S5 (260 mg, 0.93 mmol) and Boc-Val-OSu (292 mg, 0.93 mmol) were combined in DMF (1 mL) and the resulting mixture stirred at rt for 6 h. The reaction mixture was purified directly by reverse phase chromatography, eluting 15-55% acetonitrile in water with a 10 mM ammonium bicarbonate modifier adjusted to pH 10. Fractions containing product were combined and concentrated in vacuo to afford S6 as an off-white solid (394 mg, 0.82 mmol, 89% yield).