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The radical reductions and allylations of a series of a-halo-P-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 1OO:l are obtained. The reactions require initiation with EbB 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.OEt2 which is apparently required in these reactions. Competition experiments provide a more detailed explanation of substrate reactivity. Traditionally radicals derived from acyclic precursors have seldom been considered as intermediates for the generation of asymmetric centers. Recently however, it has been discovered that radicals can indeed react with high levels of diastereoselectivity (1). This has been achieved mainly through the use of chiral auxiliaries (2) or by the influence of a stereogenic center adjacent to a carbonyl (1,2-asymmetric induction) (3,4, 5). The scope of these reactions has also been expanded by the use of Lewis acids (6, 7), solvent complexation (8) and intramolecular hydrogen bonding (9) to enhance and even reverse the facial bias. In this paper, we describe results we have obtained in chelation-controlled reductions, allylations and atom transfer reactions of a-halo-P-alkoxy esters, and present evidence for radical-based processes. Preliminary experiments designed to elucidate further mechanistic details will also be discussed. Scheme 1 X Ph -. *Et

Lewis acids in diastereoselective processes involving acyclic radicals