Abstract

The number of crystal structures of diastereomeric salt pairs and especially of double salts is limited in the literature.

Highlights

  • Optical resolution via diastereomeric salt formation is the most simple and robust and the most frequently used method for enantiomer purification

  • In an optical resolution procedure via diastereomeric salt formation an optically active resolving agent is present beside the racemic mixture of the targeted compound

  • The low efficiency of the optical resolutions can be associated with the presence of the double salt during the optical resolution experiments (DOB-2 and DOB-4)

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Summary

Introduction

Optical resolution via diastereomeric salt formation is the most simple and robust and the most frequently used method for enantiomer purification. By analogy of the above, there is the possibility to obtain a crystalline salt that contains both enantiomers of the starting compound in 1 to 1 ratio and the resolving agent in optically pure form. Formation of such a so called double salt[6,7,8] is not so favoured by symmetry considerations, other factors may promote its formation. We found that practically there was no enantiomer discrimination in neither of the two chloromandelic acid resolutions with 1-cyclohexylethylamine resolving agent In both cases a double salt at 1 to 1 R- and S-chloromandelic acid ratio was found. Chloromandelate (DOB-4)] salts we uniquely have in hand two completed series, the structures of the two diastereomers and the double salt of an organic salt in both cases, all the three possible combinations of the molecular configurations were available for the comparison (Scheme 1)

Materials and methods
Optical resolution experiments
Preparation of the single crystals
Single crystal X-ray diffraction measurements
Optical resolution results
Solubility of the diastereomers and the double salts
Thermal properties and calculated solid–liquid phase diagrams
Analysis of the crystal structures
Analysis of the halogen interactions
Hirshfeld analysis of the salts
Conclusions

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