Gas chromatographic separation of diastereomeric esters of α-alkyl-α-amino acids on dimethylpolysiloxane

Abstract

The separation of 28 ( R,S)-α-alkyl-α-amino acids (AAAs) with the structure H 2NC(R 1)(R 2)COOH (R 1,R 2 = alkyl, aralkyl, which may be further substituted; R 1,R 2 ≠ H) was systematically investigated by capillary gas chromatography on the stationary phase dimethylpolysiloxane (CP-Sil 5); the AAAs were used as their N(O)-pentafluoropropionyl (PFP)-AAA ( S)(+)-2-alkyl esters (alkyl = 2-butyl to 2-octyl and 3-methyl-2-butyl). In selected cases, ( R,S)-3,3-dimethyl-2-butyl esters, ( R,S)-3-octyl esters, ( R,S)-2-dodecyl esters and (−)-menthyl esters were also employed. Baseline resolution was achieved for these AAAs by derivatization with suitable alcohols. By using AAAs of known absolute configuration, it was possible to establish the order of emergence as ( S)-AAAs before ( R)-AAAs for PFP-( R,S)-AAA ( S)(+)-2-alkyl esters. This is the opposite elution order to that of α-amino acids having a C α-hydrogen atom. In spite of their opposite elution orders on achiral stationary phases, conformational analysis of PFP ( S)(+)-2-alkyl esters of ( S)-α-amino acids and ( S)-AAAs shows that they have the same bulkiness chirality.