Lewis acids in diastereoselective processes involving acyclic radicals

The radical reductions and allylations of a series of alpha-halo-beta-alkoxy esters under bidentate chelation-controlled conditions are reported and compared with the analogous reactions under non-chelating conditions. The addition of Lewis acids is shown to give excellent selectivity for the syn products in the case of reduction, and the anti products in the case of allylation. In some cases, ratios greater than 100:1 are obtained. The reactions require initiation with Et(3)B and can be inhibited by m- and p-dinitrobenzene, which imply a radical-based mechanism. Iodides, bromides and phenyl selenides are all suitable substrates. Investigations also provide a rationale for the large excess of MgBr2 . OEt(2) which is apparently required in these reactions. Competition experiments provide a more detailed explanation of substrate reactivity.