Galactose dimethyl acetal, hydrolysis

Percival and Thomson44 obtained a mixture of disaocharide esters from which, on hydrolysis and derivatization, they isolated 2,3,4,6-tetramethyl-D-galactose anilide and a mixture of methylated acids. From these acids they obtained 3,6-anhydro-2,5-dimethyl-L-galactonic acid, the structure of which was determined by almost the same means as those employed by Jones and Peat.23 On methanolysis of hexamethylagarobiose, Araki28 isolated 3,6-anhydro-2,5-dimethyl-L-galactose dimethyl acetal (XIII),... 

Nucleophilic participation by the hydroxyl group has been noted in acid-catalyzed studies of systems other than carboxyl groups. In dilute aqueous acid solution, methyl furanosides [e.g., (248)] were formed by ring closure during hydrolysis of the acetal bonds of the acyclic dimethyl acetals of glucose (247) and galactose.It was estimated that the rates of these cyclizations were faster than the expected unassisted rates of hydrolysis. 

These data allow us to complete the structure—the 4-position of glucose is unmethylated, so this is where the bond to galactose must have been. Note that although the anomeric hydroxyl of glucose will be methylated by dimethyl sulfate, this will be removed during the hydrolysis—it is an acetal, not an ether ... 

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