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

Abstract O -Seco-RA-XXIV, a new cyclic peptide, cyclo-( d -alanyl- l -glutaminyl- N , O -dimethyl- l -tyrosyl- l -alanyl- N -methyl- l -tyrosyl- N -methyl- l -tyrosyl), was isolated from the roots of Rubia cordifolia L. along with RA-XXIV. Its structure and relative stereochemistry were determined by interpretation of the spectroscopic data and X-ray crystallography, and its absolute stereochemistry by the Marfey's amino acid analysis of its acid hydrolysate. Isolation of the two peptides from the same plant source may indicate that O -seco-RA-XXIV is a possible precursor of RA-XXIV and that the formation of the diphenyl ether linkage in the cycloisodityrosine moiety is to be formed after the formation of the cyclohexapeptide chain in this series of peptides.