Structures of bicyclic hexapeptides RA-XXVII and RA-XXVIII from Rubia cordifolia L

[Display omitted] •A rare structure in which a phenylpropanoid is ether-linked to a cyclic peptide.•X-ray crystallography solved the problem that spectroscopic methods could not.•Biosynthetic route for the connection of phenylpropanoids to aromatic rings verified by [4+2] cycloaddition reaction.•Modification of the aromatic ring at Tyr-6 may reduce cytotoxic activity. Two bicyclic hexapeptides RA-XXVII and RA-XXVIII, featuring a phenylpropanoid unit attached to the aromatic ring of the Tyr-6 residue of deoxybouvardin, were isolated from the roots of Rubia cordifolia L. Their structures were determined on the basis of spectroscopic data and X-ray crystallography of the 4-O-methyl derivative of RA-XXVII, and the semisynthesis from deoxybouvardin and coniferyl alcohol. RA-XXVII and RA-XXVIII exhibited cytotoxic activity with IC50 values of 0.051 and 0.18 μM, respectively, against the HL-60 cell line, and 0.18 and 0.78 μM, respectively, against the HCT-116 cell line.