A novel bicyclic hexapeptide, RA-XVIII, from Rubia cordifolia: Structure, semi-synthesis, and cytotoxicity

A new bicyclic peptide of RA-series, RA-XVIII (3), was isolated from the roots of Rubia cordifolia L. Its structure was established to be a hydroxylated derivative of RA-VII by the semi-synthesis of 3 from deoxybouvardin, and its cytotoxicity against P-388 cells was 0.012 μg/mL.