Aspects of chiral resolution in the presence of (partial) solid solutions

Partial miscibility in the solid state is an undesireable property of chiral substances with regard to separation by crystallisation. Nevertheless, the incorporation of molecules of the counter enantiomer into the lattice of either the pure enantiomer or the racemic compound is known for various substances, e.g. [1, 2]. Partial solid solutions at the enantiomer side limit the attainable purity of this enantiomer (fig. 1) and miscibility in the vicinity of the racemic compound makes an enantiomeric enrichment more difficult (fig. 1a). While from the thermodynamic point of view chiral resolutions for systems showing miscibility in the solid state are not feasible, the kinetics of solid-solid transformations must not be neglected. The temporal course of the crystallisation is a key factor to estimate the potential of a chiral separation in a more comprehensively manner. Therefore a quantitative study is performed to assess possible process routes to cope with chiral systems exhibiting solid solutions. The chiral malic acid was chosen for investigations since the enantiomer is incorporated into the racemic compound lattice in significant quantities. The focus of research is on the roles of cooling and vaporization rates, initial purity and solvent composition. An evaluation of obtained crystallisation trajectories and crystal growth rates on the basis of the applied crystallization conditions is done. The need for complementary use of different analytical techniques (XRPD, FT-IR/ ATR, DSC) for solid phase characterization is emphasized. Different possibilities to achieve enantiomeric enrichment on the basis of a kinetic approach are outlined and evaluated experimentally. [1] Li Z.J., Grant D.J.W., Effects of excess enantiomer on the crystal properties of a racemic compound: ephedrinium 2-naphthalenesulfonate, Int. J. Pharm., 137, 1996, p. 21-31 [2] Aubin E. et al., Resolution of the ethanolamine salt of (±) mandelic acid by using the as3pc method: principle, application and results, J. Phys. IV, 122, 2004, p. 157-16