Cyclic peptides. XVIII. Syntheses of AM-toxin I analogs containing bulky L-amino acid residues instead of an L-alanine*

In order to investigate the influence of the alanine residue at position 1 of AM-toxin I (cyclic tetradepsipeptide) on necrotic activity for apple leaves, three analogs, [l -2-aminobutanoic acid¹] AM-toxin I, [l -leucine¹] AM-toxin I and [l -phenylalanine¹] AM-toxin I, were synthesized by the conventional method for peptide synthesis. Spectra of ¹H-n.m.r., u.v. and CD of the analogs were similar to those of natural AM-toxin I. The analogs showed appreciable biological activity including host-specificity, indicating that considerable variation in the size of side chain at position 1 is allowed for the induction of necrotic activity.