RA-dimer B, a New Dimeric RA-series Cyclopeptide Incorporating Two Different Types of Cycloisodityrosine Units, from Rubia cordifolia L.

RA-dimer B, a new cytotoxic RA-series peptide, was isolated from the roots of Rubia cordifolia L. Its structure was elucidated on the basis of spectroscopic analysis to be a dimeric cyclopeptide composed of deoxybouvardin and allo-RA-V. Those two cyclopeptide units are connected by an ether linkage between the phenolic oxygen atom of deoxybouvardin and the ϵa carbon atom of Tyr-6 of allo-RA-V. RA-dimer B was synthesized by the coupling reaction of deoxybouvardin with the boronic acid derivative of allo-RA-V, and subsequent deprotection, confirming the relative stereochemistry and establishing the absolute configuration of this peptide. RA-dimer B showed cytotoxic activity against human promyelocytic leukaemia HL-60, human colonic carcinoma HCT-116, and human renal cell carcinoma ACHN cells with IC values of 0.59, 0.54, and 0.74 μm, respectively.