CONFORMATIONAL STUDIES OF 7-MEMBERED CYCLIC SULFITES. 3-OXO-2,4,3-DIOXATHIEPIN AND 3-OXO-2,4,3-BENZODIOXATHIEPIN

Article abstract of DOI:10.1016/S0040-4020(01)97685-9

The conformational and dynamic properties of the 7-membered cyclic sulfites 1 and 2 were investigated by (1)H and (13)C DNMR as well as by IR spectroscopy.Compound 1 showed two spectral modifications at low temperature for both (1)H (100 MHz) and (13)C (22.6 MHz) spectra while for compound 2 a single (1)H spectral change has been observed and resolved at 300 MHz.Analysis of the low temperature NMR spectra together with the IR frequencies for the stretching vibration of the S=O bond indicate that 1 exists as a 1:1 mixture of C-a and TB conformations in CHF2Cl.Less polar solvents increase the proportion of the C-a form.In contrast, compound 2 is found to exist very predominantly as the TB form in CHF2Cl at -160 deg, whereas in the less polar CCl4 solvent a small amount of C-a form is found to be present at room temperature from the IR data.The conformational and dynamic properties determined for 1 and 2 are discussed in light of current theories relating to n -> ?^* stereoelectronic lone pair donation and dipole-dipole interactions.Furthermore the results for 1 and 2 are used as the basis for a new conformational interpretation of published IR data for dimethyl sulfite.Finally, the differences in properties of 1 and 2 are discussed.