Intermolecular Stereoselective Pummerer Reactions of 4-(p-Chlorophenyl)thiane 1-Oxides and trans-1-Thiadecalin 1-Oxides and 2-Thiadecalin 2-Oxides with Acetic Anhydride

The Pummerer reactions of conformationally fixed 4-(p-chlorophenyl)thiane 1-oxides and trans-1-thiadecalin 1-oxides and trans-2-thiadecalin 2-oxides with acetic anhydride are either stereoselective or stereospecific, both in the absence and in the presence of a scavenger of acetic acid formed, such as dicyclohexylcarbodiimide (DCC) or 2,6-lutidine. However, the 18O-tracer experiments with the 18O-labeled sulfoxide revealed the reaction to be an intermolecular rearrangement, while the kinetic experiment with α-deuterated 4-(p-chlorophenyl)thiane 1-oxides gave sizable values of kinetic isotope effect, i.e., 2.8 for the cis isomer and 3.4 for the trans isomer. In the reaction of 4-arylthiane 1-oxides with acetic anhydride, the thermodynamically controlled product is axial 2-acetoxy-4-(p-chlorophenyl)thiane while the kinetically controlled product is the equatorial isomer which is formed in the presence of the acid scavenger, DCC. In the Pummerer reaction of the thiadecalins S-oxides the equatorial α-acetoxy sulfides are the preferential products, however, the isomer preference is more pronounced in the reaction with DCC. These observations, along with other pertinent data seem to suggest that the rate-determining step in the Pummerer reaction of these simple heterocyclic sulfoxides is E2 elimination of acetic acid from the acetoxysulfonium intermediates.