Conformationally Constrained Oxides of 1,3-Dithiane: Synthesis and NMR Spectroscopic Investigations

Conformationally constrained derivatives of 1,3-dithianes (5-tert-butyl- and 4,6-dimethyl-1,3-dithiane and dithiadecalin) have been oxidised with various oxidants to yield axial and equatorial sulfoxides, disulfoxides and sulfones. Axial sulfoxides were prepared by nucleophilic hydroxide addition to the corresponding 2-alkylidene derivatives with subsequent retro-aldol-type elimination. The reaction outcome is related to the relative energies of the derivatives, as demontstrated by DFT calculations. The NMR spectroscopic data of the compounds obtained were analysed to identify (4)J couplings. It was found that only (4)J W couplings can be observed regardless of whether there is an axial or an equatorial sulfoxide, sulfide or sulfone group present in between the respective C-H moieties. A previously postulated gamma-gauche effect of axial sulfoxides (but not of equatorial sulfoxides or sulfones) leading to the shielding of carbon atoms is not unambiguously supported by the NMR spectroscopic data of conformationally fixed derivatives.