NMR experiments on acetals—XXV: Conformational energy for the chair-twist interconversion of the 1,3-dioxane system

Trans-4-t-Bu-6-R-1,3-dioxanes (R = Me, Prⁱ and cyclohexyl) show temperature-dependent values for ²J(H—2) and ³J(H—4(6), H—5) in their PMR spectra. This is the result of the presence of twist-boat conformations. With the aid of typical limit-values of ²J(H—2) for the chair and twist forms, the amount of flexible conformations were determined as a function of temperature (Table 2), allowing the determination of the enthalpy change for chair-twist interconversion in 1,3-dioxane itself (6·2 ± 0·3 kcal/mole). Typical values for ²J(H—2) were obtained from a study of low temperature spectra and from appropriate model compounds of which 4-(1′-adamantyl)-6-t-Bu-1,3-dioxane served as the model for a genuine twist form with a twofold axis through C-2/C-5.