To a solution of ((1R,3R)-1-amino-3-(4-(3-(benzyloxy)prop-1-ynyl)phenyl)cyclopentyl)methanol (25 g, 55.9 mmol) in toluene (200 mL) was added a solution of potassium hydroxide (12.54 g, 224 mmol) in water (40 mL) followed by a solution of 20% phosgene in toluene (41.5 g, 84 mmol). After about 4 hours EtOAc was added and the organic layer was separated and concentrated in vacuo. The crude product was purified by chromatography on silica gel (EtOAc/DCM) to provide (5R,7R)-7-(4-(3-(benzyloxy)prop-1-ynyl)phenyl)-3-oxa-1-azaspiro[4.4]nonan-2-one (20 g, 44.3 mmol, 79% yield) as a brown oil.
