A solution of S3 (269 mg, 0.68 mmol) in THF (8.5 mL) was cooled to 0 °C and to this was added DCC (155 mg, 0.75 mmol). The mixture was stirred at 0 °C for 30 min, then N-hydroxysuccinimide (86 mg, 0.75 mmol) was added and the resulting mixture stirred at rt for 23 h. The suspension was filtered and the filtrate concentrated in vacuo. The residue was purified by column chromatography, eluting 20-80% EtOAc in cyclohexane on a 24 g silica column. Fractions containing product were combined and concentrated in vacuo to afford S7 as a white solid (271 mg, 0.55 mmol, 81% yield).