X-Ray Crystallographic Studies of Quasi-Racemates for Absolute Configuration Determinations

The determination of the absolute configuration of chiral molecular entities by means of X-ray crystallography is overall of central importance in stereochemistry. However, the growth of enantiopure single crystals often represents an unsurmountable and frustrating hurdle. Quasi-racemic crystals of biomacromolecules, for which the absolute configuration is predetermined by the chiral monomers, allowed the X-ray crystallographic analysis of systems that are difficult to crystallize as pure enantiomers, with aims other than the determination of the absolute structure. Taking advantage of the greater propensity of quasi-racemic mixtures to co-crystallize compared to growing enantiomerically pure crystals of a single compound, we herein describe the use of X-ray crystallography of quasi-racemates for the absolute configuration determination. We expect that this approach is particularly useful to establish the sense of selectivity in the development of stereoselective methods by simplified crystallizations, while confirming the consistent selectivity with a second molecular structure within the same measurement.