Formation of squaric acid amides of anthracycline antibiotics. Synthesis and cytotoxic properties

The reaction of the anthracycline glycoside antibiotics 1–3 with the squaric acid ester 4 gave the squaric acid amide esters 5–7 under neutral conditions, whereas over pH 7 the products are the symmetric diamides (8, 9, 11, and 12). Of the prepared compounds 11 was the most active on MCF-7 human mammary adenocarcinoma cells.