Ring-Strain Effects in Sommelet–Hauser Rearrangement of Five- and Six-Membered Fused Bicyclic Ammonium Ylides

The base-induced Sommelet–Hauser (S–H) rearrangement of N-benzylic 5,5- and 6,5-fused bicyclic ammonium salts proceeded over the [1,2] Stevens rearrangement. The ring-strain resulting from the fused bicyclic ammonium ylide generated destabilized the enolate form. The contribution of the carbanion form increased. Therefore, the initial dearomative [2,3] sigmatropic reaction proceeded smoothly to give the S–H rearrangement products. The products, α-arylamino acid amide derivatives, were obtained in the pure enantio- and diastereomerically forms via N-to-C chirality transmission.