Chapter 24, Problem 40P

Interpretation Introduction

Interpretation:

The mechanism is to be written for the conversion of pentenyl glycosides to glycosyl bromides

with bromine. The reason for the alpha anomer is formed selectively is to be explained.

Concept introduction:

The alkene, on reaction with bromine in the presence of dichloromethane, undergoes addition of bromine through formation of cyclic bromonium ion.

In pyranose, the carbocation formed at anomeric carbon stabilized by delocalization of pi electrons on oxygen is called aoxygen-stabilized carbocation.

The gauche conformation for structural units of the type $\text{X}-\text{C}-\text{Y}-\text{R}$ is more stable than its anti-conformation due to anomeric effect. The $\text{X}$ and $\text{Y}$ are electronegative atoms. $\text{Y}$ must have at least one unshared lone pair.

When the electronegative substituent at the anomeric carbon is axial, the lone pair on oxygen is stabilized by delocalization.