Nitroxyl peptides as catalysts for enantioselective oxidations

The achiral, nitroxyl-contg. a-amino acid TOAC (TOAC = 2,2,6,6- tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid), in combination with the chiral a-amino acid Ca-Me valine [(aMe)Val], was used to prep. short peptides (from di- to hexa-) that induced the enantioselective oxidn. of racemic 1-phenylethanol to acetophenone. The best catalyst was an Na-acylated dipeptide alkylamide with the -TOAC-(aMe)Val- sequence folded in a stable, intramolecularly hydrogen-bonded b-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-(aMe)Val-NHiPr, based on the X-ray diffraction structure of the latter.