17O, 13C, and 1H NMR Studies of p–π Conjugation in 2-Substituted 4-Methylene-1,3-dioxolanes and 4-Methyl-1,3-dioxoles

Abstract

The 17O NMR spectra of a number of unsaturated 5-membered cyclic acetals, 2-substituted 4-methylene-1,3-dioxolanes and their endocyclic isomers, 4-methyl-1,3-dioxoles, have been recorded. The 17O NMR chemical shifts, in comparison with those of similarly 2-substituted 1,3-dioxolanes, were used to explore the variation of the strength of p–π conjugation in the unsaturated acetals as a function of the nature of substitution at C2. The 17O NMR shift data reveal that alkoxy substituents have a significantly more favorable effect on the strength of p–π conjugation in 4-methyl-1,3-dioxoles than in 4-methylene-1,3-dioxolanes. This fact appears to be responsible for the previously observed unexpectedly large effect of alkoxy substitution on the relative thermodynamic stabilities of these two classes of isomeric compounds. Additional information of the unexpected charge distribution in 4-methyl-1,3-dioxoles is provided by their 1H and 13C NMR spectra.