Conformationally constrained 2-methylidene 1,3-oxathiane S-oxides: synthesis and nucleophilic additions

The properties of vinyl sulfoxides are significantly influenced by stereoelectronic effects, where the relative orientation of sulfoxide group and alkene moiety is responsible for reactivity and selectivity, e.g., in the addition of nucleophiles. Conformationally constrained derivatives of 2-methylidene 1,3-oxathiane S-oxides allow the quantification of stereoelectronic effects. Suitable substrates were prepared by oxidation of 2-hydroxymethyl-1,3-oxathianes and pyrolysis of the respective xanthogenates. Nucleophilic additions of ethyl thiolate, piperidine, and dimethyl malonate anion are over 100 times faster to axial sulfoxides than to the respective equatorial substrates. The oxathiane derivatives turned out to be about 1000 times less reactive than the respective 1,3-dithianes.

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