Green Synthesis of 2-thioxothiazolidin-4-one Derivatives in Deep Eutectic Solvents via Knoevenagel Condensation

Recently, more and more researchers are resorting to green methods and techniques to avoid environmental pollution. Accordingly, many researchers have been working on the development of new green synthetic procedures trying to avoid the use of toxic organic solvents. A sustainable concept of green and environmentally friendly solvents in chemical synthesis nowadays encompasses a relatively new generation of solvents called deep eutectic solvents (DESs). DESs often have a dual role in the synthesis, acting as both, solvents and catalysts. In this study, DESs are used in the Knoevenagel synthesis of rhodanine derivatives, with no addition of conventional catalysts. A model reaction of rhodanine and salicylaldehyde was performed in 20 different DESs at 80 °C, in order to find the best solvent, which was further used for the synthesis of the series of desired compounds. A series of rhodanines was synthesized in choline chloride: acetamide (ChCl:acetamide) DES with good to excellent yields (51.4 – 99.7 %).