Abstract
Recent synthetic aspects of three cyclic peptides are reviewed. On the syntheses of gramicidin S, an antibacterial cyclodecapeptide, and its analogs, several methods for cyclization were performed using various deca- and pentapeptide precursors. Among them, the succinimide active ester and azide methods gave desired products in good yield. Furthermore, the yield of the products was found to depend highly on amino acid sequence of the precursors. Five analogs of Cyl-2, a phytotoxic cyclotetrapeptide, were synthesized. Comparison of Cyl-2 with the analogs by NMR implied that Cyl-2 has an unique conformation. AM-toxin I, II and III are phytotoxic cyclotetradepsipeptides containing dehydroalanine. The proposed structures of the toxins could be synthetically confirmed by different methods. Conversion of serine residue in cyclotetradepsipeptide precursors to dehydroalanine via β-elimination was unsatisfactory, whereas reasonable improvement was obtained by using Hofmann degradation of α, β-diaminopropionic acid.
