To a stirred solution of S1 (3.4 g, 12.6 mmol) in DMF (50 mL) was added Cs 2 CO 3 (6.15 g, 18.9 mmol) and tert-butyl 4-bromobutanoate (2.5 mL, 13.2 mmol). The resulting mixture was stirred at rt for 17 h. The mixture was diluted with water (200 mL) and extracted with ethyl acetate (3 × 80 mL). The combined organics were washed with 5% aqueous LiCl (3 × 50 mL), passed through a hydrophobic frit and concentrated in vacuo. The oil was purified by flash column chromatography, eluting 0-15% EtOAc in cyclohexane on a 80 g silica column over 20 CV. Fractions containing product were combined and concentrated in vacuo to afford S2 as a light yellow oil (4.99 g, 12.1 mmol, 96% yield).