Except for the Tollens test, basic aqueous conditions are generally avoided with sugarsbecause they lead to fast isomerizations.(a) Under basic conditions, the proton alpha to the aldehyde (or ketone) carbonylgroup is reversibly removed, and the resulting enolate ion is no longer asymmetric.Reprotonation can occur on either face of the enolate, giving either the original structureor its epimer. Because a mixture of epimers results, this process is called epimerization.Propose a mechanism for the base-catalyzed equilibration of glucose to a mixture ofglucose and its C2 epimer, mannose.*(b) Propose a mechanism for the isomerization of a ketose to an aldose, via the enediolintermediate, shown immediately above. Note that the enediol has two ¬OH protons,and removing one or the other gives two different enolate ions.
Except for the Tollens test, basic aqueous conditions are generally avoided with sugars
because they lead to fast isomerizations.
(a) Under basic conditions, the proton alpha to the
group is reversibly removed, and the resulting enolate ion is no longer asymmetric.
Reprotonation can occur on either face of the enolate, giving either the original structure
or its epimer. Because a mixture of epimers results, this process is called epimerization.
Propose a mechanism for the base-catalyzed equilibration of glucose to a mixture of
glucose and its C2 epimer, mannose.
*(b) Propose a mechanism for the isomerization of a ketose to an aldose, via the enediol
intermediate, shown immediately above. Note that the enediol has two ¬OH protons,
and removing one or the other gives two different enolate ions.
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