- Role of secondary structure in the asymmetric acylation reaction catalyzed by peptides based on chiral Cα-tetrasubstituted α-amino acids
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In a recent series of papers, Miller and co-workers were able to show that His(π-Me)-based, terminally protected peptides are potent catalysts of the asymmetric acyl transfer reaction, useful for the kinetic resolution of alcohols. In a structure-supporting solvent, one of the most active compounds, an Aib-containing tetrapeptide, is folded in a doubly intramolecularly H-bonded β-hairpin motif incorporating a type-II′ β-turn conformation. In this work, we have expanded the study of the Miller tetrapeptide by examining a set of analogues and shorter sequences (dipeptide amides), characterized by chiral Cα-tetrasubstituted α-amino acids of diverging bulkiness and optical configuration. Peptide synthesis in solution, conformational analysis by FT-IR absorption and 1H NMR techniques, and screening of catalytic activity as well have been performed. Our results confirm the close relationship between the β-hairpin 3D-structure and the catalytic activity of the peptides. A tetrapeptide analogue slightly more selective than the Miller compound has been found. However, the terminally protected, industrially more appealing, dipeptide amides are poorly effective.
- Formaggio, Fernando,Barazza, Alessandra,Bertocco, Andrea,Toniolo, Claudio,Broxterman, Quirinus B.,Kaptein, Bernard,Brasola, Elena,Pengo, Paolo,Pasquato, Lucia,Scrimin, Paolo
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p. 3849 - 3856
(2007/10/03)
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- Nitroxyl peptides as catalysts of enantioselective oxidations
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The achiral, nitroxyl-containing α-amino acid TOAC (TOAC = 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid), in combination with the chiral α-amino acid Cα-methyl valine [(αMe)Val], was used to prepare short peptides (from di- to hexa-) that induced the enantioselective oxidation of racemic 1-phenylethanol to acetophenone. The best catalyst was an Nα-acylated dipeptide alkylamide with the -TOAC-(αMe)Val- sequence folded in a stable, intramolecularly hydrogen-bonded β-turn conformation with large, lipophilic (hydrophobic) N- and C-terminal blocking groups. We rationalized our findings by proposing models for the diastereomeric intermediates between (R)-[and (S)]-1-phenylethanol and the catalyst Fmoc-TOAC-L(αMe)Val-NHiPr, based on the X-ray diffraction structure of the latter.
- Formaggio, Fernando,Bonchio, Marcella,Crisma, Marco,Peggion, Cristina,Mezzato, Stefano,Polese, Alessandra,Barazza, Alessandra,Antonello, Sabrina,Maran, Flavio,Broxterman, Quirinus B.,Kaptein, Bernard,Kamphuis, Johan,Vitale, Rosa Maria,Saviano, Michele,Benedetti, Ettore,Toniolo, Claudio
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