A 35.12 g (80.46 mmol) portion of ethyl 3-[1-benzyloxy-3-(tert-butoxycarbonylamino)cyclobutan-3-yl]-4-nitrobutanoate was dissolved in 700 ml of ethanol to which, under an atmosphere of nitrogen, was subsequently added 50 ml of Raney nickel. After replacing the atmosphere with hydrogen, this was stirred at 50° C. for 5 hours. After cooling in an ice bath, the reaction solution was filtered through celite and then the solvent was evaporated under a reduced pressure. Thereafter, the resulting residue was purified by a silica gel column chromatography to give 20.53 g (70.8%) of the title compound.
