Cyclic peptides. V. Synthesis of AM-toxin II and its analog

A cyclic tetradepsipeptide with a sequence corresponding to AM-toxin II (a phytotoxic peptide) was synthesized in order to confirm the proposed structure by a conventional method. This was mediated through a dehydration reaction of cyclodepsipeptide containing a D-Ser residue, by methanesulfonyl chloride containing sulfur dioxide. The synthetic peptide and natural AM-toxin II were identical as regards t.l.c., crystal form, several kinds of spectra measurements and biological activity in causing necrosis on apple leaves. An analog, [L-Phe1]-AM-toxin, containing the lower homolog (L-phenylalanine) instead of L-2-amino-5-phenylpentanoic acid in position 1 of natural toxin II, was synthesized simultaneously. [L-Phe1]-AM-toxin showed extremely weak activity; the relationship between the bulkiness of an aromatic side chain and the biological activity of AM-toxin II is discussed.