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 4J couplings. It was found that only 4J 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 γ‐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.

The pKa values of conformationally fixed 1,3‐dithianes were investigated with experimental and theoretical methods to gain an insight into the influence of the relative orientation of the acidic proton and the stabilising S=O or S–C bonds. Experimental values were determined with a modification of the spectrophotometric method using overlapping indicators developed by Bordwell; theoretical values were obtained with the proton exchange methods using DFT calculations. It turned out that stereoelectronic stabilisation is important for sulfides and sulfoxides, whereas charge effects become dominant in sulfones. Deprotonation of the equatorial proton is favoured for most of the compounds except for the equatorial/axial bis‐sulfoxide.

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H and 13 C NMR Spectra 1) 4,4,5,5-Tetramethyl-2-phenyl-1,3-dioxolane (2)2) 2-(3-Chloropropyl)-4,4,5,5-tetramethyl-1,3-dioxolane ( 17)3) 2-(2-Bromoethyl)-4,4,5,5-tetramethyl-1,3-dioxolane (18) 4) 4-Iodo-5-methyl-5- ((4,4,5,5- Experimental ProceduresGeneral. Flash column chromatography 1 was carried out using Merck SiO 2 60 (230-400 mesh), and thin layer chromatography (TLC) was carried out using commercially available Merck F 254 precoated sheets. 1 H and 13 C NMR spectra were recorded on a Bruker AVANCE 300 or on an AM-400 spectrometer. Chemical shifts are given in ppm and were referenced using residual signals of the solvent as internal standard ( 1 H: CHCl 3 , 7.26; 13 C: CDCl 3 , 77.16). IR spectra were recorded on a Bruker IFS-88 spectrometer. Mass spectra were recorded on a Finnigan MAT-90 mass spectrometer.General procedure (GP). Pinacol (6 eq.) is placed in a flask, evacuated and flushed with Ar (4×). TFA (5 mL / 6 mmol pinacol) is added and the solution is stirred until pinacol is completely dissolved. The solution is cooled (0 °C) and the glycol acetal (1 eq.) is added. After stirring at 0 °C for 30 min, the mixture is carefully dissolved in an excess of a satd. aq.NaHCO 3 soln and extracted with EtOAc (3×50 mL). The combined organic layers are washed with brine (3×50 mL), dried (Na 2 SO 4 ) and concentrated. The crude product is purified by flash column chromatography (silica gel). 4,4,5,5-Tetramethyl-2-phenyl-1,3-dioxolane (2). 2 2-Phenyl-1,3-dioxolane (1) and pinacol were reacted according to the GP. Chromatography (hexanes/EtOAc/Et 3 N, 250:10:1) afforded 2 (286 mg, 68%) as a colorless oil. R f = 0.75 (hexanes/EtOAc, 3:1). IR (ATR): ν ~ (cm −1 )