A large-scale process for the preparation of tert-butyl 4-oxoazepane-1-carboxylate (1), a useful intermediate for many pharmaceutical products under development, is described. This process is based on the ring expansion of tert-butyl piperid-4-one-1-carboxylate using ethyl diazoacetate. The entire process can be operated safely, less expensively, and conveniently without the necessary purification for any of the intermediates. The production scale of over 33 kg of the final product was achieved with good overall yield. The main focuses of optimization were the large-scale preparation of ethyl diazoacetate and the ring expansion at low temperature.