Higher selectivity at higher temperatures! Effect of precursor stereochemistry on diastereoselectivity in radical allylations. Insight into the role of the Lewis acid

A derailed investigation of the effects of Lewis acid, temperature, and trapping efficiency of functionalized allylstannanes on diastereoselective radical allylation reactions of alpha-bromoxazolidinone imides 1 and 2 was conducted. Results indicate that despite the addition of Lewis acids, a bidentate chelated radical intermediate 21 may be accessible from only one diastereomer of starting material due to steric interactions in 20 that are not present in chelated intermediate 17. It is shown that application of the appropriate Lewis acid, increasing temperatures, and slower allylstannane traps all facilitate formation of 21. Thus highly stereoselective radical allylations (>50:1, Tables 2 and 3) can be performed at room temperature as well as low temperatures.