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 ¹H and ¹³C 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).