Stereochemistry and Mechanism of the Ring-Opening Reaction of Cyclopropylenones with LiCu(Me)2

The chemical shifts of the diastereotopic hydrogens of the ethyl group of the cyclopropane ring-opening product of conjugate addition of LiCu(CH3)2 to cyclopropyl enone 1 were computed. Comparison of computed and observed 1H NMR chemical shifts of the diastereotopic hydrogens of the ethyl group of 2 established the stereochemistry of the ring-opening product as 2d-b. This provides evidence that the reaction proceeds by conjugate addition of the cuprate to the enone, followed by ring-opening of the cyclopropylmethyl copper species and reductive elimination.