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Synthesis of biologically active cyclic peptides

1. The extent of racemization and the coupling yield in peptide synthesis were studied under high dilution conditions. The azide method yielded the best results. 2. Five linear penta‐peptide precursors related to gramicidin S were subjected to cyclization in order to study how the difference in the sequence influences the yield and the ratio of cyclic dimer to monomer. The azide with the sequence of ‐L‐Pro‐L‐Val‐L‐Orn(Z)‐L‐Leu‐D‐Phe‐ afforded diZ‐gramicidin S in a high yield of 63%. 3. Alternaria mali toxin III, a cyclotetradepsipeptide phytotoxin, was synthesized. The activated linear tetradepsipeptide containing a D‐Dap(Z) (N3‐Z‐D‐2,3‐diaminopropionic acid) residue at the N‐terminus afforded the cyclic precursor (53%). The Dap residue in the precursor was converted into a ΔAla residue by Hofmann degradation to give the desired product.