Studies of Unusual Amino Acids and Their Peptides. IX. The Synthetic Study of Bottromycins B1 and B2

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Abstract
Bottromycins B1 and B2, peptide antibiotics, were synthesized according to the structures proposed by Nakamura et al. B1 was obtained by the condensation of the imidate, Piv–Tle–Val–ImPro–OEt·HCl with the ester, H–Gly–Phe(βMe)–β-Tha–OH in the presence of Et3N, and B2 by the replacement of the Z group of Z–Tle–Val–ImPro–Gly–Phe(βMe)–β-Tha–OMe by the trans-4-methyl-2-pentenoyl group. The structures of the synthetic B1 and B2 were confirmed by elemental analyses, amino acid analyses, and Cl-mass spectra. However, the synthetic bottromycins differed from the natural ones in many respects, e.g., IR spectra, behavior in TLC, and pKa values. Most significantly, no antimicrobial activities were detected in the synthetic B1. On the basis of these results, it is concluded that the structures proposed by Nakamura et al. for bottromycins are erroneous and should be revised.