The asymmetric dirhodium(II)-catalyzed intramolecular C À H insertion of a-diazo acetamides in water is described for the first time. The use of natural a-amino acids as chiral ligands allowed the preparation of novel dirhodium(II) homochiral complexes by a simple procedure consisting of the in situ ligand exchange starting from dirhodium tetraacetate. The catalytic system was further reused up to 7 cycles and b-lactams were obtained in good yields and enantiomeric excess.