap MS. Infrared (IR) spectra were recorded on a Perkin Elmer FT-IR spectrometer.
The benzaldehydes a-k utilized for the synthesis of 8a-k were purchased from commercial sources.

Synthesis of imidazo[1,2-a]pyrimidine-2-carbaldehyde 3
The compound 2 (1 g, 4.95 mmol) was dissolved in water (30 mL) and CaCO 3 was added (5 g, 24.75 mmol) and refluxed for 1 h. The reaction mixture was cooled to room temperature and extracted with ethylacetate and evaporated under reduced pressure to obtain the crude compound 3, which was purified by column chromatography (silica gel: 60-120 mesh, eluent:

Synthesis of imidazo[1,2-a]pyrimidine-2-carboxylic acid 4
To a solution of compound 3 (2 g, 13.60 mmol) in DMF (15 vol), Oxone was added (40.80 mmol) at 5°C and stirred for 2 h. The completion of the reaction was monitored by TLC. As soon as the reaction was completed, the reaction mixture was diluted with water (45 mL) followed by ethyl acetate. The organic layer was washed with water followed by brine solution, dried over sodium sulphate, filtered and evaporated to produce compound 4 as pale yellow syrupy liquid. The crude compound was used in the next step without further purification. Yield: 1.10 g, 50%.

General experimental procedure for the synthesis of hydrazone derivatives (8a-k)
Respective benzaldehydes a-k (1.0 mmol) were added to an ethanol solution containing 7(100 mg, 0.553 mmol) and the contents were stirred at reflux temperature for 6 h. Ethanol was evaporated from the reaction mixture and the residue was extracted with pet-ether, to obtain pure compounds. Yields of the products varied between 80 and 92%.