SUBJECTS mutarotation

A number of other reactions in this general area such as the mutarotation of D-(+)-glucose431,432 and aldose-ketose isomerizations433 are also subject to catalysis by metal ions. 

A reaction exhibiting general acid catalysis, the ester hydrolysis (XXXVIII), has been discussed in Sect. 2.3. The present section deals with a classic reaction which is subject to both general acid and base catalysis in homogeneous media, the mutarotation of D-glucose. 

When a reaction is susceptible to catalysis by Bronsted acids and bases, the phenomenon is characterized as generalized acid or basic catalysis. One of the earliest reactions discovered to be subject to such generalized acid-base catalysis was the mutarotation of optically active glucose ... 

Determination of the catalytic activity of the hydroxyl ion is difficult, because the mutarotation becomes too rapid for accurate measurement when the concentration of hydroxyl ions rises above 10 /unolar. With a minute concentration of hydrogen ions, accurate measurements require rigorous exclusion of carbon dioxide and other impurities. Evaluation of the rate constant is also complicated by ionization of the sugar, a subject that was not considered by the early workers. 

One of the first reactions to be examined in a reverse micelle was the mutarota-tion of 2,3,4,6-tetramethyl-a-D-glucose in solutions of dodecylammonium salts of carboxylic acids in benzene containing small amounts of water [134], The substrate is relatively hydrophilic, and should be partitioned into the aqueous interiors of the micelles where it is exposed to the ammonium and carboxylate groups of the surfactant which could act as general acid-base catalysts, (Mutarotation in water is subject to general acid-base catalysis,)... 

The term intramolecular catalysis introduced by Bender is also widely used to describe neighboring group effects, especially when analogous intermolecular catalysis is observed. Thus this term is commonly used when referring to reactions that are subject to acid, base, and/or nucleophilic catalysis such as hydrolysis of esters, amides, and acetals the mutarotation of aldoses or the enolization of ketones. It is rarely used when referring to nucleophilic substitution reactions at saturated carbon. 

The mildest type of reaction of the sugars induced by acids is the interconversion between a- and /3-isomers or between ring isomers. This type of change has been discussed above under the general subject of mutarotation. Dilute acids at room temperatures have little or no additional action on the sugars, but hot concentrated acids produce profound changes. 

Methyl a-D-glucopyranoside is a stable compound that does not undergo mutarotation under neutral or basic conditions. However, when subjected to acidic conditions, an equilibrium is established consisting of both methyl a-D-glucopyranoside and methyl (f-D-glucopyranoside. Draw a mechanism that accounts for this observation. 

Draw the structure of the initial product of /3-maltose when it is subjected to (a) Br2 oxidation (b) phenylhydrazine (3 equivalents) (c) conditions that effect mutarotation. 

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