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Title: Quininium mandelates—a systematic study of chiral discrimination in crystals of diastereomeric salts
Authors: Báthori, Nikoletta B 
Nassimbeni, Luigi R. 
Oliver, Clive L 
Keywords: Quininium mandelates;Crystals;Diastereomeric salts;Salts
Issue Date: 2011
Publisher: Chemical Communications
Abstract: The resolution of racemic modifications by a chiral resolving agent is generally achieved either by diastereomer salt formation or by inclusion with a chiral host compound. The former method is the most common, and has been reviewed.1 The cinchona alkaloids, of which quinine is the most abundant, have been used extensively as resolving agents for acids1,2 and the pairs quinine/quinidine and cinchonidine/ cinchonine have been described as ‘‘quasi-enantiomeric’’.3 Larsen4,5 has described the structures of the salts formed by cinchonine and cinchonidine with both (R)- and (S)-mandelic acids. She concluded that cinchoninium (R)-mandelate is the less soluble salt and its structure displays disorder in the –CH=CH2 moiety. In contrast, cinchonidinium-(S)-mandelate is the less soluble salt. The thermal and solubility parameters of the four salts were reconciled with their crystal structures and it was noted that the packing of the cinchonidinium salts was significantly different from their corresponding cinchoninium salts. We have taken a somewhat different approach to the question of enantiomeric resolution in order to understand the mechanism of the molecular recognition that drives the differentiation of the resolving agent for one particular enantiomer. We have thus set up a series of competition experiments where the resolving agent, quinine (QUIN) was exposed to mixtures of mandelic acid, where the mole fraction of the starting mixture was varied systematically. The ensuing solutions were allowed to crystallise and the mole fraction of the entrapped enantiomer was measured by analysing the crystal structure.
Appears in Collections:Appsc - Journal Articles (DHET subsidised)

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