Approach for the Synthesis of Potent Antimicrobials Containing Pyrazole, Pyrimidine and Morpholine Analogues

. The present study is in the interest of some synthesized novel derivatives containing 4-(1,3-diphenyl-1 H -pyrazol-4-yl)- N -(morpholinomethyl)-6-arylpyrimidin-2-amines pooled with different bio-active heterocycles such as pyrazole, pyrimidine and morpholine derivatives. The structures of newly synthesized compounds were elucidated by IR, 1 H NMR, 13 C NMR and mass spectral data. The synthesized compounds were evaluated for their in vitro antimicrobial activity against different bacterial and fungal strains using Mueller-Hinton Broth dilution method. On the basis of SAR studies, it was observed that the presence of electron withdrawing groups remarkably enhanced the antimicrobial activity of synthesized compounds.

The pyrimidine base is found in thymine, cytosine and uracil; which are the essential building blocks of nucleic acids and is also found in Vitamin B 1 . The literature indicated that compounds having pyrimidine nucleus possess broad range of biological activities, like 5-fluorouracil as anticancer, idoxuridine and trifluridine as antiviral, zidovudine and stavudine as anti-HIV, trimethoprim, sulphamethiazine and sulphadiazine as antibacterial, sulphadoxin as antimalarial. Pyrimidine derivatives have remarkable pharmaceutical importance because of their biological activity as anti-HIV [6], antitubercular, antimicrobial [7], antidiabetic [8] and antiinflammatory [9]. It has been found that several DHPM derivatives possess biological activity, which has led to their use as antiplasmodial [10] and anticancer [11]. Substituted aminopyrimidine nuclei is the key structure marketed drugs such as antiatherosclerotic aronixil, anti-histaminic thonzylamine, antianxielytic buspirone, antihypertensive minoxidil and prazosin, anti-psoriatic enazadrem, Common examples of pyrimidine based drugs are: Bacimethrin (Antibiotics), Viomycin (Antitubercular drugs), Alobarbital (Sedative/hypnotic), Retrovir (Antiviral and anti-HIV), etc (Fig. 1).
Promising diverse pharmacological activities are shown by various N-functionalized morpholines. They are reported to exert a number of important physiological activities, such as antibacterial, antifungal [12], platelet aggregation inhibitors [13], antiemetic [14] and antidepressants [15], Phenmetrazine was used as an appetite suppressant. It was firstly replaced by its equivalent phendimetrazine which functions as a prodrug (Fig. 1). In accordance with our previous work [16][17][18] and medicinal significance of pyrazole, pyrimidine and morpholine motifs, we decided to combine all these moieties in to one single frame with hybrid approach as potential antimicrobial agents.

Materials and methods
The required chemicals were purchased from E. Merck. Melting points were recorded on Gallenkamp apparatus and were left uncorrected. The completion of reaction and the purity of all compounds was checked on aluminum-coated TLC plates 60, F 245 (E. Merck) using various solvent systems as mobile phase and visualized under ultraviolet (UV) light, or iodine vapor. Elemental analysis (% C, H, N) was carried out by a Perkin-Elmer 2400 CHN analyzer. IR spectra were also recorded on Perkin Elmer FT-IR spectrophotometer. 1 H NMR and spectra were recorded on Varian Gemini 400 MHz and 13 C NMR spectra on Varian Mercury-400, 100 MHz in DMSO-d 6 as a solvent and tetramethylsilane (TMS) as an internal standard. Mass spectra were scanned on a Shimadzu LCMS 2010 spectrometer.

Chemistry
Synthetic strategies adopted to achieve the target compounds are depicted in Scheme 1. Here, scaffold 5 is a part of synthesis of new chemical entities in the form of antimicrobial agents. According to Scheme 1, the key chalcone derivatives 3-(1,3-diphenyl-1H-pyrazol-4-yl)-1-arylprop-2-en-1-ones (3a-o) are used as precursors for the synthesis of title compounds (5a-o). Compounds  Amongst synthesized compounds (5a-o), many compounds exhibited antimicrobial potency that diverged from good to excellent. In preliminary screening, compounds 5a-o against the test microbes are listed in Table 1 along with MIC values of reference compounds ampicillin (for bacteria) and griseofulvin (for fungi). On the basis of antibacterial screening results from Table 1, compounds 5e, 5f, 5n and 5o were noticeably displayed outstanding antibacterial activity. For several pathogens, these compounds were more active than the reference drugs and the remaining compounds of the series possessed feeble antibacterial activity. Tabular data revealed that the presence of functional group at para position enhanced antibacterial activity as compared to ortho and meta substituted compounds.

Structure-activity relationship studies
SAR studies revealed that the antimicrobial activity in heterocyclic class of bioactive molecules depends on the nature of the peripheral substituents and their spatial relationship within this skeleton. The presence of chloro, fluoro and nitro substituents at para position on substituted ring system had enriched the antibacterial activity of compounds compared to those of electron donating substituents. Compounds tend to lose its potency by changing the substitution pattern to ortho and meta position. Integration of electron donating groups such as methyl, methoxy and hydroxy

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ILCPA Volume 69 reduced the antibacterial property. In rationality with the above results, electron withdrawing groups like fluoro and chloro on ortho and para substituted position showed optimal activity. It is determined from Table 1 that, a compound without any substitution does not display antimicrobial activity against a panel of all microorganisms.

Conclusion
We have synthesized newly compounds (5a-o) by conventional method with enlargement in yield of reactions. The synthesized compounds were screened for their in vitro antibacterial and antifungal activity against several bacterial and fungal strains. It is concluded from biological activity table that structural and electronic diversity of these products affects their biological activities. Compounds 5d, 5e, 5f, 5g, 5n and 5o are the most characteristic derivatives identified in present study because of their prominent in vitro antimicrobial potency. SAR studies reveal that when ortho and para positions are substituted by electron withdrawing group's fluoro, chloro, bromo and nitro; compounds revealed effective antimicrobial activity.