To a suspension of acid 54B (210 mg, 0.649 mmol) in 15 mL of acetonitrile at rt was added DCC (134 mg, 0.649 mmol) followed by p-nitrophenol (180 mg, 1.30 mmol). The suspension was refluxed for 1 h, cooled to rt and filtered. The filtrate was concentrated under reduced pressure and the residue dissolved in EtOAc, washed with water and brine, dried (MgSO4), filtered and concentrated under reduced pressure. The residue was purified by flash chromatography (silica gel, EtOAc/hexanes, 75:25) to afford the title compound (121 mg) as a white foam. 1H NMR (CD3OD) 1.94-2.05 (m, 1H), 2.18, 2.22 (s, 3H), 2.24-2.33 (m, 1H), 3.34-3.40 (m, 1H), 3.67-3.75 (m, 1H), 4.08, 4.16 (d, J=6.05, 1H), 4.35, 4.42 (m, 1H), 7.33, 7.38 (d, J=8.25, 1H), 7.70 (m, 1H); 13C NMR (CD3OD) 15.90, 16.11, 36.95, 37.05, 44.88, 45.06, 70.91, 70.94, 72.46, 72.90, 115.19, 115.53, 116.60, 116.66, 129.30, 129.46, 132.74, 133.03, 137.41, 137.61, 138.08, 138.37, 138.42, 138.53, 159.69, 159.97, 172.10, 172.43; HPLC a) column: Phenominex LUNA C18 4.6×50 mm, 4 min gradient, 10% MeOH/90% H2O/0.1% TFA to 90% MeOH/10% H2O/0.1% TFA; 1 min hold, 4 mL/min UV detection at 220 nm, 2.32 min retention time; HPLC b) column: Shimadzu Shim-Pack VP-ODS C18 4.6×50 mm, 4 min gradient, 10% MeOH/90% H2O/0.1% TFA to 90% MeOH/10% H2O/0.1% TFA, 1 min hold; 4 mL/min, UV detection at 220 nm, 2.15 min retention time (100%); HPLC c) column: Daicel Chiralcel OD 4.6×250 mm, Isocratic 25% Isopropanol/Hexanes, 30 min, 1 mL/min, UV detection at 220 nm, 17.65 min retention time (99%); MS (ES) m/z 306 [M+H]+.
