Specific acid catalysis of acetals, metals and glycosides

Specific acid catalysis of acetals, ketals and glycosides 

In the case where both alkoxyl groups are identical, only one mechanism can be written  

The buildup of the intermediate simple hemiacetal is known in a couple of cases [98,99], but not where the only pathway for hydrolysis is specific acid catalysis. In 

There is no doubt that simple aldopyranosides react via exocyclic protonation (top pathway). The evidence for this includes  

02 for -xylosides [107], and +0.026 + 0.012 for a-mannosides [108]. The slightly bigger )S g values for the -glycosides could well be real if so they imply more bond breaking in the transition state for a-glycoside hydrolysis. 

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