Theoretical study of stereochemistry of methoxy(methylthio)methane as a model of thioacetal segment in thiosaccharides

The stereochemistry of methoxy(methylthio)methane - a model of the thioacetal segment in thiosaccharides - has been studied by semiempirical quantum -chemical methods. The conformational map of the rotation around the central C-O and C-S bonds has been calculated by the PCILO method. Decomposition of the energy for the rotation around the C-O (C-S)bond (the methyl group being fixed in the antiperiplanar position) in terms of the Fourier series gives the values 7.12 (2.53) kJ mol-1 for the respective rotational barrier. The geometry of five stable conformers, whose stability decreases in the order (sc,sc) > (ap,sc) > (-ac,sc) > (ap,ap), has been optimized by the MNDO method. Ratio of the individual conformers in an isolated molecule calculated on the basis of the MNDO energies is 92 : 5 : 2 : 1 : 0, the mean dipole moment being 3.6 . 10-30 Cm. The solvent effect has been calculated by a method involving electrostatic and dispersion interactions and cavity contribution. Population of the conformers in diluted solutions depends on nature of the solvent, proportion of the antiperiplanar positions being increased with increasing polarity. The calculated ratios of the conformers are 85 : 8 : 3 : 4 : 0 and 63 : 14 : 7 : 16 : 0 in acetonitrile and water, respectively. The calculated values of the Gibbs energies have also been used for assessment of magnitude of anomeric and exo-anomeric effects in α- and β-1-thiopyranosides and α- and β-5-thiopyranosides.