Studies on the Stereochemical Characterization of N-Methylated Amino Acids

Various methods that could be used to determine the absolute configuration of N,N- dimethylated amino acids were investigated. Two methods utilizing 1H nuclear magnetic resonance (N.M.R.) spectroscopy were successful. The first involved the conversion of the N,N- dimethylated amino acids into diastereoisomers by reaction with methyl (S)-(+)- mandelate . In the second method, the N,N- dimethylated amino acids were amidated with 3,5-dinitroaniline, and 1H n.m.r. analysis was carried out with the addition of a chiral solvating agent, (R)-(-)-1-(9-anthryl)-2,2,2-trifluoroethanol [(R)-(-)-atfe ]. Another method involving the use of high-performance liquid chromatography ( h.p.l.c .), with bis (L-aspartic cyclohexylamide )copper(II) complex CuII (L- Aspchex )2, as the chiral mobile phase was also examined but was found to be unsuccessful in resolving the D- and L-isomers of N,N- dimethylated amino acids.