STEREOCHEMICAL DICHOTOMY IN THE STEVENS REARRANGEMENT OF AXIALLY TWISTED DIHYDROAZEPINIUM AND DIHYDROTHIEPINIUM SALTS - A NOVEL ENANTIOSELECTIVE SYNTHESIS OF PENTAHELICENE

Evidence is presented indicating that the stereochemistry of the Steve ns rearrangement of the axially chiral onium salts 1a-d and 5 is drama tically structure-dependent. Thus, the binaphthyl ammonium salts (S)-( +)-1a-c react with a strong base with exclusive (100% de) formation of the corresponding rearranged amines (R,3R)-(+)-2a-c, demonstrating a complete transfer of the (S) axial dissymmetry/asymmetry into (R) asym metry of the newly formed carbon center. Exactly opposite stereochemis try was established in an earlier study by Mislow of the biphenyl anal ogue (S)-(+)-5, yielding the rearranged products (S,9S)-(+)-6 and (R,9 S)-(-)-7 with exclusive (S) configuration at the carbon center. Rearra ngement of the sulfonium salt 1d is found to be intermediate between t he two extremes, yielding a mixture of diastereoisomeric products 2d a nd 3d, which differ in configuration at the asymmetric carbon center. A direct proof is thus provided that two stereochemically different pa thways can participate in the Stevens rearrangement. An explanation is suggested in terms of competition between suprafacial (concerted) and antarafacial (nonconcerted) mechanism.