A mixture of compound 2W (5.3 g, 25 mmol), methanol (50 ml) and dioxane solution of 4N HCl (10 ml) was placed in a sealed tube and the reaction was heated at 70°C overnight. Upon completion of the reaction, the reaction mixture was concentrated, followed by addition of tetrahydrofuran (20 ml), dichloromethane (50 ml), triethylamine (16 ml) and di-tert-butyl dicarbonate (11 g). The mixture was stirred at room temperature overnight and then concentrated. The concentrated residue was diluted with ether (200 ml) and washed with deionized water (100 ml). The organic layer was separated, dried with anhydrous sodium sulfate, filtered and concentrated again. Finally, the residue was purified by silica gel column chromatography using 1:1 (v/v) ethyl acetate/hexanes and 2:1 (v/v) ethyl acetate/hexanes sequentially as eluent to give the target product 3W (4.35 g, 71% yield).
