Absolute Configuration Determination from Low ee Compounds by the Crystalline Sponge Method. Unusual Conglomerate Formation in a Pre-Determined Crystalline Lattice

Abstract

When chiral compounds with low enantiomeric excess (ee, R:S=m:n) were absorbed into the void of the crystalline sponge (CS), enantiomerically pure [(R)_m (S)_n ] chiral composites were formed, changing the centrosymmetric space group into non-centrosymmetric one. The absolute configuration of the analyte compounds was elucidated with a reasonable Flack (Parsons) parameter value. This phenomenon is characteristic to the "post-crystallization" in the pre-determined CS crystalline lattice, seldom found in common crystallization where the crystalline lattice is defined by an analyte itself. The results highlight the potential of the CS method for absolute configuration determination of low ee samples, an often encountered situation in asymmetric synthesis studies.

Keywords: X-ray crystallography; absolute configuration determination; crystalline sponge method; host-guest systems.