X-RAY CRYSTALLOGRAPHIC EVIDENCE IN SUPPORT OF A PROPOSED CHIRAL RECOGNITION MECHANISM

A new family of pi-basic chiral selectors has been developed and emplo yed in the separation of enantiomers by liquid chromatography. These c hiral selectors, derived from (S)-proline and designed from mechanisti c considerations, show high levels of discrimination between the enant iomers of N-(3,5-dinitrobenzoyl)amino acid esters and amides. A consid erable amount of chromatographic data has been assembled, all of it co nsistent with the proposed chiral recognition mechanism. Moreover, thi s mechanism is supported by induced chemical shift differences and int ermolecular NOE data previously obtained in solution with an equimolar mixture of (S)-1 and (S)-2. A crystalline 1:1 complex of (S)-1 and (S )-2 has been obtained and analyzed by X-ray crystallography. The struc ture of this complex in the solid state illustrates the essential feat ures of the mechanism proposed to; account for chiral recognition betw een chiral stationary phase (CSP) 3 and the enantiomers of 2 and relat ed analytes. In addition, the orientation of the two components in the solid state is in close agreement with the structure of the more stab le diastereomeric complex deduced from solution-state NMR evidence rel ating to the same system.