Dissolve triphenylphosphine (41.97 g, 158.4 mmol) in fresh toluene (660 mL) and add bromine (8.14 mL, 158.4 mmol) until a yellow color persists. Add dropwise a solution of ditert-butyl(1S,2S,4S,5R,6R)-2-(tert-butoxycarbonylamino)-4-hydroxy-bicyclo[3.1.0]hexane-2,6-dicarboxylate (32.75 g, 79.2 mmol) in toluene (176 mL) and anhydrous pyridine (528 mL) during 45 min. Stir the reaction at 75° C. overnight. Cool to room temperature, dilute with ethyl acetate, filter and concentrate to dryness. Slurry the crude in methyl tert-butyl ether, filter to remove the solids and concentrate the filtrate to dryness. Purify the crude by silica gel chromatography (750 g) eluting with hexane:ethyl acetate (0:100 to 80:20) to obtain the title compound as a white solid (29.52 g, 78%). MS (m/z): 498, 500 (M+23).
