Protonation glycosides

The mechanism of cleavage of the a-(l—A) linkage by starch lyase involves removal of the proton on C-2, with subsequent formation of a double bond between C-2 and C-l, and simultaneous displacement of the protonated glycosidic oxygen atom. This results in the release of the starch chain with one less glucosyl unit and the formation of the enol of 1,5-anhydro-D-fructose (Figure 7.20). 

Consequently, the glycosidic linkage in kasuganobiosamine must be -configuration, showing conversion of the anomeric proton during methanolysis. 

Fig. 14.—Catalysis of glycosidic oxygen protonation by hydrated magnesium...

An Sn 1 -like reaction, on the other hand, is much more favorable, because the glycosyl cation intermediate is stabilized by charge distribution between C-1 and the ring-oxygen atom. The unfavorable formation of an ion pair on bond cleavage can be avoided by protonation of the glycosidic oxygen atom and thus the requirement for acid catalysis (see Scheme 1). 

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