Reported herein is a general platform of a stereodefined access to gamma-lactams via Cp*Ir-catalyzed olefin difunctionalization, where in situ generated Ir-nitrenoid is utilized as a key motif of 1,3-dipoles to enable amido transfer in a syn-selective manner. Computational studies suggested that the stereodefined process can be attributed to the proposed working mode of concerted [3 + 2] cyclization. Frontier molecular orbital (FMO) analysis implied that a low-lying lowest unoccupied molecular orbital (LUMO) of the Ir-imido fragment engages in the olefin interaction. Mechanistic understanding on the nitrene transfer process led us to develop mild catalytic protocols of stereoselective difunctionalization of alkenyl dioxazolones to furnish alpha-(haloalkyl)- or (oxyalkyl)lactam products which are of high synthetic and medicinal utility. Product stereochemistry (threo and erythro) was found to be designated by the olefin geometry (E/Z) of substrates.