Structure determination of allo-RA-V and neo-RA-V, RA-series bicyclic peptides from Rubia cordifolia

Abstract

Two bicyclic hexapeptides, allo-RA-V and neo-RA-V, and one cyclic hexapeptide, O-seco-RA-V were isolated from the roots of Rubia cordifolia. Their groß structures were elucidated on the basis of spectroscopic and X-ray crystallographic analysis. The absolute stereochemistry of allo-RA-V and neo-RA-V were confirmed by their total syntheses, and the absolute stereochemistry of O-seco-RA-V by chemical correlation with deoxybouvardin (RA-V). Comparison of 3D structures highly active RA-VII with less-active peptides, allo-RA-V and neo-RA-V suggests that the orientation of the Tyr-5 and/or Tyr-6 phenyl rings plays a significant role in the biological activity.