Reactions with Hydroxy Compounds O-Glycosides

A highly stereospecific access to glycosides is possible by C-1 bromination of acetylated sugars. 

In the reaction of peracetylated sugar with HBr, due to the strong anomeric effect, a-halogenide is formed almost exclusively (cf. Formula 4.103). 

This then reacts, probably through its glycosyl cation form. Due to the steric influence of the acetylated group on C-2, the 1,2-trans-glycoside is preferentially obtained, e. g., in the case of D-glucose, P-glucoside results. 

The hydrolysis rate is dependent on the aglycone and the monosaccharide itself. The most favored form of alkylglycoside, a-pyranoside, usually is the isomer most resistant to hydrolysis. This is also true for arylglycosides, however, due to steric effects, the P-pyranoside isomer is synthesized preferentially and so the P-isomer better resists hydrolysis. 

The influence of the sugar moiety on the rate of hydrolysis is related to the conformational stability. Glucosides with high conformational stability are hydrolyzed more slowly (cf. data compiled in Table 4.17). 

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