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, Pr i and cyclo hexyl) show temperature‐dependent values for 2 J (H—2) and 3 J (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 2 J (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 2 J (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.