Novel Squaramides and Squaramates Containing a Five-Membered Heterocyclic Ring: Synthesis, Structure, and Cytotoxicity.

With the introduction of Navarixin in clinical trials, the role of squaric acid derivatives as bioisosteres gained popularity. Because of their distinctive electronic properties and hydrogen-bonding capacity, these compounds hold considerable promise for medicinal chemistry applications. In this study, a series of novel furan- and thiophene-containing squaric acid derivatives was synthesized via base-catalyzed nucleophilic substitution and characterized by spectroscopic techniques. The structures of three compounds were additionally confirmed by X-ray crystallography. Density functional theory calculations showed good agreement with the experimental vibrational spectra. In silico evaluation predicted favorable drug-like characteristics, including compliance with Lipinski's rule of five and high gastrointestinal absorption. The cytotoxic activity of the synthesized compounds was assessed against HeLa, HT-29, HL-60, A-549, and MCF-7 cancer cell lines, as well as the non-cancerous CCL-1 cell line. Several derivatives displayed moderate to strong antiproliferative activity with selectivity toward malignant cells. Compound 3d exhibited the most pronounced improvement (five-fold) over Navarixin in HL-60 cells 5.81 µM, while compounds 3a and 3c demonstrated superior potency and selectivity in A-549 cells (10.33 µM and 9.65 µM). These findings identify squaric acid derivatives as promising candidates for further anticancer drug development and structure-activity relationship studies.