Radical Cyclizations of Cyclic Ene Sulfonamides Occur with β‑Elimination of Sulfonyl Radicals to Form Polycyclic Imines

Radical cyclizations of cyclic ene
sulfonamides provide stable
bicyclic and tricyclic aldimines and ketimines in good yields. Depending
on the structure of the precursor, the cyclizations occur to provide
fused and spirocyclic imines with five-, six-, and seven-membered
rings. The initial radical cyclization produces an α-sulfonamidoyl
radical that undergoes elimination to form the imine and a phenylsulfonyl
radical. In a related method, 3,4-dihydroquinolines can also be produced
by radical translocation reactions of N-(2-iodophenylsulfonyl)­tetrahydroiso-quinolines.
In either case, very stable sulfonamides are cleaved to form imines
(rather than amines) under mild reductive conditions.