Increased Chemical Purity Using a Hydrate

Crystallization is usually a highly selective process, and this selectivity is exploited in the pharmaceutical and other industries as a method of chemical purification. Relatively little is known about how polymorphism and solvate formation can influence the selectivity of crystallizations. We describe a piperidene pharmaceutical intermediate that can crystallize as either a hydrate or an anhydrous form, with very different purities. The exploitation of this effect is described, and an explanation in terms of the crystal structures is proposed.