Non-chair six-membered-ring conformations. Preference for a twist-boat (or skew) structure in α--sorbopyranose derivatives

The conformational preferences for 2,3-O-isopropylidene-α--sorbopyranose derivatives 3–6 were determined by using ¹H NMR data and empirical force field calculations. Proton NMR studies of 3–6 indicate that a twist-boat (or skew) conformation (³S₀) prevails over possible chair forms for each compound. Force-field calculations (MM2, MNDO, AM1) on a model 2,3-O-isopropylidene-α--sorbopyranose system (18) indicate that the ³S₀ conformation is among the low-energy structures. X-Ray crystallographic analysis of α--sorbopyranose sulfamate 3, a compound with potent anticonvulsant activity, demonstrates that the ³S₀ skew conformation is manifested in the solid state, as well.