O-Seco-RA-XXIV, a possible precursor of an antitumor peptide RA-XXIV, from Rubia cordifolia L.

► This work deals with important new antitumor peptides. ► A significant clue for the biosynthetic route of RAs and bouvardins is provided. ► The structure was unambiguously established by X-ray crystallography. ► The coordinates from the crystallography will be useful for the subsequent SAR study.