Sugars anhydride formation

Pursuing this idea further, it is seen that anhydride formation in the sugars resolves itself into an exchange of anions on a carbonium cation,... 

The formation of anhydro sugars must involve attack by the internal hydroxyl group, instead of generation of a carbonium ion, as the rate at which they are formed is very much greater than the rate of elimination of halide either in fully protected halo sugars or in those in which anhydride formation is not sterically possible. 

The 1C conformation (51) of most of the D-hexoses is highly unfavored, because of the large, axial, hydroxymethyl substituent at C-5. Stabilization of the structure of D-idose is effected by the formation of the 1,6-anhydro derivative (52), which is formed in about 80% yield by thermodynamic equilibration in aqueous acid solution, the highest yield of 1,6-anhydride for any of the hexoses. It is obvious that, in 6-deoxy-(n or L)-idose, no such anhydride formation can occur and, since the 5-(7-methyl group requires somewhat less space than the 5-(hydroxymethyl) group of the hexose, a considerable proportion of the 1C conformation of 6-deoxy-D-idose may be present in this sugar. 

Acetal condensations are generally, and most conveniently, conducted at room temperature. Higher temperatures may lead, especially with reducing sugars and their derivatives, to undesirable side-reactions, such as inversion, anhydride formation, and hydrolysis of acid-labile substituents. 

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