Preparation of beta-lactones by 4-exo-trig radical cyclization

The inherent instability of the intermediate radicals in four-membered rings means the construction of these rings by radical cyclization is problematic. We recently reported the first preparation of b-lactones by 4-exo-trig radical cyclization. The photochemical cyclization was successfully achieved via a a-ethenoxy radical, generated from a N-hydroxypyridine thione ester (1), in the presence of tributylstannane. Preliminary investigations of this reaction indicate that the highest yields are obtained when groups capable of stabilizing a carbon-centred radical are present at the 3-position of the alkenoate acceptor.

The scope of this reaction is currently being extended with functionalization of X and Y with dithiane and by employing allyl tributyl tin as a radical acceptor. Thus, preparation of these functionalized b-lactones will permit further synthetic transformations on the cyclized product hence providing an entry to bioactive b-lactones.