To a solution of methyl 2-[[(1,1′-biphenyl)-4-yl]sulfonyl]amino-4-pentenoate 16a (2.0 g, 5.8 mmol) in CH2Cl2 (15 mL), NaHCO3 (0.58 g, 7 mmol) and water (10 mL) at 0° C., is added slowly m-chloroperbenzoic acid (3.3 g, 11.6 mmol, 57-86%). The reaction mixture is stirred overnight. The resulting mixture is diluted with aqueous NaHCO3, and the mixture is extracted three times with CH2Cl2. The combined CH2Cl2 layers are washed with NaHCO3, brine, dried over MgSO4 and concentrated to an oil which is purified by column chromatography eluting with EtOAc/hexane (4:6) to give the desired product as a mixture of two isomers (ratio 2:3).
