Figure 1.

(a) Idealized energy profile for the S_N1 mechanism without considering proton transfer reactions. The two-conformer hypothesis of per-O-methyl glycosyl cations is shown. The intermediate B0 geometry is most similar to that of the starting glycoside. The 2D projection shows the interconversion of selected C (chair), B (boat), H (half-chair), and S (skew-boat) conformations. (b) Main conformations and Gibbs free energies of mannopyranosyl cations (⁴H₃, ³E, and B_2,5). (c) Single point calculations upon rotation around the H2–C2–O2–C_Ac torsion. (d) Optimized ⁴H₃ and ⁵S₁ conformations and ΔG differences (kcal/mol) for 2-O-Me and 2-O-isopropyl Glcp. Calculations are carried out at the PBE0-D3/def2-TZVP level (CH₂Cl₂). Configurations are defined by a three-letter code (Gal, Glc, or Man). Ring size is denoted by p (pyranose).