The reason for the formation of both glycosides is shown in Scheme 3. The protonation of the anomeric carbon –OH group followed by elimination of water gives a planar sp2 hybridized oxocarbenium ion. This can react with alcohol from both faces to give the β -glycoside and the α-glycoside.
Scheme 3
11.7 Anomeric Effect
When a pyranose or furanose ring closes, the hemiacetal carbon atom is converted from a flat carbonyl group to an asymmetric carbon. Depending on which face of the (protonated) carbonyl group is attacked, the hemiacetal –OH group can be directed either up or down. These two orientations of the hemiacetal –OH group give diastereomeric products called anomers, and the hemiacetal or acetal carbon atom is called the anomeric carbon atom. The preference of certain substituents bonded to the anomeric carbon for the axial position is called the anomeric effect. Ano is Greek for “upper”; thus, anomers differ in configuration at the upper-most asymmetric carbon. The anomeric carbon is the only carbon in the molecule that is bonded to two oxygen atoms. The anomer with the anomeric –OH group down (axial) is called the α -anomer, and the one with the anomeric –OH group up (equatorial) is called the β -anomer (Scheme 4).
Scheme 4
In fructose, the α -anomer has the anomeric - OH group down, trans to the terminal –CH2OH group, while the β -anomer has it up, cis to the terminal –CH2OH group (Scheme 5).
Scheme 5