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Stereoconversion of amino acids and peptides in uryl-pendant binol schiff bases
- Stereoconversion of amino acids and peptides in uryl-pendant binol schiff bases
- Park H.; Nandhakumar R.; Hong J.; Ham S.; Chin J.; Kim K.M.
- Ewha Authors
- SCOPUS Author ID
- Issue Date
- Journal Title
- Chemistry - A European Journal
- vol. 14, no. 32, pp. 9935 - 9942
- SCI; SCIE; SCOPUS
- (S)-2-Hydroxy-2′-(3-phenyl-uryl-benzyl)-1,1′-binaphthyl-3- carboxaldehyde (1) forms Schiff bases with a wide range of nonderivatized amino acids, including unnatural ones. Multiple hydrogen bonds, including resonance-assisted ones, fix the whole orientation of the imine and provoke structural rigidity around the imine C=N bond. Due to the structural difference and the increase in acidity of the a proton of the amino acid, the imine formed with an L-amino acid (1-L-aa) is converted into the imine of the D-amino acid (1-D-aa), with a D/L ratio of more than 10 for most amino acids at equilibrium. N-terminal amino acids in dipeptides are also predominantly epimerized to the D form upon imine formation with 1. Density functional theory calculations show that 1-D-Ala is more stable than 1-L-Ala by 1.64 kcal mol-1, a value that is in qualitative agreement with the experimental result. Deuterium exchange of the a proton of alanine in the imine form was studied by 1H NMR spectroscopy and the results support a stepwise mechanism in the L-into-D conversion rather than a concerted one; that is, deprotonation and protonation take place in a sequential manner. The deprotonation rate of L-Ala is approximately 16 times faster than that of D-Ala. The protonation step, however, appears to favor L-amino acid production, which prevents a much higher predominance of the D form in the imine. Receptor 1 and the predominantly D-form amino acid can be recovered from the imine by simple extraction under acidic conditions. Hence, 1 is a useful auxiliary to produce D-amino acids of industrial interest by the conversion of naturally occurring L-amino acids or relatively easily obtainable racemic amino acids. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.
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