Why Is the Resolution of Certain Racemic Modifications
Inefficient? Formation of Diastereomeric Double Salts of Brucinium
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Abstract
Fractional crystallization of diastereomeric salts remains
the
most frequently used method for a separation of racemic compounds,
and yet it has been performed by trial and error. For a better understanding
of the chiral discrimination mechanism that is useful for the rationalization
of optical resolution and help in choosing the crucial experimental
parameters, structural investigations of products of both successful
and unsuccessful racemic resolution are important. The former and
the latter typically involve diastereomeric salts precipitating fractionally
and the formation of solid solutions of diastereomeric salts or diastereomeric
double salts, respectively. In this contribution, a mechanism of recognition
leading to formation of the diastereomeric double salts is proposed
based on crystal structures of three brucinium double salts (bis(brucinium) <i>N</i>-(3,5-dinitrobenzoyl)-dl-alaninate methanol 3-solvate,
bis(brucinium) <i>N</i>-(3,5-dinitrobenzoyl)-dl-alaninate 5.75-hydrate, and bis(brucinium) <i>N</i>-(3,5-dinitrobenzoyl)-dl-serinate 0.10-hydrate) as well as of some relevant diastereomeric
salts (containing the d- or l-enantiomer of the
alanine or the serine derivative)