Conformational analysis of cyclic sulphites. 2-Oxo 1,3,2-dioxathiane-4-spiro-4-tert-butyl-cyclohexanes

Three series of 4-spirosulphites, a new class of cyclic sulphites, were synthesized and five couples of diastereoisomers isolated. Their structural analysis, using ¹H NMR coupling constants and SO stretching vibration as conformational probes, shows a large variety of ananchomeric chair forms and multicomponent equilibria for the cyclic sulphite moiety, the cyclohexane part of the molecule remaining in the chair form. Related to the occurrence of severe interactions involving the 5-methyl or 6-tert. butyl substituent, several twist forms were encountered, with 2,5-axis and isoclinal SO or 1,4-axis and axial or equatorial SO as a consequence of the weak free energy difference between chair and twist conformations in the cyclic sulphite series.