Synthesis of some new 1-(5-((1H-pyrazol-1-yl)methyl)-2-aryl-1,3,4-oxadiazol-3(2H)-yl) ethanone derivatives and study their antimicrobial activity

Ethyl 2-(1H-pyrazol-1-yl)acetate (1) was synthesized by the reaction of ethylchloroacetate with 1H-pyrazole, Then compound (1) reuxed with hydrazine hydrate to get 2-(1H-pyrazol-1-yl) acetohydrazide (2). Compound (2) was reaction with appropriate aryl aldehyde to get schiff bases N'-arylidine-2-(1H-pyrazol-1-yl)acetohydrazide derivatives (3a-3f). schiff’s base (3a-3e) were cyclized by reux with acetic anhydride to get new 1-(5-((1H-pyrazol-1-yl)methyl)-2-aryl-1,3,4-oxadiazol-3(2H)-yl)ethanone derivatives (4a-4e). The structures of the synthesized compounds were characterized by IR, 1H-NMR, 13C-NMR, mass spectra, and elemental analysis data. Synthesized compounds (4a-4e) were evaluated as antibacterial agents against some common pathogenic bacteria Gram-positive (Staphylococcus aureus, Streptococcus pyogenes) and Gram-negative bacteria (Escherichia coli, Psuedomonas aeruginosa). The result of antibacterial activity was compared with standard drugs (Ciprooxacin and Tetracycline).


Introduction
Heterocyclic compounds have considerable attention from researchers because of their key role in the medicinal and pharmaceutical eld. Pyrazole is a ve-member ring heterocyclic compound, has some structural features with two nitrogen atoms in an adjacent position, and is also known as azole [1]. It has only one endocyclic double bond and is basic. Pyrazole is a versatile lead compound for designing potent bioactive agents. The studies of pyrazole derivatives revealed that they are very useful in pharmaceutical and agrochemical research. Pyrazoline derivatives displayed various types of pharmacological activities such as antibacterial, antifungal, antitubercular antiviral [2], antihistaminic [2], antidepressant [2], antiin ammatory, antiarthritis [3], tranquillizing [4], anticancer [5,6], antihypertensive [7], anti-arrhythmic [8], analeptic [9], anticonvulsant [10], antidiabetic activities [11], and antioxidant etc [1][2][3][4][5]. The pyrazole derivatives are quite stable and have inspired chemists to synthesize the new pyrazole derivatives. Mostly pyrazole is synthesized taking ethyl acetoacetate and substituted hydrazine as starting chemicals.
Pyrazolone can be considered as an intermediate compound for the synthesis of various cyclic compounds of high biological activity.
1,3,4-Oxadiazole is a heterocyclic compound containing an oxygen atom and two nitrogen atoms in a ve-membered ring. Substituted 1,3,4-oxadiazole derivatives have demonstrated a broad spectrum of biological properties in both pharmaceutical and agrochemical elds. More widely studied and interest was shown by researchers because of their many important chemical and biological activities [12][13][14][15].
1,3,4-oxadiazole derivatives have been possessed diverse biological activities such as antimicrobial, antitubercular, antimalarial, antiviral [16][17][18], analgesic, anti-in ammatory, CNS depressant, anticonvulsant, hypnotic, sedative, muscle relaxants [19], insecticidal, herbicidal [20], antitumor, cytotoxic [21,22], lipid peroxidation inhibitor, diuretic, antioxidant [23], etc. Therefore, 1,3,4-oxadiazoles have attracted researchers all over the world to work in this area of new drug development. An enormous amount of research was undertaken to synthesize these classes of compounds by employing traditional methods, introducing new innovative methods and techniques, to reach the target molecules, and study their biological applications. In this study, we considered the synthesized derivatives of oxadiazole derived from 1H-pyrazole as in the scheme (I) and study its antimicrobial effect on some and Grampositive and Gram-negative bacteria.

Materials
All the chemicals were procured from Merck, Loba Chem, and SD Fine Chemicals. The purity of compounds was checked by thin-layer chromatography (TLC). Infrared (IR) spectra were recorded on Synthesis of N-(arylidine)-2-(1H-pyrazole-1-yl)acetamide (3a-3e): A mixture of compound 2 (0.1 mol) and appropriate aromatic aldehyde (0.1 mol) in (50 ml) of absolute ethanol, the mixture was re ex for 4 hrs and concentrated and cooled the reaction mixture [26]. The crude product was obtained, separated by ltration, and recrystallized from ethanol.
Synthesis of 5-(1H-pyrazol-1-yl)methyl)3-N-acetyl-2-(aryl)-1,3,4-(2H)-oxadiazole (4a-4e): A mixture of appropriate Schiff base (A3-8) (0.1mol) with acetic anhydride (50 ml) was re exed for 6 hr, then the solvent evaporated. The residue was poured into crushed ice [26]. The crude product was obtained, was separated by ltration, and recrystallized from ethanol. Antibacterial activity [27] The antibacterial activity of the synthesized compounds was determined by the cup plate method. In this method, the sample solution diffuses from a vertical-cavity through the solid agar layer of a petri dish in a manner that growth of the added microbe is prevented entirely in a circular area or a zone around the cavity containing a solution of the sample if the added sample possesses antibacterial activity. For determining antibacterial activity, freshly prepared liquid agar medium (35 mL/Petri dish) was transferred into the petri dishes and allowed the medium to solidify. Then, the 200 µL standardized culture (99 mL Nutrient broth media + 1mL culture) of microorganism was spread on each Petri dish by L-shaped spreader. With the help of the borer (5 mm), three bores were made on each plate. The synthetic compounds diluted with dimethyl sulfoxide (DMSO) at three concentrations (50, 100, and 200µg/mL) were added to each well separately. The Petri dishes were kept aseptically for approximately 4 to 5 h for the diffusion of the sample. Following diffusion, all the Petri dishes were incubated for 24 h at a temperature of 37º C. After the stipulated period of 24 h, the activity of compounds in terms of zone of inhibition was observed against two Gram-positive bacteria (Staphylococcus aureus, Streptococcus pyogenes) and two Gram-negative bacteria (Escherichia coli, Puedomonas aeruginosa). The antibacterial activity of the synthesized compounds is reported in Table 1.