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Racemic Mixtures and the Resolution of Enantiomers02:30

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A racemic mixture, or racemate, is an equimolar mixture of enantiomers of a molecule that can be separated using their unique interaction with chiral molecules or media. Racemic mixtures are denoted by the (±)- prefix. This ‘optical rotation descriptor’ applies to the whole solution of a racemic mixture rather than a specific stereoisomer. Enantiomers typically have the same physical and chemical properties. Hence, they are not easily separable. However, enantiomers can exhibit...
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The chiral α-carbon of the carbonyl compound is the stereocenter of the molecule. As shown in the figure below, when such a carbonyl compound undergoes racemization under an acidic or basic condition, an achiral enol is formed.
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The rate of acid-catalyzed hydration of alkenes depends on the alkene's structure, as the presence of alkyl substituents at the double bond can significantly influence the rate.
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The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.
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The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
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The Claisen rearrangement is a [3,3] sigmatropic rearrangement of allyl vinyl ethers to unsaturated carbonyl compounds. The rearrangement is a concerted pericyclic reaction proceeding via a chair-like transition state.
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Highly Stereoselective Synthesis of 1,6-Ketoesters Mediated by Ionic Liquids: A Three-component Reaction Enabling Rapid Access to a New Class of Low Molecular Weight Gelators
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Chirality amplification step of temperature cycle-induced deracemization.

Ryusei Oketani1, Riku Naito1, Ichiro Hisaki1

  • 1Division of Chemistry, Graduate School of Engineering Science, The University of Osaka, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan. r.oketani.es@osaka-u.ac.jp.

Chemical Communications (Cambridge, England)
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Summary

Chirality enrichment during temperature-induced deracemization occurs during crystal dissolution. The enantiomeric excess (ee) changes are linked to the mass and surface area ratio of enantiomeric crystals.

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Area of Science:

  • Chiral chemistry
  • Crystallization science
  • Physical organic chemistry

Background:

  • Deracemization is crucial for obtaining enantiopure compounds.
  • Understanding the dynamics of chiral enrichment is key for process optimization.

Purpose of the Study:

  • To quantitatively analyze the timing of chirality enrichment during temperature cycle-induced deracemization.
  • To investigate the factors influencing the evolution and devolution of enantiomeric excess (ee).

Main Methods:

  • Monitoring crystalline enantiomeric excess (ee) during a single heating-cooling cycle.
  • Quantitative analysis of crystal mass and surface area ratios.

Main Results:

  • Chirality enrichment was found to proceed during the dissolution steps of the temperature cycle.
  • The observed changes in ee (evolution and devolution) can be accurately described by the mass and surface area ratio between enantiomeric crystals.

Conclusions:

  • Dissolution is the critical stage for chiral enrichment in this deracemization process.
  • The mass and surface area ratio of enantiomeric crystals is a predictive parameter for ee changes.