OBSERVATIONS RELEVANT TO THE DIFFERENTIAL INTERCALATION OF ENANTIOMERS BETWEEN THE STRANDS OF BRUSH-TYPE CHIRAL STATIONARY PHASES

Differential penetration of the enantiomers of certain analytes betwee n the strands of ''brush-type'' liquid chromatographic chiral stationa ry phases (CSPs) has been invoked in several instances to explain the progressive change in separation factors as one proceeds, chromatograp hically, through a homologous series of these analytes. These ''interc alative effects'' are presumed to arise as a consequence of the semi-o rdered, side-by-side bonding of selector strands in conventional brush -type CSPs. Thus, they are not expected td occur in the absence of the stationary phase. A recently developed proline-derived CSP affords la rge separation factors for the enantiomers of some esters, thioesters, and amides of N-acylated alpha-amino acids, the separation factor bei ng dependent upon the length of the C-terminal substituent of the anal yte. For example, the separation factors increase with the length of t he linear thioalkyl substituent when N-(3,5-dinitrobenzoyl)leucine thi oesters are chromatographed on the proline-derived CSP. However, the l ength of this substituent does not affect the enantiomeric purity atta ined when the enantiomers of these thioesters are stereochemically equ ilibrated in solution in the presence of the proline-derived selector employed in the CSP. These observations further support the concept of differential intercalation of enantiomers between the strands of brus h-type CSPs and may have implications for asymmetric syntheses conduct ed with immobilized catalysts or reagents.