Chapter 9, Problem 9.76P

Interpretation Introduction

Interpretation:

The complete, detailed mechanism of the given glycoside formation is to be drawn. Products for the glycoside formation are to be shown paying attention to stereochemistry.

Concept introduction:

A glycosidic linkage or bond is a type of covalent bond that joins two sugar molecules or one sugar molecule and one alcohol molecule. A glycoside is formed when $\text{OH}$ group of a sugar molecule has been replaced by $\text{OR}$ group. The alkoxy group can be placed at an axial position (below the plane; designated as alpha) or an equatorial position (above the plane; designated as beta). The glycosidic bond is formed between the hemiacetal or hemiketal group of a saccharide and the hydroxyl group of another saccharide molecule. An acetal group involving sugar is produced in the laboratory by treating the sugar with alcohol under acidic conditions. This is an example of a ${\text{S}}_{\text{N}}\text{1}$ reaction. ${\text{S}}_{\text{N}}\text{1}$ reaction is a unimolecular substitution nucleophilic reaction which consists of two steps. First step is the heterolysis step in which the bond between carbon and leaving group breaks and a carbocation intermediate is formed. If the substrate molecule does not possess a good leaving group, then it can be converted to a good leaving group by deprotonation in the presence of an acid. The next step is the substitution step in which the incoming nucleophile attacks the carbocation intermediate to form the substitution product. As the resulting carbocation intermediate has a planar geometry, a mixture of stereoisomers is produced.