Thionation of an antitumour cyclic pentapeptide, astin B, from Aster tataricus

Astin B, an antitumour cyclic pentapeptide, isolated from the roots of Aster tataricus, was thionated with Lawessonʹs reagent to give [Ser-3-Φ(CS-NH)-β-Phe-4]astin B (thioastin B). NMR spectroscopy showed that S atom was introduced at Ser3. The conformation of thioastin B was studied by NOE experiments, temperature effects on NH protons and vicinal NH-Cα coupling constants. Thioastin B was shown to retain the restricted bioactive conformation of astin B and had a more promising antitumor activity than astin B.