Steric Effects of N-Alkyl Group on the Base-Induced Nitrogen to Carbon Rearrangement of Orthogonally Protected N-Alkyl Arylsulphonamides

The rearrangement of a total of 56 members of 22 series of orthogonally protected <i>N</i>-alkyl arylsulphonamides of general structure 4-XC₆H₄SO₂NR¹CO₂R² [X = H, CH₃, F, Cl, Br, CH₃O, CN, CF₃ or C(CH₃)₃; R¹ = CH₃, CH₂CH₃, CH₂CH₂CH₃, CH(CH₃)₂ or CH₂CH(CH₃)₂; R² = CH₃, C₂H₅ or C(CH₃)₃] when treated with lithium di-isopropylamide in tetrahydrofuran at -78 °C has been studied. The competition between directed ortho metalated rearrangement, to form 4-X-2-(R²O₂C)C₆H₃SO₂NHR¹ and the production of a substituted saccharin, is strongly influenced by the size of R¹ and R², especially in the series with X = CH₃. When R¹ = CH₃ or to a lesser degree, C₂H₅, formation of the saccharin competes to a significant extent, especially when the migrating group is CO₂CH₃ or CO₂C₂H₅. In contrast, when R¹ is a larger alkyl group, particularly if it is branched at either the α- or β-carbon atom [CH(CH₃)₂ or CH₂CH(CH₃)₂], the increased steric hindrance essentially prevents cyclisation, thus facilitating rearrangement to 4-X-2-(R²O₂C)C₆H₃SO₂NHR¹ in high yield. The size of the migrating CO₂R² group also exerts an effect on the competition between the reactions: when R² = C(CH₃)₃, clean rearrangement is possible even when R¹ = CH₃ in each series of X. These results have implications for further elaboration and rearrangement of 4-X-2-(R²O₂C)C₆H₃SO₂NHR¹ in order to prepare substituted saccharins containing a 6-CO₂R³ group.