In order to investigate the relationship between the structure of a peptide backbone and the formation of an intramolecular disulfide bond (S–S) in the peptide, a linear analog (1) of gramicidin S and a heterodetic cyclic peptide containing an S–S bond (2) were synthesized by the conventional solution method. Since a disulfide bonded compound (2) was easily formed by a treatment of the acetamidomethyl(Acm)-protected linear peptide (1) with iodine in good yield (70%), it is suggested that the mutual position between Cys1 and Cys8 residues becomes approximate, owing to the contribution of –d-Phe–Pro– part of the sequence. A heterodetic cyclic peptide (2) showed about 1/16–1/8 fold activity compared to that of GS; 1 as inactive. By the construction of an S–S bridge on the peptide backbone (1), an inactive peptide derivative was effectively converted to an active analog (2). The CD spectrum pattern also suggests that the heterodetic cyclic peptide (2) has a GS-like structure.
One-pot synthesis of gramicidin S, cyclic decapeptide, was successfully achieved by treatment of the corresponding linear decapeptide with (5-nitropyridyl)diphenyl phosphinate, a new condensing reagent, in pyridine. Similarly, the phosphinic ester can be successfully employed in the Young test as well as syntheses of dipeptides.