GAS-CHROMATOGRAPHIC SEPARATION OF STEREOISOMERIC ESTERS OF ALPHA-AMINO-ACIDS AND ALPHA-ALKYL-ALPHA-AMINO ACIDS ON CHIRAL STATIONARY PHASES

The separations of stereoisomers of (a) N-pentafluoropropionyl (PFP)-(R,S)-alpha-amino acid (AA) (R,S)-alkyl esters and (b) N-trifluoroacetyl (TFA)-(R,S)-alpha-alkyl-alpha-amino acid (AAA) (R,S)-alpha-alkyl esters were investigated by capillary gas chromatography on Chirasil-L-Val, and Chirasil-D-Val, respectively. The order of elution of the four possible stereoisomers (a) was found to be R-S < R-R < S-R < S-S for PFP-(R,S)-AA 2-butyl and 2-octyl esters, and R-S < S-R < R-R < S-S for PFP-(R,S)-AA 3-methyl-2-butyl esters (the first letter in the diastereomers refers to the configuration of the amino acid and the second to the configuration of the alcohol). For (b) it was found that the S-S diastereomers eluted first and the R-S diastereomers last (fourth) from the column. In the few instances in (b) in which all four stereoisomers were separated, with the TFA-(R,S)-alpha-butyl-Ala (R)-2-butyl esters the S-R diastereomer eluted second and the R-R diastereomer eluted third whereas with the TFA-(R,S)-alpha-butyl-Ala (R)-2-octyl esters and TFA-(R.S)-alpha-methylleucine (R)-2-octyl esters, the R-R diastereomers eluted second and the S-R diastereomers third. Further, the temperature dependence of the elution order of the four stereoisomers of PFP-(R,S)-Ala (R,S)-2-octyl esters on Chirasil-D-Val showed that the S-R diastereomer eluted first and the R-R diastereomer last (fourth) at all temperatures; however, at 125-degrees-C S-S eluted before R-S and at 150-degrees-C S-S and R-S eluted together, whereas at 175-degrees-C R-S eluted before S-S. The order of elution of stereoisomeric derivatives of AA and AAA from chiral stationary phases is rationalized by superimposed diastereomeric and enantiomeric resolution effects on the chiral stationary phase. That the chiral alcohol contributes to the resolution of stereoisomeric N-acyl-(R,S)-AA (R,S)-2-alkyl esters on a chiral phase is demonstrated by the resolution of enantiomeric TFA-Gly (R)-alkyl esters and TFA-Gly (S)-alkyl esters on Chirasil-D-Val.