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Discrimination in resolving systems: ephedrine-mandelic acid.

E J Valente1, J Zubkowski, D S Eggleston

  • 1Department of Chemistry, Mississippi College, Clinton 39058.

Chirality
|January 1, 1992
PubMed
Summary

Mandelic acid resolution using ephedrine yields diastereomeric salts with distinct solubilities. X-ray analysis reveals isosteric crystal structures with helical hydrogen-bonded ion columns, providing insights into chiral compound separation.

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Area of Science:

  • Crystallography
  • Chiral Chemistry
  • Organic Chemistry

Background:

  • Chiral resolution is crucial for separating enantiomers of compounds like mandelic acid.
  • Diastereomeric salt formation with chiral resolving agents, such as ephedrine, is a common method.
  • Understanding the solid-state structures of these salts aids in optimizing resolution processes.

Purpose of the Study:

  • To elucidate the crystal structures of diastereomeric salts formed between mandelic acid and ephedrine.
  • To investigate the structural basis for the differing solubilities and melting points of these salts.
  • To analyze the hydrogen bonding patterns and packing in the crystalline structures.

Main Methods:

  • Resolution of mandelic acid using (-)-(1R,2S)-ephedrine in aqueous ethanol.

Related Experiment Videos

  • Single crystal X-ray diffraction analysis of the isolated diastereomeric salts (L- and M-salts).
  • Characterization of mixed crystals containing both mandelic acid isomers.

    • Main Results:

    • Two diastereomeric salts, (-)-ephedrinium (2R)-mandelate (L-salt) and (-)-ephedrinium (2S)-mandelate (M-salt), were isolated with differing solubilities.
    • Both L- and M-salts exhibit isosteric crystal structures, crystallizing in the monoclinic system (space group C2).
    • A helical hydrogen-bonded network of alternating protonated ephedrine and mandelate ions forms columns along the crystallographic 2-fold screw axis.

    Conclusions:

    • The isosteric nature of the diastereomeric salts, despite solubility differences, is attributed to subtle variations in hydrogen bonding and crystal packing.
    • The observed helical hydrogen-bonded chains provide a structural framework for understanding the chiral recognition during resolution.
    • The study demonstrates the utility of X-ray crystallography in characterizing intermediates of chiral resolution and informing process optimization.