Free-radical cyclizations

Abstract

INTRODUCTION Interest in free radical cyclizations has increased enormously in the last few years and this Symposium gives a very good opportunity to review this rapidly developing field. An exhaustive review will, however, greatly exceed the time available and I should like to make it quite clear that the selection which had to be made has been directed partly by the need to illustrate general points but also by our own particular interests. It seems convenient to divide this talk into two parts covering two of the main types of reactions which free radicals undergo to generate new bonds: (i) Addition of free radicals to unsaturated bonds. We shall consider here only double bonds although triple bonds also undergo cyclization reactions. (ii) Aromatic substitution reactions. We shall restrict ourselves to the formation of new carbon—carbon bonds. We shall not consider here the formation of cyclopropane rings which could be envisioned either as microcyclizations or homoallylic participations. We will have to omit also the very interesting cyclizations by radical coupling reactions, as well as a few recent very elegant cyclizations of radical anions.