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Microwave-Assisted Synthesis of Benzothiazole Derivatives using Glycerol as Green Solvent

Five-membered heterocyclic rings, such as benzothiazole derivatives, are present in natural products, in synthetic agrochemical and pharmaceutical compounds and in organic synthetic intermediates. They have been widely used in therapeutic areas including anti-tumour agents, Gram-positive selective antibacterials, HIV reverse transcriptase inhibitors, and orexin-1 receptor antagonists. Classical methods for the synthesis of benzothiazoles involve the condensation of oaminothiophenols with substituted nitriles, aldehydes, carboxylic acids, acyl chlorides, or esters using strong oxidants as catalysts, such as animal bone meal (ABM) and Lewis acids doped ABMs, FeCl3·6H2O, mesoporous mixed metal oxide nanocrystals, Lawesson’s reagent, H2O2/Fe(NO3)3, Na2S2O5, and other methods. Although successful, many of these processes suffer from drawbacks such as drastic reaction conditions, low yields, tedious work-up procedures or using toxic metal salts as catalysts. The main disadvantages are the use of toxic organic solvents and catalysts that are destroyed in the work-up and cannot be recovered or reused. Hence, the search for the better method is still being actively pursued. Recently, due to environmental and safety considerations, reduction of costs and the simplicity of the process, attention has been drawn to reactions using green solvents. Glycerol is usually produced as a major by-product of the biodiesel industry. Indeed, glycerol is abundant, biodegradable, cheap, non-toxic, non-flammable and has a very high boiling point. It is highly polar and immiscible with hydrocarbons and ethers. It is suitable for biphasic catalysis and for product isolation by liquid–liquid extraction. All of these properties indicated that glycerol meets the requirements for consideration as a green solvent. Microwave-assisted organic synthesis has had a profound impact on the way that chemists approach organic synthesis. This enabling technology has now been generally accepted in industrial research laboratories and is no longer an academic curiosity. Several organic transformations have been successfully realised employing this technique. In contrast to conventional heating, microwave-assisted organic synthesis has proven to dramatically shorten reaction times, to deliver cleaner reaction mixtures and hence increases overall yields. In many instances, CEM focused microwave heating under sealed vessel conditions has had a dramatic influence on reducing reaction times, increasing product yields, improving selectivity, using less solvent, and enhancing product purities by reducing unwanted side reactions compared to conventional synthetic methods. To the best of our knowledge, the synthesis of benzothiazole derivatives in glycerol under microwave radiation conditions