Cyclic peptides. XII. Synthesis of AM-toxin III and its analogs

A cyclic tetradepsipeptide with a sequence corresponding to AM-toxin III (a phytotoxic peptide) was synthesized by a conventional method in order to confirm the proposed structure. This was mediated through a deamination reaction of precursor cyclotetradepsipeptide containing a D-2, 3-diaminopropionic acid residue by the Hofmann degradation method. The synthetic peptide and natural AM-toxin III were identical in regard to t.l.c., crystal form, mass spectrum and biological activity in causing necrosis on apple leaves. Two analogs, [l-O-methyl-L-tyrosine]-AM-toxin and [l-L-tyrosine]-AM-toxin showed extremely weak activity; the relationship between the bulkiness of an aromatic side chain at position 1 of AM-toxin III and its biological activity is discussed.