Reaction Steps
To a stirred solution at −20° C. of 5 (1.49 g, 2.62 mmol) in anhydrous THF (30 mL) under nitrogen was slowly added a solution of TBAF (2.88 mL, 2.88 mmol, 1.0M in THF). The reaction mixture was allowed to warm-up to the room temperature over 1 h and was stirred for additional 22 hours. MeOH was added and the reaction mixture was concentrated, diluted with AcOEt, and successively washed with saturated aqueous solution of NaHCO3, H2O, a saturated aqueous solution of NH4Cl and brine, dried over MgSO4, filtered, and concentrated. The residue was purified by flash chromatography on silica gel (MeOH/CH2Cl2, 5/95→10/90 plus several drops of NH4OH) and triturated with a mixture of AcOEt/CH2Cl2/hexane to afford the title compound 6 (956 mg, 2.10 mmol, 80% yield) as a pale yellow solid. 1H NMR (400 MHz, DMSO-d6) δ(ppm): 10.76 (s, 1H), 9.39 (s, 1H), AB system (δA=7.58, δB=7.44, JAB=8.0 Hz, 4H), 7.56 (d, J=15.7 Hz, 1H), 7.42-7.34 (m, 2H), 7.33 (d, J=8.0 Hz, 1H), 7.12 (d, J=2.3 Hz, 1H), 7.05 (td, J=7.2, 1.2 Hz, 1H), 6.98-6.90 (m, 2H), 6.90 (d, J=15.8 Hz, 1H), 6.77 (dd, J=8.0, 1.4 Hz, 1H), 6.60 (ddd, J=7.5, 7.5, 1.4 Hz, 1H), 4.98 (bs, 2H), 4.43 (t, J=5.4 Hz, 1H), 3.78 (s, 2H), 3.56 (td, J=6.3, 5.6 Hz, 2H), 2.94-2.84 (m, 2H), 2.82-2.74 (m, 2H), 2.68 (t, J=6.5 Hz, 2H).
