Compound 13 (22.6 g, 0.1 mol), benzyl chloride (57 g, 0.46 mol) and anhydrous potassium carbonate (120 g, 0.87 mol) were mixed in dry DMF (500 mL). The mixture was heated at 75° C. while stirring under nitrogen overnight. TLC confirmed that the reaction was complete. The reaction mixture was cooled, filtered, and the filtrate was evaporated to remove the volatiles. The residue was dissolved in 1 L of recycling [?] methylene chloride and a silica gel plug was used to remove some impurities with low Rf. The fully benzyl-protected product was obtained as a thick, yellow oil after removing the solvent: raw yield 53 g (90%).
