THE EXOCYCLIC EFFECT - PROTECTING GROUP STRATEGY TO ENHANCE STEREOSELECTIVITY IN HYDROGEN-TRANSFER REACTIONS OF ACYCLIC FREE-RADICALS

To enhance the diastereoselectivity of the hydrogen transfer reaction of acyclic substrates bearing 1,2- or 1,3-diols, the feasibility of a strategy employing bifunctional protecting groups has been demonstrate d. This strategy is based upon the ''exocyclic effect'' or the signifi cant improvement of anti-selectivity exhibited by the reductions of su bstrates in which the two substituents (R-1 and Y) at the stereogenic center alpha to the radical center are linked together. A rationale fo r the excellent facial discrimination of these exocyclic radicals is o ffered based on an analysis of transition state models, which consider s both steric and electronic factors.