Role of Secondary Structure in the Asymmetric Acylation Reaction Catalyzed by Peptides Based on Chiral Cα-Tetrasubstituted α-Amino Acids

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Abstract
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.