Total Synthesis and Cytotoxicity Evaluation of an Oxazole Analogue of Tubulysin U

Tubulysins are strongly cytotoxic natural tetrapeptides with potent antiproliferative, antimitotic, and antiangiogenic activities which might find use in oncology. We herein report the first total synthesis of a stereoisomerically pure oxazole analogue of tubulysin U, which was found to be more cytotoxic than the thiazole-containing natural product. Additionally, we have developed an improved and scalable synthetic route towards the Tup fragment of the tubulysins.