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EPIMERIZATION OFTEN HAPPENS BEFORE E1cb ELIMINATION

EPIMERIZATION OFTEN HAPPENS BEFORE E1cb ELIMINATION. EPIMERIZATION. This term usually applies to the inversion of a hydrogen atom on a stereocenter next to a carbonyl group and occurs in a basic solution. This one stereocenter is inverted in a molecule

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EPIMERIZATION OFTEN HAPPENS BEFORE E1cb ELIMINATION

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  1. EPIMERIZATION OFTEN HAPPENS BEFORE E1cb ELIMINATION

  2. EPIMERIZATION This term usually applies to the inversion of a hydrogen atom on a stereocenter next to a carbonyl group and occurs in a basic solution. This one stereocenter is inverted in a molecule that has other stereocenters that are not inverted . This creates an “epimer”. Epimers are a type of diastereomer.

  3. EPIMERIZATION NaOH / H2O R R S + R R R fast -H+ epimers ( also diastereomers ) -H+ fast - +H+ Rapid proton exchanges invert the stereocenter between the carbonyls. planar anion Removal of a proton makes an anion which is planar (resonance system). When the proton is replaced, it can add back to either side of the planar ion. eliminates slowly see next slide E1cb If one of the epimers has a lower energy, it will predominate in the equilibrium.

  4. PLANAR ANION H+ TOP H .. O Rotated for clarity. O CH3O Ph also a part of the pi system H+ BOTTOM

  5. DEUTERIUM EXCHANGE AND EPIMERIZATION deuterium adds back to either side of planar resonance structure fast fast E1cb planar sp2 slow

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