New antitumour cyclic astin analogues: synthesis, conformation and bioactivity.

Astins, antitumour cyclic pentapeptides, were isolated from the Aster tataricus. Their chemical structures, consist of a 16-membered ring system containing a unique beta,gamma-dichlorinated proline [Pro(Cl)2], other non-coded amino acid residues and a cis conformation in one of the peptide bonds. The astin backbone conformation, along with the cis peptide bond in which the beta,gamma-dichlorinated proline residue is involved, was considered to play an important role in their antineoplastic activities on sarcoma 180A and P388 lymphocytic leukaemia in mice, but the scope and potential applications of this activity remain unclear. With the aim at improving our knowledge of the conformational properties influencing the bioactivity in this class of compounds, new astin-related cyclopeptides were synthesized differing from the natural products by the presence of some non-proteinogenic amino acid residues: Aib, Abu, -(S)beta3-hPhe and a peptide bond surrogate (-SO2-NH-). The analogues prepared c(-Pro-Thr-Aib-beta3-Phe-Abu-), c[Pro-Thr-Aib-(S)beta3-hPhe-Abu], c[Pro-Abu-Ser-(S)beta3-hPhe psi(CH2-SO2-NH)-Abu] and c[Pro-Thr-Aib-(S)beta3-hPhe psi(CH2-SO2-NH)-Abu] were synthesized by classical methods in solution and tested for their antitumour effect. These molecules were studied by crystal-state x-ray diffraction analysis and/or solution NMR and MD techniques.