Tetrabutylammonium hydrogen sulfate (0.020 g, 0.059 mmol), benzenesulfonyl chloride (0.055 mL, 0.43 mmol) and 50% aqueous NaOH solution (0.20 mL) were added to a solution of 54 (0.095 g, 0.29 mmol) in CH2Cl2 (3 mL). After stirring for 2 hours, the mixture was poured into saturated aqueous NaHCO3 solution and extracted with CH2Cl2 three times. The combined organic solutions were washed with saturated aqueous NaHCO3 solution, dried (MgSO4), and concentrated to afford a tan syrup. MeOH was added to the syrup and the mixture was cooled in an ice bath. The precipitate was filtered off, washed with MeOH and dried to afford 55 (0.088 g) as a brown solid. The filtrate was concentrated and purified by means of preparative TLC with hexane:AcOEt as eluent to afford additional 55 (0.016 g, total yield: 0.104 g, 77%) as a brown solid. 1H NMR (400 MHz, CDCl3) δ 3.08-3.12 (m, 4H), 3.80-3.84 (m, 4H), 7.00 (d, J=0.26 Hz, 1H), 7.38-7.44 (m, 2H), 7.48-7.54 (m, 1H), 7.72 (s, 1H), 8.08-8.11 (m, 2H), 8.12 (d, J=0.26 Hz, 1H).
