Transition-Metal-Free Cyclopropanation of 2‑Aminoacrylates with <i>N</i>‑Tosylhydrazones: A General Route to Cyclopropane α‑Amino Acid with Contiguous Quaternary Carbon Centers

Cyclopropanation of 2-aminoacrylates with N-tosylhydrazones could proceed smoothly under transition-metal-free conditions via a [3 + 2] cycloaddition process. This robust protocol exhibits excellent generality, delivering a wide spectrum of cyclopropane α-amino acid esters bearing contiguous quaternary carbon centers in high yields and diastereoselectivities. With these readily available products, the steric convergence of cyclopropane α-amino acids could be readily obtained.