When a pyranose is in the chair conformation in which the group and the C-1 OH group

Question:

When a pyranose is in the chair conformation in which the group and the C-1 OH group are both in the axial position, the two groups can react to form an acetal. This is called the anhydro form of the sugar. (It has lost water.) The anhydro form of D-idose is shown here. In an aqueous solution at 100 °C, about 80% of D-idose exists in the anhydro form. Under the same conditions, only about 0.1% of D-glucose exists in the anhydro form. Explain.