Structures of cytotoxic bicyclic hexapeptides, RA-XIX, -XX, -XXI, and -XXII, from Rubia cordifolia L.

Novel bicyclic hexapeptides, RA-XIX, -XX, -XXI, and -XXII, were isolated from the roots of Rubia cordifolia L. The structures of RA-XIX and RA-XX were established by semisynthesis from a cycloisodityrosine, derived from previously reported RA-VII, and those of RA-XXI and RA-XXII by chemical correlation with RA-XX and previously reported RA-VIII, respectively. The IC50 values of these new peptides against P-388 leukemia cells were 0.013–0.63 μg/mL.