“…Conventionally, NaOH and KOH are used as catalysts [16][17][18][19][20], but more complex systems such as pyrrolidine [21], zinc prolinate [22], L-lysine [23], prolinethioamide derivatives [24] and ionic liquids [25][26][27][28][29] are found to be able to catalyze the reaction. In most cases, the disadvantages are toxicity, difficult preparation, ability to hydrolyze in aqueous solutions, non-recyclability and high cost of the catalysts, generation of large amount of wastes, sometimes modest yields of target products.…”