13C nuclear magnetic resonance studies of saturated heterocycles: II—substituent effects on the 13C chemical shifts of methyl substituted 1,3‐dithianes and their application to the determination of conformational equilibria

Abstract

AbstractThe 13C chemical shifts for 1,3‐dithiane and 9 methyl substituted derivatives are reported. Only three of the methyl‐1,3‐dithianes were conformationally anancomeric and hence the conformational equilibria must be taken into account when deriving the values of the different substituent effects on the 13C chemical shifts. The best fit for each substituent effect was obtained when –ΔGθ (5a‐Me) for 5‐methyl‐1,3‐dithiane was given the value 3.8 ± 0.3 kJ mol−1 and when the difference between –ΔGθ (2a‐Me) and –ΔGθ (5a‐Me) for cis‐2,5‐dimethyl 1,3‐dithiane equalled 3.4 ± 0.4 kJ mol−1. The conformer populations chosen from our earlier paper1 were then suitable for all the other conformational equilibria in question. The magnitude of the derived substituent effects are compared with those for cyclohexane and 1,3‐dioxane.