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.