A number of 5, 5‐disubstituted 1,3‐dioxanes (mostly methyl, alkyl) were prepared and their PMR spectra studied at various temperatures. Conformational equilibria and thermodynamic parameters were obtained (table I), using correct [7] shift data. The results obtained with data for H‐2 hydrogens are more accurate then those for H‐4,6 hydrogens. Some ambiguity however exists concerning equilibria for 5‐Me‐5‐n alkyl‐m dioxanes and different sets of values remain possible (table III). In general however it seems that the method of calculation as previously introduced [7] gives more then satisfactory results. Accuracy seems to be very high and without correction of the shift parameters for their temperature dependance, the linearity of the total free energy differences with temperature is often very bad. Even after correction it remains bad however for 5‐Me‐5‐r Bu‐1, 3‐dioxane. Some spectral features of the latter compound are discussed and it is shown that H‐4a (H‐6a) resonates at lower field then H‐4e (H‐6e).

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Barriers for ring inversion in some 5,5-gem. substituted m . dioxanes (dialkyl-and spiro derivatives) were measured and compared with some 2,2-gem. substituted derivatives. The results are compatible with a lowering of AG# when steric compression is higher in the ground state (table I). An exceptional low value is found in 5,5-di isopropyl-m-dioxane (ca. 8 kcal/mole), whereas normally the geminal 5substitution raises the inversion barrier. Some spectral features of 5,Spentamethylene-and 5,5-dimethyl-2,2-pentamethylene-m-dioxane are commented.

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Für die Dioxolane (I) und (II) wird gezeigt, daß das Epimerenverhältnis weitgehend von den 2‐ und 4‐Substituenten unabhängig ist und ca. 60/40 (cis/ trans) beträgt.