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 repo rted and compared with the analogous reactions under non-chelating con ditions. The addition of Lewis acids is shown to give excellent select ivity for the syn products in the case of reduction, and the anti prod ucts 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 m echanism. Iodides, bromides and phenyl selenides are all suitable subs trates. Investigations also provide a rationale for the large excess o f MgBr2 . OEt(2) which is apparently required in these reactions. Comp etition experiments provide a more detailed explanation of substrate r eactivity.