ABSTRACT
The vinylic radical participates in a tandem cyclization either through a radical mechanism or through an anion mechanism (Scheme 23). The radical pathway seems the more likely, with reduction of the final bicyclic radical being facilitated by the electron-withdrawing group. The stereochemistry at this center is determined by preferred protonation from the least hindered side. Ketones and esters can both function as the activating group. Generally, formation of six-membered rings is less favored than that of five-membered rings, and if R3 =/- H the yields are also lowered [222]. More examples are given in Refs. 223 and 224.
