A novel deep eutectic solvent (ETPP-Br/THF-TCA-DES) was prepared by a mixture of ethyl triphenylphosphonium bromide (ETPP-Br) and tetrahydrofuran-2,3,4,5-tetra-carboxylic acid (THFTCA, mole ratio 7:3), characterized by FT-IR, TGA/DTA, densitometer, eutectic point, and 1H NMR techniques and used as a capable and new catalyst for the synthesis of two sets of compounds: (1) the four new [a(1–4)] and the eleven [a(5–15)] known alkyl 1,2,6-trisubstituted-4-[(hetero)arylamino]- 1,2,5,6-tetrahydropyridine-3-carboxylates and (2) the two new [b(1–2)] and the eight [b(3–10)] known 1,3-thiazolidin-4-ones in DES with short reaction time, high yields, and easy recycling and separation of the DES catalyst. There is a nice consistency between the proposed structure of the DES compound, the integration values of the 1H NMR peaks and the ratio of ETPP-Br to THF-TCA obtained from the eutectic point phase diagram. Also, the decrease in splitting patterns of the peaks in DES, compared to the two starting materials can be the good evidence of the hydrogen bond formation between the two components.
