The molecular conformations of the stereoisomers of 4-methyl-5,6-benzo-2-oxo-1,3,2-dioxathiepin and of the 4,7-dimethyl derivative

Abstract

The conformational and dynamic properties of the stereoisomers of 4-methyl-5,6-benzo-2-oxo-1,3,2-dioxathiepin (5) and of the 4,7-dimethyl derivative (6) have been investigated by 1H and 13C dnmr methods, as well as by ir spectroscopy. Analysis of the nmr spectra at low temperature (−125 °C), together with the conformational interpretation of the stretching vibration bands of the S = O bond in the ir spectra, indicate that the most stable conformations detected for solutions in CHF2Cl are C-a (82%) and TB-t-Mec (18%) for 5a, TB-c-Mec, (96%) and TB-c-Mea, (4%) for 5b, C-a for 6a, TB-t-Mec, c-Mec, for 6b and C-e for 6c. The substituent effects of a methyl group at the benzylic position on both the conformational and dynamic properties of the seven-membered sulfite are discussed.