Conformational analysis: XIV—A ¹H n.m.r. conformational study of methyl substituted 2‐oxo‐1,3,2‐dioxathians to confirm the predominance of chair forms in the trimethylene sulphite series

Seven isomeric 4,5,6‐trimethyl‐2‐oxo‐1,3,2‐dioxathians, cis ‐4‐ trans ‐6‐dimethyl‐ r ‐2‐oxo‐1,3,2‐dioxathian and two isomeric 4,5,5,6‐tetramethyl‐2‐oxo‐1,3,2‐dioxathians were prepared and their 1 H n.m.r. spectra analysed. The values of the vicinal coupling constants reported earlier for the cis ‐4‐ trans ‐6 compound were shown to be erroneous. In all cases the values of the vicinal coupling constants (and those of the chemical shifts) are indicative of a single chair conformation or a chair‐chair equilibrium, in contrast to earlier reports on the significant contribution of twist forms but in agreement with an electron diffraction study. The chair form is ≳ 31 kJ mol −1 thermochemically more stable than the twist form.