Planarity as a factor in determining the rate constant and mechanism of acetal hydrolysis

Investigations of the hydrolysis of methyl-α-D-altropyranoside (II), methyl-2,6-anhydro-α-D-altropyranoside (I), and methyl-α-D-glucopyranoside (III) have established that ground-state planarity of the C-2, C-1, O, C-5 region of the glycoside attained in (I) does not lower the energy barrier sufficiently to alter the mechanism from an A1 to an SE2 process (the increase in the rate constant associated with the A1 process for the 2,6-anhydroglycopyranoside is much smaller than that established for the 2-deoxyglycopyranosides).