[H-].[Na+].[CH3:3][N:4]([CH2:6][C:7]1[CH:12]=[CH:11][C:10]([OH:13])=[CH:9][C:8]=1[CH3:14])[CH3:5].CS(O[CH:20]1[CH2:23][N:22]([C:24]([O:26][C:27]([CH3:30])([CH3:29])[CH3:28])=[O:25])[CH2:21]1)(=O)=O.O>CN(C=O)C>[CH3:5][N:4]([CH2:6][C:7]1[CH:12]=[CH:11][C:10]([O:13][CH:20]2[CH2:21][N:22]([C:24]([O:26][C:27]([CH3:30])([CH3:29])[CH3:28])=[O:25])[CH2:23]2)=[CH:9][C:8]=1[CH3:14])[CH3:3]
A mixture of NaH (55-65% disp. in oil, 1.0 g, 23 mmol) in dry DMF (25 mL) under nitrogen was cooled by an ice-bath. A solution of 20A (2.9 g, 18 mmol) in DMF (20 mL) was added drop-wise over 10 minutes. The mixture was stirred for one hour and then tert-butyl 3-[(methylsulfonyl)oxy]azetidine-1-carboxylate (5.3 g, 21 mmol) in DMF (15 mL) was added. The mixture was heated to 80° C. over night and then cooled to RT. Water (200 mL) was added and the mixture was extracted three times with EtOAc (100 mL). The combined organic phases were dried (MgSO4) and the solvent was removed by evaporation. The residue was purified by column chromatography on SiO2 using 5-75% EtOAc/heptane as eluent. The product was further purified using 0.5% NH3 in MeOH/DCM as eluent. There was obtained 3.1 g (55%) of 20B as a semi-solid. 1H NMR (600 MHz, CDCl3): δ 1.44 (s, 9H), 2.21 (s, 6H), 2.32 (s, 3H), 3.29 (s, 2H), 3.93-4.02 (m, 2H), 4.21-4.32 (m, 2H), 4.79-4.89 (m, 1H), 6.48 (d, 1H), 6.55 (s, 1H), 7.12 (d, 1H).
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