Alpha Epimerization

Carbohydrates - back

Base catalyzed unfavorable reactions of sugars

Attacks aldoses - Creates epimers

• H-C2-OH <--(NaOH/H2O)--> HO-C2-H

Mechanism:

• OH nucleophile attacks the acidic alpha proton from the carbonyl
  • The remaining electrons forms an enolate intermediate (carbanion and ENOLATE)
  • The point where it loses its stereochemistry = C2 is no longer asymmetric
• Enolate double bond nucleophile attacks a proton from water
  • Regenerates the base
  • Epimerization: causes equal confirguration (1:1 E and Z like configurations or R and S configurations)

Alpha Epimerization Fundamentals

• Base catalysis occurs the same way attacking the acidic alpha proton (C2 proton)
• The deviation in the products occurs form the enolate oxygen
  • -1 Reform the carbonyl from attacking the carbon it is attached to
    • RESULTING IN ENOLATE PRODUCTION
  • -2 Reduced to an alcohol from attacking a proton from water
    • RESULTING IN ENEDIOL PRODUCTION
• Under strong basic conditions a combination of enediol and epimerization leads to complex mixture of sugars

Concepts:

Terminology:

Additional Information: