Hydrophilic and hydrophobic pockets at surfaces of strychninium self-assemblies in pseudopolymorphous crystals of strychninium salts

Abstract

Racemic mixtures of N-3,5-dinitrobenzoyl-DL-alanine, as well as N-4-nitrobenzoyl-DL-glutamic acid were resolved by fractional crystallization of strychninium diastereomeric salts. Crystals of strychninium N-3,5-dinitrobenzoyl-D-alaninate methanol solvate (1), and distrychninium N-4-nitrobenzoyl-D-glutamate tetrahydrate (3) were grown during these resolutions. Crystals of stychninium N-3,5-dinitrobenzoyl-D-alaninate 3.5 hydrate (2) and distrychninium N-4-nitrobenzoyl-D-glutamate 4.6 hydrate (4) were obtained during recrystallization of 1 and 3, respectively. Crystal structures of 1–4 were determined. In all of the crystals and in most of the known crystals of strychninium salts, strychninium cations form similar pillars. Depending on how the pillars are linked, the surfaces of the resulting self-assemblies reveal various donor/acceptor capabilities. The amide O atom can participate in weak (C–H⋯O) or strong (e.g. O–H⋯O) hydrogen bonds. Depending on the kind of hydrogen bond, weak or strong, various components are recognized by the amide O atom of the strychninium cations. These may be resolved compounds (or other strychninium cations) or solvent molecules. Comparison of the crystal structures of strychninium and brucinium salts (brucine is the dimethoxy derivative of strychnine) indicates that both strychninium and brucinium cations can form similar self-assemblies. However, both alkaloids reveal preferences to form differing assemblies. While strychninium cations are usually assembled in a way that allows the amide O atom to participate in weak C–H⋯O hydrogen bonds, common brucinium corrugated monolayer sheets provide an opportunity for the amide O atom to participate in strong e.g. O–H⋯O hydrogen bonds. The different preferences of both alkaloids show additional possibilities in the endeavor to achieve an efficient resolution of racemic mixtures.