To a solution of S4 (6.22 g, 12.4 mmol) in DMF (20 mL) was added piperidine (2.45 ml, 24.8 mmol), and the resulting solution stirred at rt for 1 h. The reaction mixture was diluted with water (100 mL) and filtered under vacuum, washing the solid with additional water (100 mL). The filtrate was washed with diethyl ether (50 mL), then EtOAc (50 mL), and the aqueous layer was then concentrated in vacuo to afford S5 as a white solid (3.45 g, 11.9 mmol, 97% yield).