13C Chemical Shifts - Sensitive Detectors in Structure Determination. 1.13C NMR Studies of Saturated Heterocycles. 4. Methyl-Substituted 1,3-Dioxanes

The 13C chemical shifts for 1,3-dioxane and 50 methyl-substituted derivatives are reported. Substituent effects on them are derived and their magnitude shown to be closely related to the exact ring geometry and other conformational aspects. The results confirm the high chair-twist preference and support the view that 1,4-twist is appreciably more stable than 2,5-twist. Only 2,4-diaxially substituted compounds normally attain a twist form whereas 4,6-diaxial substitution alone is not enough but deform the chair form. A combination of the detailed knowledge on the stereochemistry of the 1,3-dioxane ring and on the type of significant shift parameters leads inevitably to the conclusion that a proper understanding of their interdependence offers a useful tool for conformational and configurational analysis.