M. Abbasi, Ayesha Mumtaz, Aziz‐ur‐Rehman
May 9, 2018
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Abstract
Oxadiazoles are the heterocyclic compounds containing one oxygen and two nitrogen atoms in a five membered ring,1 possessing a diversity of useful biological effects.2 Oxadiazole is considered to be resultant from furan by replacement of two methane (–CH=) groups by two pyridine type nitrogen atoms (–N=) at position 3 and 4.3 Oxadiazole is a very weak base due to the inductive effect of the extra heteroatom.4 The replacement of (–CH=) groups in furan by two pyridine type nitrogen (–N=) reduces aromaticity of the resulting oxadiazole ring to such an extent that the oxadiazole ring exhibits the character of conjugated diene.5 Due to relatively low electron density on the carbon atom, the oxadiazole ring is extremely resistant towards electrophillic substitutions at carbon atom; however the attack of electrophile occurs at nitrogen, if oxadiazole ring is substituted with electron releasing groups. Nucleo-philic attack is quite difficult in oxadiazole ring; however, halogen substituted oxadiazoles can undergo nucleophilic substitution with replacement of halogen atom by nucleophiles.6 These derivative compounds have been found to exhibit diverse biological activities such as analgesic,7 anti-inflammatory,8 antimicrobial,9 anti-HIV,10 antimalarial,11 antifungicidal,12 and other biological properties. Some 1,3,4-oxadiazole derivatives have also been applied in the fields of photosensitizers,13 liquid crystals,14 and organic light-emitting diodes (OLED).15 Consequently, the synthesis of compounds containing this heterocyclic core has attracted considerable attention, and a wide variety of methods has been used for their assembly. The most common synthetic protocol toward the preparation of these compounds involves the dehydrative cyclization of diacylhydrazides using usually strong acidic reagents such as thionyl chloride,16 phosphorus pentoxide,17 phosphorus oxychloride,18 and sulfuric acid.19