Seventeen-Membered Water Cluster Resulting from Recognition of Solvated Anions on Brucinium Corrugated Layers

Formation of diastereomeric salts
remains the most important method
for the separation of racemic acids and bases. Selection of a suitable
resolving agent in this method is a key for successful resolution.
There are primary, secondary, and tertiary chiral amines among frequently
used resolving agents for the separation of racemic acids. Cations
of most of them and anions of resolved acids are linked to each other
by a characteristic system of hydrogen bonds resulting in common cationic–anionic
self-assemblies. In this respect, brucine and strychnine are unique,
because incorporation of anionic species into a crystal lattice of
their salts usually does not affect common cationic self-assembly.
The uniqueness of both resolving agents is also reflected in a high
frequency of solvated salt formation. In this paper, we show that
the presence of water molecules incorporated into the crystal lattice
of the brucinium salt may result from recognition of the resolved
compound together with its closest aqueous environment on the common
brucinium corrugated layers. Performing racemic resolution of model
compounds and studying structural relations between succeeding crystalline
fractions, we also point out factors responsible for the successful
separation of N-(4-nitrobenzoyl)­alanine by fractional
crystallization of brucinium diastereomeric salts.