RA-XXV and RA-XXVI, Bicyclic Hexapeptides from Rubia cordifolia L.: Structure, Synthesis, and Conformation.

Two RA-series bicyclic hexapeptides, RA-XXV (4) and RA-XXVI (5), which have no N-methyl group at Tyr-5, were isolated from the roots of Rubia cordifolia L. Their amino acid compositions and sequences were determined by interpretation of MS, and 1D and 2D NMR data and their relative structures were elucidated by XRD analysis of 4 and RA-XXVI acetate (6). The absolute stereochemistry of 4 was established by the total synthesis of 4, and that of 5, by the chemical correlation with 4.