A Modular and Scalable Route to Protected Cyclopropane Amino Acid Building Blocks

An improved method for the synthesis of noncanonical cyclopropane amino acids from common laboratory reagents is described, avoiding the use of neurotoxic oxidants or precious metal catalysts. Intramolecular isocyanate trapping via a Hofmann rearrangement permits the synthesis of bicyclic carbamates in an enantioenriched and diastereopure manner. Subsequent ring-opening of these species allows access to cyclopropane amino acids which can be further functionalized via oxidation and S_N2 pathways and incorporated into peptides via solid-phase peptide synthesis.