Squaric acid derivatives with cytotoxic activity-a review.

3,4-Dihydroxycyclobut-3-ene-1,2-dione (squaric acid, SQ) is the most important representative of the oxocarbon acids family. Squaric acid derivatives can be promising pharmaceutical agents, due to their unique structural properties, from which novel drugs benefit: a planar aromatic ring, the ability to form hydrogen bonds, good reactivity and similarity with carboxylate, phosphate and amide groups. These properties make it suitable for three major applications in cancer treatment.