Synthesis of Some Symmetrical Novel Bis-thiosemicarbazides, 1,2,4-Triazoles, 1,3,4-Thiadiazoles, and Their Derivatives

Conjugated Oligo-Aromatic Compounds Bearing a 3,4,5-Trimethoxy Moiety: Investigation of Their Antioxidant Activity Correlated with a DFT Study

A series of heterocyclic compounds bearing the well-known free radical scavenging 3,4,5-trimethoxybenzyloxy group, was synthesized. The key compound 4-(3,4,5-trimethoxybenzyl-oxy)benzohydrazide was converted into thiosemicarbazide derivatives, which were subsequently cyclized with NaOH to provide 1,2,4-triazole derivatives. Alternative treatment of the acid hydrazide with carbon disulfide in the presence of KOH led to the corresponding 1,3,4-oxadiazole and various alkylated derivatives. The newly synthesized compounds were purified and the structures of the products were elucidated and confirmed on the basis of their analytical and spectral data. Their antioxidant activities were evaluated using 2,2-diphenyl-1-picrylhydrazyl (DPPH(•)) and Ferric Reducing Antioxidant Power (FRAP) assays. The thiosemicarbazide derivatives were highly active in both antioxidant assays with the lowest IC50 value for DPPH radical scavenging. Theoretical calculations based on density functional theory (DFT) were performed to understand the relative importance of NH, SH and CH hydrogens on the radical scavenging activities of these compounds.

Efficient synthesis of Some Novel Symmetrical and Unsymmetrical Pyridine bis-1,2,4-triazoles and bis-1,3,4-thiadiazoles

A series of novel bis-1,2,4-triazoles and bis-1,3,4-thiadiazoles bearing pyridine rings was synthesised, by multistep reaction sequences, in good to high yields. Reaction of pyridine-2,5-bis- and pyridine-2,6-bis-carbohydrazides with alkyl- or aryl-isothiocyanates gave the corresponding bis(thiosemicarbazide) derivatives, which could be cyclised under strongly basic conditions to give novel pyridine bis(1,2,4-triazole-3-thiols). However, under strongly acidic conditions the corresponding bis(2-amino-1,3,4-thiadiazoles) were obtained. The synthetic routes and spectroscopic characterisation of the new compounds are reported.