Indium-mediated regio- and diastereoselective reduction of norbornyl α-diketones

A novel, efficient, and regio-as well as diastereoselective conversion of non-enolizable bicyclic α-diketones into synthetically useful acyloins mediated by indium metal is described. The reduction is highly diastereoselective, leading exclusively to endo-acyloins, (endo-hydroxyl groups) in excellent yields, and tolerates a variety of sensitive substituents, such as acetate, ester, and bridgehead halogens. The regioselectivity in the reductions of monosubstituted α-diketones varied from 70:30 to 100:0 for the two possible isomeric alcohols. The methodology is extended to the synthesis of highly functionalized cyclopentane carboxaldehydes, potential building blocks in organic syntheses, by cleavage of the acyloins by treating them with Pb(OAc)₄ in MeOH/PhH. Allylindium additions to carboxaldehydes 22 have been found to be highly diastereoselective.