Shakir, Raied Mustafa (2014) Synthesis and antioxidant properties of some new di-tert-butylphenol derivatives bearing heterocyclic ring / Raied Mustafa Shakir. PhD thesis, University of Malaya.
Abstract
The hindered phenolic compounds are known as one of the important antioxidants. Eleven new derivatives of aryl 1,3,4-oxadiazoles containing 2,6-di-tert-butylphenol were synthesized to increase the antioxidant ability. These oxadiazoles (coded as 2.12-2.22) were synthesized through the condensation of different aryl hydrazides with 3,5-di-tert-butyl-4-hydroxybenzoic acid in the presence of POCl3 as a dehydrating agent. The DPPH and FRAP assays showed that the antioxidant ability of these compounds depends on the type of substituents and their position in the phenyl ring. The sequence of substituents in order of their antioxidant activity are found to be 4-OH > 2-Me > 2,4-di-Me > 4-Me > OMe ≈ OEt >4-Cl > 2-Cl >4-Br ≈ 3,4-di-Cl ≈ 3,5-di-Cl. The second series consists of seven bis-oxadiazole-bis-2,6-di-tert-butylphenol (coded as 2.31-2.37) and were synthesized through the condensation of seven different dihydrazides with 3,5-di-tert-butyl-4-hydroxybenzoic acid using POCl3 as the dehydrating agent. Compounds 2.33 and 2.35 showed significant antioxidant ability while the other five compounds recorded low activity. Increasing the steric hindrance and the resonance of the compunds would lead to increase the antioxidant ability. As the effect of steric hindrance of 2,4-di-tert-butylphenol group is less than 2,6-di-tert-butylphenol, in order to have hight antioxidant ability, compounds containing 1,3,4-oxadiazole ring (coded as 3.5-3.8) at position six of 2,4-di-tert-butylphenol were synthesized. The formation of oxadiazoles occurred through the synthesis of hydrazones of 2,4-di-tert-butyl derivatives (coded as 3.1-3.4). The cyclization of these hydrazones to oxadiazoles were achieved by using bromine in glacial acetic acid as the oxidising reagent. The antioxidant activity of the hydrazones and oxadiazoles were quite similar. Two bis-hydrazones (coded as 3.9-3.10) were also synthesized and then cyclized to form bis-oxadiazoles. The bis-oxadiazoles gave low antioxidant activity for both DPPH and FRAP. A new series of 1,3,4-oxadiazole-5- iii thione (coded as 3.15) and their alkyl derivatives (coded as 3.16-3.18) were synthesized at the sixth position of 2,4-di-tert-butylphenol. The 1,3,4-oxadiazole-5-thione (3.15) showed 91.53% DPPH inhibition and this value was slightly higher than that of ascorbic acid (90.65%). The compound (3.15) also exhibited FRAP value higher than gallic acid. However, the alkylated derivatives of compound (3.15) showed lower DPPH and FRAP values. Of these alkylated derivatives, the compound alkylated by 4-bromobenzyl bromide (3.18) exhibited the highest value. The 5-amino-oxadiazole ring (coded as 3.19) were synthesized from the reaction between 3,5-di-tert-butyl-2-hydroxybenzohydrazide and cyanogen bromide. Whereas the 5-amino-1,3,4-thiadiazole ring compounds (coded as 3.21) was synthesized by two methods: the first was from the cyclization of the corresponding thiosemihydrazones (coded 3.20) and the second from cyclization of 3,5-di-tert-butyl-2-hydroxybenzoic acid with thiosemicarbazide. Both compounds 3.19 and 3.21 gave slightly similar antioxidant activity in DPPH and FRAP. Generally, the DPPH values were lower than that of FRAP when compared to the reference compounds. The last series of compounds synthesized were those containing 1,2,4-triazole ring, namely 4-amino-1,2,4-triazole-5-thione (3.22) and 4-substituted-phenyl-1,2,4-triazole-5-thione (3.26-3.28). The antioxidant activity of these compounds was higher than BHT. The intermediate compounds (3.26-3.28) gave good antioxidant activity quite similar to that of ascorbic acid.
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