To a solution of intermediate 11 (200 mg, 0.886 mmol) in dichloromethane (7.5 mL) were added 3,4-dimethoxybenzenesulfonyl chloride (320 mg, 1.320 mmol) and triethylamine (220 mg, 2.169 mmol) under a N2 atmosphere. The reaction mixture was stirred at room temperature for 4 h. Water (10 mL) followed by dichloromethane (10 mL) were added to the reaction mixture and the organic layer was separated, dried on sodium sulfate, filtered and concentrated under reduced pressure. The residue was purified by column chromatography using silica gel to give the compound 12b as a solid (200 mg, 53.0%). HPLC: 99% (Rt 6.507 min).
