Oxadiazole and Thiadiazole are the sub-family of azole compounds, which are fairly common in a variety of pharmaceutical molecules. The molecular formula of Oxadiazoles is C2H2N2O, which can be synthesized with microwave method in moderate yields. There are four isomers of oxadiazole: 1,2,3-Oxadiazole, 1,2,4-Oxadiazole, 1,2,5-oxadiazole and 1,3,4-oxadiazole(Fig. 1), while the 1,2,3-isomer is not stale and readily decomposes to diazoketone tautomer via ring opening. Thiadiazole structurally are five-membered heterocyclic compounds containing two nitrogen and a sulfur atoms, and two double bonds, to give an aromatic ring. Four possible structures exist depending on the relative positions of the heteroatoms 1,2,3-thiadiazole, 1,2,4-thiadiazole, 1,2,5- thiadiazole, and 1,3,4-thiadiazole (Fig. 1) .
Fig. 1 The structures of oxadiazole and thiadiazole isomers
Electron-transporting materials: Scientists have widely explored the applications of oxadiazole derivatives and found out it is suitable for electron-transporting, hole-blocking (ETHB) materials in electroluminescence devices due to their electron-deficient nature, their high thermal stability and high photoluminescence quantum yield (PLQY). The combination of oxadiazoles and pyridines or pyrimidines also has been studied for the bilayer devices.
Pharmaceutical Chemistry: It is well known in the literature that nitrogen and oxygen containing compounds are essentially used in medicine for the treatment of different kinds of fungal and bacterial infections along with the treatment of gastric ulcer, cancer, etc. In 2017, Modi group focused on 1,3,4-oxadiazoles and succeeded to synthesized a range of oxadiazoles derivatives via microwave activation, which afforded better yields compared to the conventional heating method. The compounds were screened later and behaved moderate to excellent microbial and cytotoxic activities. Besides, Spisani group discoved the oxadiazol-3-one derivative was revealed as a potent and selective myocardial calcium channel modulator. A 3D QSAR model (Fig 2)was obtained and validated with homologous literature data, and a virtual receptor scheme was derived for the unknown binding site. The following pharmacophoric features favorably affect the potency: one positively charged center, three lipophilic groups, and two hydrogen-bonding acceptor groups.
Fig 2. 3D QSAR model and the structure of the oxadiazol-3-one
Some of the drug molecules that contain thiadiazole ring are sulfamethizole, acet-azolamide, cefazolin, xanomeline, timolol, and recently developed SCH-202676 (Fig.3). Thiadiazole are applied as bioisosteres of thiazole, oxazole, oxa-diazole, and benzene for the development of analogs with improved biological profiles. 1,2,4-Thiadiazoles and 1,3,4-thiadiazoles have been widely used by researchers due to their diverse chemical and biological potential.
Fig.3 Thiadiazole ring containing drugs
Optical Applications: Yang group have synthesized three new monomers containing 2,5-diphenyl-1,3,4-thiadiazole containing vinyl, and later carefully characterized their electrochemical and fluorescent properties., which can emit strong blue or green fluorescence have good solubility in CHCl3 or DMF. In addition, photoinduced intermolecular charge transfer, the key to photoluminescence, needed a molecular assembly containing electron-donor units and electron-accepting units as well as an intermolecular pi-conjugated structure. The aromatic 1,3,4- thiadiazole ring allows good pi-electron conjugation and was demonstrated as a good candidate for its electron-withdrawing property.
Electrochemistry: With phenyl units and 1,3,4-thiadiazole rings linked through acetylene groups, a π system extending throughout the molecular structure called a poly(aryl-ethynylene) (PAE) system was obtained. Polymers involving this system showed rather low intramolecular reorganization energies and can be considered as candidates for n-type semiconductors.