Chemical Modification via Thioamide Intermediates and Conformation-Activity Relationships of an Antitumor Bicyclic Hexapeptide RA-VII

Three epimeric analogues at D-Ala-1, Ala-2, and Ala-4 of RA-VII, an antitumor bicyclic hexapeptide from the plants of the genus Rubia, were prepared from its thioamides through oxazole formation on the peptide backbone and subsequent partial hydrolysis. Several other RA-VII analogues, in which D-Ala-1, Ala-2 or Ala-4 was replaced by glycine or D-Ala-1 by D-2-aminobutyric acid or D-norvaline, or in which a 1, 2, 4-triazole cis-amide bond surrogate was incorporated at residues 2 and 3, were prepared by connection of the cycloisodityrosine unit with corresponding tetrapeptides and subsequent macrocyclization. The cycloisodityrosine unit was obtained by degradation of the bis (thioamide) of RA-VII. Studies on conformation and cytotoxicities indicated that in RA-VII series peptides, the original D, L, L-configurations at residues 1, 2 and 4 are essential for the bioactive conformation, and that the minor conformer of RA-VII having cis Ala-2/Tyr-3 and Tyr-5/Tyr-6, and trans Ala-4/Tyr-5 takes little, if any, part in expressing the activity.