Relative thermodynamic stabilities of isomeric dimethyl acetals derived from 2-acetyltetrahydrofuran. An experimental and computational study

In the presence of an acid catalyst, the dimethyl acetal of 2-acetyltetrahy drofuran (1) is converted into a mixture of three isomeric acetals composed of the reactant and two diastereomers of 2-methoxy-2-(1 ' -methoxyethyl)te trahydrofuran (2). The relative thermodynamic stabilities of these acetals have now been determined by chemical equilibration. The least stable isomer is 1, in the liquid phase 4-6 kJ mol(-1) less stable than the two diastere omers. The geometry-optimized structures and relative energies of the title compounds were also studied by theoretical calculations (ab initio and DFT ). Comparison of the theoretically determined relative stabilities of the d iastereomers with the corresponding experimental data suggests the more vol atile (and more stable) diastereomer to exist as a racemic mixture of the ( R,S) and (S,R) configurations.