Rearrangements enediol

Mechanism 23-2 Base-Catalyzed Epimerization of Glucose 1115 Mechanism 23-3 Base-Catalyzed Enediol Rearrangement 1115 23-9 Reduction of Monosaccharides 1116... 

The carbonyl group in D-galactose may be isomerized from Cl to C2 by brief treatment with dilute base (by the enediol rearrangement, Section 23-8). The product is the C4 epimer of fructose. Draw the furanose stmcture of the product. 

Epimerization and the Enediol Rearrangement One of the most important aspects of sugar chemistry is the inability, in most cases, to use basic reagents because they cause unwanted side reactions. Two common base-catalyzed side reactions are epimerization and enediol rearrangement. 

Another base-catalyzed side reaction is the enediol rearrangement, which moves the carbonyl group up and down the chain, as shown in Mechanism 23-3. If the enolate ion (formed by removal of a proton on C2) reprotonates on the Cl oxygen, an enediol intermediate results. Removal of a proton from the C2 oxygen and reprotonation on Cl gives fructose, a ketose. 

Show how another enediol rearrangement can move the carbonyl group from C2 in fructose to C3. 

Tollens test cannot distinguish between aldoses and ketoses because the basic Tollens reagent promotes enediol rearrangements. Under basic conditions, the open-chain form of a ketose can isomerize to an aldose, which reacts to give a positive Tollens test. 

A test for reducing sugars, employing the same silver-ammonia complex used as a test for aldehydes. A positive test gives a silver precipitate, often in the form of a silver mirror. Tollens reagent is basic, and it promotes enediol rearrangements that interconvert ketoses and aldoses. Therefore, both aldoses and ketoses give positive Tollens tests if they are in their hemiacetal forms, in equilibrium with open-chain carbonyl structures, (p. 1118)... 

Base-Catalyzed Epimerization of Glucose 1115 Base-Catalyzed Enediol Rearrangement 1115... 

Fructose is the ketose that results from the enediol rearrangement of mannose that shifts the carbonyl group to C2. 

In addition to forming its C-2 epimer in a basic solution, o-glucose also undergoes an enediol rearrangement, which forms o-fructose and other ketohexoses. 

MECHANISM FOR THE BASE-CATALYZED ENEDIOL REARRANGEMENT OF A MONOSACCHARIDE... 

In a bask solution an aMose forms a Another enediol rearrangement, initiated by a base removing a proton from C-3 of... 

Both aldoses and ketoses are oxidized to aldonic acids by Tollens reagent (Ag", NH3, HO ), so Tollens reagent cannot be used to distinguish them. Tollens reagent only oxidizes aldehydes, but since the oxidation reaction is carried out in a basic solution, a ketose is converted into an aldose by an enediol rearrangement (Section 21.5), and the aldose is then oxidized by Tollens reagent. 

Enediol rearrangement (Section 21.5). The mechanism of the reaction is shown on page 1023. 

We have already encountered several base-catalyzed reactions, such as the interconversion of keto and enol tautomers (Section 18.3), the Claisen condensation (Section 18.13), and the enediol rearrangement (Section 21.5). A base catalyst increases the rate of a reaction by removing a proton from the reactant. For example, the dehydration of a hydrate in the presence of hydroxide ion is a base-catalyzed reaction. Hydroxide ion (the base) increases the rate of the reaction by removing a proton from the neutral hydrate. 

The Mechanism for an Enzyme-Catalyzed Reaction That Is Reminiscent of the Base-Catalyzed Enediol Rearrangement 1125... 

THE MECHANISM FOR AN ENZYME-CATALYZED REACTION THAT IS REMINISCENT OF THE BASE-CATALYZED ENEDIOL REARRANGEMENT... 

For example, the first step in glycolysis is an Sj. 2 reaction, the second step is identical to the enediol rearrangement that students learned when they studied carbohydrate chemistry, the third step is another Sj 2 reaction, and the fourth step is a reverse aldol addition. 

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