Conformational preference of the s=O group.3. Continued evidence for a very strong s-s=o anomeric interaction from the nmr spectroscopic study of 44,5,5-tetramethyl-1,2-dithiane 1-oxide¹ 1 For Part 2,see: Juaristi, E.; Cruz-Sánchez, J.S. J. Org. Chem. in press.

The ¹H and ¹³C NMR behavior of the title compound (4) was studied in order to determine the conformational preference of the S=O group in this heterocycle. The NMR spectra were assigned by a combination of homo- and heteronuclear double irradiation, as well as nuclear Overhauser enhancement experiments. From the results of variable temperature, solvent effects and shift reagent experiments, it is concluded that the axial conformer dominates the equilibrium to such an extent that no contribution of the equatorial isomer is recorded. This result suggest that ΔG° (4-ax 4-eq)≥ 1.7 kcal mol, and because the axial conformer suffers from a syn-diaxial Me/S=O interaction, a ΔG° ≥ 3.0 kcal mol for the conformational equilibrium in the parent 1,2-dithiane mono-S-oxide (1) can be extrapolated. Some discussion of the possible mechanism responsible for such strong anomeric effect is presented.