Thiane oxides conformational analysis

Thiadiazoline S-oxides, synthesis of 275 Thiane dioxides formula of 382 synthesis of 468 Thiane oxides 750-752 conformational analysis of 465-467 formula of 382... 

Other studies have also established the preference of the chair conformation with the oxygen in the axial position the rationale for this preference is different from the attractive interaction between the sulfoxide oxygen and the syn-axial hydrogens proposed previously . Rather, a repulsion effect is advocated the equatorial oxygen is squeezed between four vicinal hydrogens, while there are only two corresponding repulsions if it is in the axial position. The correlation between the predicted and observed conformational/orientational preferences in 3,3-dimethylthiane oxide (e.g., equatorial preference in the chair conformation) corroborates this interpretation. The axial preferences of the sulfur-oxygen bond in the thiane oxide is reversed in 3,3-dimethylthiane oxide because of the syn-axial interaction. 4,4-Dimethylthiane oxide, however, maintains a predominance of the axial isomers as deduced from the analysis of NMR data . ... 

Much of the sustained interest in the conformational analysis of sulfoxides was initiated by the discovery of the axial preference of the sulfinyl oxygen in thiane-1-oxide. Indeed, although Henbest and Khan [12] reported that the thermally-induced equilibrium of 4-/-butylthiane-l-oxides (1) afforded a cis.trans ratio of 20 80, a reexamination by Johnson and McCants established that the isomer bearing the axial oxygen is more stable [13] (Scheme 3.2). 

More recently, Abraham and coworkers [18] reexamined the conformational behaviour of thiane-l-oxide (3) by analysis of lanthanide-induced shifts (LIS) in the appropriate NMR spectra. Their results compare reasonably well with those obtained by Barbarella et al. [17], suggesting the applicability of the LIS method to sulfoxide conformational analysis. 

More recently, Nagao and colleagues [40] described the conformational analysis of cis- and rrans-4-benzyloxy- and 4-siloxy-substituted thiane-1-oxides (cis-(12) and trans- 12)) at -80°C in CD2CI2. The results are collected in Table 3.2, and show that in the cases of c/s-4-RO-thiane-l-oxide (Scheme 3.23A), their RO-ax conformer preferences are 45% (12a), 66% (12b), and 67% (12c), respectively. In the cases of trans-4-RO-thiane-l-oxide (Scheme 3.23B), the preferences of RO for the axial position are higher. 

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