NMR Experiments on acetals 34: The conformational equilibrium in trans‐6‐alkyl‐4‐methyl‐ and trans‐6‐alkyl‐4‐phenyl‐1,3‐dioxanes. The conformational free energy of the 4‐methyl and 4‐phenyl groups in 1,3‐dioxanes

The conformational chair-chair equilibrium in trans-6-alkyl-4-methyl-, and trans-6-alkyl-4-phenyl-1,3-dioxanes has been determined by NMR spectroscopy. The equilibrium constant was calculated with the aid of coupling constants, and involved the use of model compounds. The experimental order of conformational preferences of the alkyl groups is i-Pr > Et > Pr ≈ i-Bu ≈ neoPe ≈ Me, in agreement with theory. Base catalyzed epimerization of cis-, and trans-6-methyl-4-phenyl-1,3-dioxane, together with the conformational equilibrium in the trans isomer, investigated here allows the evaluation of ΔG°303 values of 4-Me and 4-Ph: 2950 and 2850 cal./mole, respectively.