[1+1+1] Cyclotrimerization for the Synthesis of Cyclopropanes

The synthesis of small rings by functionalization of C(sp(3))−H bonds remains a great challenge. We report for the first time a copper‐catalyzed [1+1+1] cyclotrimerization of acetophenone derivatives under mild reaction conditions. The reaction has a broad scope for the stereoselective synthesis of cyclopropanes by trimerization of acetophenone. The developed transformation is based on an extraordinary copper‐catalyzed cascade process that allows saturated carbocycles to be obtained for the first time by cyclotrimerization through functionalization of C(sp(3))−H bonds. The cascade of sixfold C(sp(3))−H bond functionalization allows the synthesis of cyclopropanes in a highly stereoselective approach.