3-Phenyl-1-oxa-2-azaspiro[4.5]dec-2-ene-8-carboxylic acid (400 mg, 1.5 mmol) was slowly added to a solution of N-ethyldiisopropylamine (662 mg, 4.6 mmol), 1-hydroxybenzotriazole hydrate (206 mg, 1.5 mmol), O-(benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium tetrafluoroborate (496 mg, 1.5 mmol) and 4-tert-butylaniline (230 mg, 1.5 mmol) in abs. THF (11 ml). The reaction mixture was stirred overnight and the solvent removed under vacuum. The residue was taken up in EtOAc (75 ml). The organic phase was washed successively with sat. aq. NaCl solution, sat. aq. NaHCO3 solution, sat. aq. ammonium sulphate solution and sat. aq. NaCl solution, dried over magnesium sulphate and the solvent removed under vacuum. The desired product 3-phenyl-1-oxa-2-azaspiro[4.5]dec-2-ene-8-carboxylic acid-(4-tert-butylphenyl)amide was obtained as a diastereomer mixture in a yield of 220 mg (37% of the theoretical amount).
