[C:1]([OH:5])([CH3:4])([CH3:3])[CH3:2].[CH3:6][C:7]([CH3:10])([O-:9])[CH3:8].[K+].[C:12]1([CH3:18])[CH:17]=[CH:16][CH:15]=[CH:14]C=1.[N-:19]=[C:20]=[O:21].CC([O:26]C)(C)C>O>[C:1]([O:5][C:20]([NH:19][C:15]1([C:14]([O:9][C:7]([CH3:10])([CH3:8])[CH3:6])=[O:26])[CH2:16][CH:17]=[CH:12][CH2:18]1)=[O:21])([CH3:4])([CH3:3])[CH3:2]Add t-butanol (35 g, 471 mmol) to a solution of potassium t-butoxide (1M in THF, 471 mL, 471 mmol) under nitrogen. Cool the reaction solution was cooled to 0-5° C. and add the toluene solution containing 1-(isocyanate)-1-(tert-butoxycarbonyl)cyclopent-3-ene over 60 minutes, maintaining the temperature at 0-10° C. Warm the reaction to 23° C., stir for 2 hours, and assay by GC for the disappearance of the isocyanate starting material. Add the reaction mixture to a mixture of deionized water (1.2 L) and MTBE (1.2 L) at 15° C. Stir the solution for 20 minutes and separate the layers. Wash the organic layer with a 20% brine solution (250 mL) and separate the layers. Transfer the organic layer concentrated via distillation, to approximately 250 mL. Add heptane (500 mL) and concentrate the solution to a total volume of 250 mL. Cool the resulting slurry solution to 0° C., stir for 2 hours, and filter. Wash the filter cake with cold heptane (2×100 mL) and vacuum dry to give 34.54 g (52% yield from 1-(carboxylic acid)-1-(tert-butoxycarbonyl)cyclopent-3-ene) of 1-(tert-butoxycarbonylamino)-1-(tert-butoxycarbonyl)cyclopent-3-ene as a white solid.
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