Diastereoselective conjugate additions to different α,β-unsaturated N-acyl oxazolidinones using various monoorganocuprate reagents, Li[RCuI], are described. The TMSI activated conjugate addition reactions provided high yields (80–98%) and reversed major diastereomers (70–96% de) compared to the conventional copper(I)-promoted additions of Grignardreagents or the addition of Li[RCuI] to precomplexed MgBr2/imides