Synthesis and Structural Revision of the Cyclic Hexapeptide Dimers Antatollamides A and B.

The synthesis and structural revision of the dimerized cyclic hexapeptides antatollamides A ( 1 ) and B ( 2 ) are reported. These are unique peptides with two proline residues and bicyclic peptides combined by a disulfide bond. Cyclization and disulfide bond formation of the linear peptide led to antatollamide A ( 1 ). However, the 1 H and 13 C NMR spectra of synthetic antatollamide A ( 1 ) were not consistent with those of isolated antatollamide A ( 1 ). Meanwhile, the NMR spectra of the monomeric cyclic hexapeptide cyclo (Pro-Pro-Phe-dCys-Ile-Val) ( 3 ) and the isolated antatollamide A ( 1 ) were identified completely. In addition, we found that isolated antatollamide B ( 2 ) is cyclo (Pro-Pro-dPhe-dCys-Ile-Val) ( 4 ).