To a solution of 4 (305 mg, 0.29 mmol, 1 eq) in THF (10 mL) at 0° C. was added a solution of LiOH (10.3 mg, 0.43 mmol, 1.5 eq) in H2O (2 mL) dropwise. The mixture was stirred for 2 h, then quenched with 1N HCl (3 mL) and brine (10 mL). The organic layer was separated and the resulting aqueous layer was further extracted with EtOAc (2×15 mL) and (15 mL). The combined organic extracts were dried over MgSO4 and the solvent was removed in vacuo. The resulting carboxylic acid was then dried on the high-vacuum pump for 2 h. To a solution of MNBA (118 mg, 0.34 mmol, 1.2 eq) and DMAP (84 mg, 0.69 mmol, 2.4 eq) in CH2CL2 (200 mL) was added a solution of the crude carboxylic acid in CH2Cl2 (130 mL) and THF (20 mL) dropwise over 3 h. The reaction mixture was then left to stir up at rt overnight. The solvent was then removed in vacuo and the residue was further purified by silica gel column chromatography with hexane/EtOAc (1:9→0:1) to yield 5 as a white solid (171 mg, 58%).
