Cyclic peptides. XI. Synthesis of AM-toxin analogs by intramolecular condensation of pyruvyl-tripeptide amides.

In order to establish possible routes for the synthesis of AM-toxins, cyclotetradepsipeptides containing Dha, Pyr-L-Ala-L-Hmb-L-Tyr-NH2 were prepared and treated with TFA or anhydrous HF. The product was identified to be cyclo (-alpha-Hyala-L-Ala-L-Hmb-L-Tyr-), resulting from intramolecular condensation of the alpha-keto and amide group with the formation of the alpha-Hyala residue. Cyclo (-alpha-Hyala-L-Ala-L-Hmb-L-Phe-) and cyclo (-alpha-Hyala-L-Ala-L-Val-L-Phe-) were obtained by acid-treatment of the corresponding pyruvyl-tripeptide amides. The cyclic tetrapeptides did not show the activity of AM-toxin.