Synthetic Studies on Quinoxaline Antibiotics. III. Synthesis of Nortriostin A, a Triostin A Analog Lacking N-Methyl Groups on the Cystine and Valine Residues

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Abstract
Nortriostin A, which is an analog of a cyclic octadepsipeptide antibiotic triostin A and contains one cystine and two valine residues substituted for the N,N′-dimethylcystine and N-methylvaline residues of the antibiotic, was synthesized with Z–d-Ser[Boc–Ala–Cys(MeBzl)–Val]–OH and Z–d-Ser[H–Ala–Cys(MeBzl)–Val]–OTce as key intermediates. The synthetic analog showed no antimicrobial activity at a concentration of 100 μg/ml and was found to exist as a single structure in solution as examined by 1H-NMR spectroscopy. The latter observation suggests that the conformer equilibrium known to occur with triostin A is a consequence of the presence of N-methyl peptide bonds in the antibiotic molecule.

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