Od rozpoznania molekularnego do rozdziału racemicznego metodą frakcyjnej krystalizacji soli diastereomerycznych From molecular recognition to racemic resolution by fractional crystallization of diastereomeric salts
Synthesis and isolation of enantiopure compounds remains an important
challenge in medicinal, pharmaceutical, alimentary as well as material chemistry.
Amongst many methods leading to enantiopure compounds, formation and fractional
crystallization of diastereomeric salts of an enantiopure resolving agent is the
most popular one. Since its discovery many years went by, and yet it has been performed
by the trial and error method.
Selection of the suitable resolving agent is crucial for racemic resolution efficiency.
Hence, this review is concerned with common features of crystal structures
of salts, in which resolving agents, frequently used for separation of racemic acids,
are involved. Among them, there are crystal structures of salts of primary, secondary
and tertiary chiral amines (1-arylethylamines, ephedrine, pseudoephedrine, deoxyephedrine,
chinine, chinidine, cinchonine, cinchonidine, strychnine and brucine).
In most of them, cations and anions are involved in characteristic systems of hydrogen
bonds leading to formation of cationic-anionic self-assemblies. In this respect,
brucine and strychnine appear as unique, because presence of anions usually does
not affect formation of common cationic self-assemblies stabilized by weak hydrophobic
interactions.
Comparison of the common structural motifs indicates different factors being
crucial in molecular recognition and in diastereomeric diversification. Formation of
common hydrogen bonded cationic-anionic self-assemblies plays a dominant role
in molecular recognition of a given acid by most of the studied amines. In turn, formation
of common cationic self-assemblies stabilized by hydrophobic interaction
plays a crucial role in molecular recognition of a given acid by brucine or strychnine.
Diastereomeric diversification can be realized in various ways. In extreme cases,
when molecular recognition in crystals of particular diastereomeric salts is similar,
diastereomeric diversification is governed by different factors, depending on factors
governing molecular recognition. Hydrophobic interactions play an important
role in diastereomeric diversification when molecular recognition is governed by
hydrogen bonds. In turn, when molecular recognition is governed by hydrophobic
interactions, different hydrogen bond systems in crystals of particular salts are
responsible for their diastereomeric deversification.
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