Synthesis of Tripeptide Derivatives with Three Stereogenic Centers and Chiral Recognition Probed by Tetraaza Macrocyclic Chiral Solvating Agents Derived from D-Phenylalanine and (1S,2S)-(+)-1,2-Diaminocyclohexane via 1H NMR Spectroscopy

Enantiomers of a series of tripeptide derivatives with three stereogenic centers (+/-)-G1-G9 have been prepared from D- and L-alpha-amino acids as guests for chiral recognition by H-1 NMR spectroscopy. In the meantime, a family of tetraaza macrocyclic chiral solvating agents (TAMCSAs) 1a-1d has been synthesized from D-phenylalanine and (1S,2S)-(+)-1,2-diaminocyclohexane. Discrimination of enantiomers of (+/-)-G1-G9 was carried out in the presence of TAMCSAs 1a-1d by H-1 NMR spectroscopy. The results indicate that enantiomers of (+)-G1-G9 can be effectively discriminated in the presence of TAMCSAs 1a-1d by H-1 NMR signals of multiple protons exhibiting nonequivalent chemical shifts (Delta Delta delta) up to 0.616 ppm. Furthermore, enantiomers of (+/-)-G1-G9 were easily assigned by comparing H-1 NMR signals of the split corresponding protons with those attributed to a single enantiomer. Different optical purities (ee up to 90%) of G1 were clearly observed and calculated in the presence of TAMCSAs 1a-1d, respectively. Intermolecular hydrogen bonding interactions were demonstrated through theoretical calculations of enantiomers of (+/-)-G1 with TAMCSA 1a by means of the hybrid functional theory with the standard basis sets of 3-21G of the Gaussian 03 program.