Studies of Unusual Amino Acids and Their Peptides. VIII. The Syntheses of an Iminohexapeptide as a Model of Bottromycin and Its Related Iminopeptides

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Abstract
As a model of bottromycin, the structure of which had been proposed by Nakamura et al., an iminohexapeptide, Z–Val–Val–ImPro–Gly–Phe–Phe–OMe, and its related iminopeptides were synthesized in good yields. These syntheses were achieved by the condensation of the imidates, Z–ImPro–OEt, Z–Val-ImPro–OEt·HCl, or Z–Val–Val–ImPro–OEt·HCl, with the peptide esters, Gly–Phe–OMe·HBr or Gly–Phe–Phe–OMe·HCl, in the presence of triethylamine. All of the resulting iminopeptides were isolated as the hydrobromide or the hydrochloride. The pKa values of these iminopeptide salts were measured to be 9.75–9.3 in methanol-water; they showed that the pKa rule of Nakamura et al. could not be applied in the cases of these iminopeptides.