Free Radical Addition to Enoates

Free radicals can be generated in different ways [93]. The standard procedure is to treat a bromide or iodide with tributylstannane and a radical starter such as AIBN [93ej. Another possibility is to use an excess of samarium diiodide with HMPA as an additive. The difference lies in the enormous affinity of samarium to 

It is obvious that the tin-mediated reaction shows low stereocontrol independent of the substrate. This is quite different for the samarium-induced reaction. Here, the donicity of the oxygens in the substrate 276, 279, 282 and hence the tendency toward complex formation are important Consequently, free OH and acetate have high donicity they may form samarium-chelate species such as 285 and 286, both of which give cis-cychzation products such as 277 and 283. If complexation with the oxygen is stericaUy hindered as in acetal 279 the stereoselectivity is gone. 

---