Compound 52 (0.16 g, 0.605 mmol) was dissolved in anhydrous THF (5 mL). To the clear solution were sequentially added triethylamine (0.18 mL, 1.21 mmol) and 4-dimethylaminopyridine (15 mg, 0.121 mmol). The mixture was cooled to about −10° C. for 15 min by a methanol-ice bath. To the cold solution was slowly added methanesulfonyl chloride (51 μL). The solution was further stirred at the temperature (about −10° C.) for an additional 30 min, then allowed to slowly warm up to room temperature by removing the cooling bath. The reaction mixture was concentrated under reduced pressure. The residue was chromatographed over silica gel, eluting with a mixture of ethyl acetate (0-35%) and hexanes (100-65%), to afford 0.212 g (83%) of the product 53 as a white solid. 1H NMR (CDCl3) δ 1.44 (s, 9H), 1.52-1.70 (m, 4H), 2.68 (t, J=7.4 Hz, 2H), 3.13-3.18 (m, 2H), 3.40 (s, 6H), 4.53 (br s, 1H), 7.27 (s, 4H). m/z (APCI)=321 [C17H28N2O6S2−Boc+H]+.
