Cyclic peptides from higher plants. Part 21. Thionation of the antitumour cyclic pentapeptides, astins A, B and C, from Aster tataricus

Abstract

Thionation of the potent antitumour cyclic pentapeptides, the astins A, B and C, with Lawesson's reagent gave [Ser-3-ψ(CS-NH)-β-Phe-4]astin A (thioastin A), [Ser-3-ψ(CS-NH)-β-Phe-4]astin B (thioastin B) and [Ser-3-ψ(CS-NH)-β-Phe-4]astin C (thioastin C), respectively. Conformational analysis of the thioastins was conducted by comparing the 2D NMR data, temperature effects on NH protons, vicinal NH-CαH coupling constants, and NOE results, with those of the corresponding astins. Thioastins showed more promising antitumour activity than the corresponding astins.