Abstract
A series of 1,3-oxazole, 1,3-thiazole, isomeric 1,2,4-oxadiazole, 1,3,4-oxadiazole, and 1,2,3,4-tetrazole heterocycles was synthesized. All the compounds shared as a common feature the presence of a 4-hydroxyphenyl substituent. The structures of the synthesized compounds were confirmed by MS, 1H-NMR, and elemental analysis. In vitro antimicrobial activity for all the newly synthesized compounds at concentrations of 200-25 μg/mL was evaluated against Gram+ve organisms such as methicillin-resistant Staphylococcus aureus (MRSA), Gram–ve organisms such as Escherichia coli (E. coli), and the fungal strain Aspergillus niger (A. niger) by the cup plate method. Ofloxacin and ketoconazole (10 μg/mL) were used as reference standards for antibacterial and antifungal activity, respectively. Compounds 15, 16, and 20 showed notable antibacterial and antifungal activities at higher concentrations (200 μg/mL), whereas 17-19 were found to display significant antibacterial or antifungal activity (25-50 μg/mL) against the Gram+ve, Gram–ve bacteria, or fungal cells used in the present study.
Highlights
IntroductionHeterocycles containing nitrogen(s) and an oxygen or sulfur atom constitute an important class of compounds in the field of medicinal chemistry due to their interesting and diverse clinical applications [1].For example, considerable evidence has accumulated during the past two decades demonstrating the various pharmacological effects of 1,3,4-oxadiazoles, which include antibacterial [1], antifungal [2], anthelmintic [3], antitubercular [4], anticancer [5], anti-HIV [6], antioxidant [7], analgesic [8], anti-inflammatory [9] and anticonvulsant [10] activities
All novel heterocyclic compounds were synthesized by ring-closing reactions, with each derivative being prepared via a different synthetic pathway (Scheme 1)
Carbonyl compounds can be efficiently converted into 5-substituted oxazoles in a one-pot reaction with tosylmethyl isocyanide (TosMIC) [13]
Summary
Heterocycles containing nitrogen(s) and an oxygen or sulfur atom constitute an important class of compounds in the field of medicinal chemistry due to their interesting and diverse clinical applications [1].For example, considerable evidence has accumulated during the past two decades demonstrating the various pharmacological effects of 1,3,4-oxadiazoles, which include antibacterial [1], antifungal [2], anthelmintic [3], antitubercular [4], anticancer [5], anti-HIV [6], antioxidant [7], analgesic [8], anti-inflammatory [9] and anticonvulsant [10] activities. In spite of the large number of antibiotics and chemotherapeutics currently available for medical usage, the spectre of increasing bacterial resistance has made it necessary to continue the search for new antimicrobial substances To this end a large number of oxadiazole derivatives have been prepared, many of which have shown a wide spectrum of antimicrobial activity. Some oxadiazoles with different substituents, especially with a 4-hydroxyphenyl moiety at different locations on the five-membered heterocyclic ring, produced fungicidal and bactericidal agents of various potencies [11,12]. These observations promoted us to develop, synthesize, and evaluate a novel series of diverse heterocycles of more simplified chemical structures that include 1,3-oxazole, 1,3-thiazole, isosteric oxadiazoles, and tetrazole heterocyclic ring systems, all bearing a 4-hydroxyphenyl moiety
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