TY - JOUR
T1 - Stereoconversion of amino acids and peptides in uryl-pendant binol schiff bases
AU - Park, Hyunjung
AU - Nandhakumar, Raju
AU - Hong, Jooyeon
AU - Ham, Sihyun
AU - Chin, Jik
AU - Kim, Kwan Mook
PY - 2008/11/10
Y1 - 2008/11/10
N2 - (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.
AB - (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.
KW - Amino acids
KW - Chiral auxiliaries
KW - Epimerization
KW - Schiff bases
UR - http://www.scopus.com/inward/record.url?scp=55449112158&partnerID=8YFLogxK
U2 - 10.1002/chem.200801036
DO - 10.1002/chem.200801036
M3 - Article
C2 - 18816560
AN - SCOPUS:55449112158
SN - 0947-6539
VL - 14
SP - 9935
EP - 9942
JO - Chemistry - A European Journal
JF - Chemistry - A European Journal
IS - 32
ER -